Product Training

Caterpillar Bulldozer
D8R series
Service Training
Meeting Guide 699 SESV1699
June 1998

TECHNICAL PRESENTATION

D8R TRACK-TYPE TRACTOR

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OPERATOR'S STATION

• Clear full-circle view The operating environment of the D8R is designed to give the operator a
clear full-circle view of the job site and the machine. The cab is isolation
mounted, which reduces the noise and vibration to a sound level below
85dB(A).
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3
2

• D8R features: In the operator's station (1), the fully adjustable suspension seat (2)
provides maximum comfort and less operator fatigue. The operator can
1. Operator's station adjust the seat height, front to rear position, tilt, and seat back angle. The
seat is angled 15° to the right to provide maximum visibility of the
2. Seat
implement operation. The back and seat cushion assembly can be
3. Bolt separated from the suspension base by removing one 9/16 in. stop bolt (3)
located on the lower right front of the seat.
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4
2

• Steering control The dual twist tiller control (1) combines machine steering, directional
components: changes, and gear selection into a single control. Rotating the
transmission speed selector (2) selects a transmission speed. The
1. Steering and
directional control transmission speed selector is always in one of the three transmission
speeds.
2. Transmission
speed selector The operator can select three conditions by rotating the twist handle (3).
Rotating the twist handle to the forward position selects the FORWARD
3. Twist handle direction. Rotating the twist handle to the reverse position selects the
REVERSE direction. Both positions are detented and will stay in the
4. Parking brake and selected direction until the operator manually selects a different position.
transmission lock
The NEUTRAL position is between the FORWARD and the REVERSE
• Tiller controls
positions. This position is also detented.
steering, speed and
direction With the machine in FORWARD, moving the twist handle (3) toward the
front causes the machine to turn left, while moving the twist handle to the
rear causes the machine to turn right. When the operator releases the
twist handle, a centering spring returns the handle to the center (NO
STEER) position.

Moving the parking brake and transmission lock lever (4) up locks the
transmission in NEUTRAL and ENGAGES the parking brake.

➥
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WARNING

With the engine running and the machine stationary, moving the
twist lever control toward the front or the rear of the machine can
cause the machine to steer. To avoid potential personal injury and/or
property damage, always put the transmission in NEUTRAL and
ENGAGE the parking brake lever.
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6 2

3
4
1

• Operator's station The dozer control lever (1) is located at the right of the operator's seat.
components: The standard implement control lever allows the operator to control all the
blade functions from one lever.
1. Dozer control lever

2. Ripper control
Moving the lever completely forward to the detent position causes the
lever blade to FLOAT. When the lever is moved partially to the forward
position, the blade will LOWER. Moving the lever to the rear of the
3. Trigger switch center (HOLD) position causes the blade to RAISE. Moving the lever to
the right tilts the right side of the blade down, and moving the dozer
control lever to the left tilts the left side of the blade down.

All machines are equipped with three implement control valve sections:
the ripper, the tilt, and the lift. The ripper control lever (2) and all the
external ripper components are attachments. To RAISE the ripper, move
the control lever (2) out of the center (HOLD) position toward the
operator’s seat. To LOWER the ripper, move the control lever out of the
center (HOLD) position away from the operator’s seat . To bring the
ripper shank closer to the machine, activate the electric trigger switch (3)
on the ripper lever, and move the control lever to the right. To move the
ripper shank away from the machine, move the lever to the left.

4. Pin puller switch An optional pin puller is available for machines equipped with a single
location shank ripper. Moving the switch (4) to the right extends the pin, and
moving the switch to the left retracts the pin. Transmission oil pressure is
used to extend and retract the pin puller cylinder.

➥
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5. Horn button The horn button (5) is located between the dozer control lever and the
location ripper control lever.

6. Cigarette lighter Above the implement control levers are the 24-Volt DC cigarette
7. Ash tray
lighter (6) and the ash tray (7).
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1
2

10

• Component locations: Depressing the large pedal (1) ENGAGES the service brakes. The
smaller pedal is the decelerator pedal (2). During normal operation, the
1. Brake pedal operator moves the governor control lever (3) into the HIGH IDLE
position and decreases the engine rpm using the decelerator pedal to
2. Decelerator pedal
control engine speed for directional shifts, modulated steering, and
precise grading control of the blade.
3. Governor control
lever
NOTE: Both pedals and the governor lever have adjustment screws.
Refer to the Service Manual module "D8R Track-type Tractor Power
Train Testing and Adjusting" (Form SENR8326) for the adjustment
procedures.
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2 4 6 7
3 5
1

9 8

11

• Instrument panel The instrument panel (on cab equipped machines) includes the air
components: conditioning and heating controls (1), the key start switch (2), the ether
starting aid button (3), the Electronic Monitoring System (EMS) action
1. Air conditioning
and heating lamp (4), the EMS panel test switch (5), the EMS action horn (not
controls visible), the machine light switches (6), the service hourmeter (7), the four
gauge cluster (8), and the EMS display (9) used for monitoring the
2. Key start switch machine systems.
3. Ether starting aid
If the machine is equipped with automatic engine pre-lubrication, the pre-
button
lubrication start function has been incorporated directly into the key start
4. Action lamp switch.

5. EMS test switch INSTRUCTOR NOTE: The automatic engine pre-lubrication system
and EMS will be discussed in more detail later in this presentation.
- Action horn (not
visible)

6. Light switches

7. Hourmeter

8. Gauges

9. EMS display
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12

• Component locations: The disconnect switch (1) and the 24-Volt DC power outlet receptacle (2)
are located on the left side of the operator's station. The receptacle can be
1. Disconnect switch
used to power many diagnostic service tools that have the 4C9031 Battery
Tool Cable. The cable can also be plugged into the cigarette lighter.
2. Receptacle
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13

ENGINE

• 3406C engine The 3406C turbocharged and aftercooled engine is rated at 228 kW
(305 hp) at 2100 rpm and supplies power through the torque divider, the
• Meets emissions power shift transmission, and the final drives to the undercarriage.
requirements
The 3406C engine meets worldwide emissions regulations for the U.S.
• Increased fuel Environmental Protection Agency, the European Union and the California
pressure
Air Resources Board. To meet these stringent worldwide regulations,
• Crankshaft journals fuel pressure has been increased from 82800 kPa (12000 psi) on the D8N
and bearings larger with the two pump system to 110400 kPa (16000 psi) on the D8R. The
compression ratio has also been increased.
• Steel spacer
Other features include:
• Gallery-cooled pistons
• Steel spacer between the block and head eliminates the need for
• Additional coolant block counterbores.
passages
• Gallery-cooled pistons and full-length, water-cooled cylinder
• Stellite-faced valves liners provide maximum heat transfer for longer component life.
• Increased torque rise • Cylinder heads also utilize additional coolant passages to provide
maximum cooling to the rear of the engine.
• Tamper resistant bolts
• Stellite-faced valves.
• Torque rise has increased from 42% to 55%.
• Tamper resistant bolts on the fuel ratio control.
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EXHAUST
STACK
D8R AIR INLET AND DUST EJECTOR LINE
EXHAUST SYSTEM

BOOST MUFFLER
PORT

AFTERCOOLER
EXHAUST
MANIFOLD
PRECLEANER

HEAD AND
VALVES

CYLINDER

TURBOCHARGER
AIR
CLEANER TURBINE WHEEL

FILTER ELEMENT INDICATOR COMPRESSOR WHEEL

14

• Increased air cleaner The air cleaner system on the D8R has been increased in size. The larger
size air inlet system helps improve the reliability of the air filter indicator and
provides increased time between air filter changes. If a D8N needs to be
• Air inlet and exhaust
components: converted to this configuration, refer to the Special Instruction
"Decreasing Air Inlet Restriction" (Form SEHS9883).
- Precleaner
The air inlet and exhaust components are: the precleaner and the dust
- Air cleaner
ejector line, the air cleaner, the filter element indicator, the turbocharger,
- Filter indicator the aftercooler, the cylinders, the valves and the valve system components,
- Turbocharger the exhaust manifold, and the muffler.
- Aftercooler
- Cylinders, head and
valves
- Exhaust manifold
- Muffler
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15

• Primary filter (not Clean inlet air is pulled through two radial seal filters in the air cleaner
shown) housing: the primary filter (not shown) and the secondary (1). Clean the
primary filter only after the yellow piston in the filter element indicator
1. Secondary filter
(2) moves into the red zone with the engine running at HIGH IDLE.
After the primary filter has been cleaned, if the indicator still moves into
2. Filter element
indicator the red zone or the exhaust smoke is black, install a new primary filter and
recheck the indicator at HIGH IDLE. If the indicator still moves into the
red zone, replace the secondary filter. Then, reset the indicator.

NOTE: The filter element indicator is threaded into an elbow which

contains a small screen to keep contaminents from entering the
engine. If the screen becomes plugged, the indicator will not function
properly.
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1 2

16

1. Aftercooler Clean inlet air from the air cleaner is pulled through the air inlet of the
turbocharger by the rotation of the compressor wheel. Compressed inlet
2. Boost pressure tap
air from the compressor side of the turbocharger is forced into the
aftercooler (1) through the air inlet pipe. The air passes over the core
assembly in the aftercooler, which lowers the air temperature to
approximately 93°C (199°F). Engine coolant flows through the core
assembly to cool the inlet air. The cooler air flows out the bottom of the
aftercooler into the cylinder head. The advantage of the cooler air is
greater combustion efficiency.

To measure the boost pressure, remove the pipe plug and install an
adaptor, which is then connected to the pressure test gauge.
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17

• Exhaust system After the air flows through the aftercooler and into the cylinder head, the
components: intake valves in the head open and direct the air to the cylinders where the
fuel is mixed in the combustion chamber. When the exhaust valves open,
1. Right side of
engine the gasses are directed to the turbine wheel of the turbocharger which
causes the compressor wheel to spin faster. After travelling through the
2. Muffler turbocharger, the exhaust gasses are directed through the muffler (2) and
the exhaust stack, which are located on the right side of the engine (1).
3. Dust ejector tube Just before entering the exhaust stack, the gasses flow past the dust ejector
tube (3). The flow creates a slight vacuum in the precleaner housing
4. Dust ejector line through the dust ejector line (4) to remove dust and dirt from the
precleaner.
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1 2

3
4

18

• Ether starting aid The ether starting aid group (1) is standard on the D8R and is located on
components: the left side of the engine. The group includes the ether valve, the
cylinder clamp assembly, the supply tube, and the atomizer assembly.
1. Starting aid group
location When starting the engine below 0°C (+32°F), review the Operation and
Maintenance Manual for the specific procedure and warnings. The
2. Starting aid starting aid solenoid (2) is activated by depressing the starting aid knob
solenoid (3) every two seconds while cranking the engine. After the engine is
running, release the key start switch and the starting aid knob. If the
3. Knob coolant temperature is above 37.8°C (100°F), ether cannot be injected into
the air inlet housing because an engine coolant temperature switch (4) is
4. Engine coolant threaded into the right rear of the engine head.
temperature switch
NOTE: The starting aid knob injects a precise amount of ether for a
two second period each time the knob is depressed.
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INJECTOR
D8R FUEL SYSTEM
CONSTANT
BLEED VALVE

TANK
FUEL
RETURN PUMP

DRAIN
SHUTOFF VALVE
VALVE

PRIMING TRANSFER
PUMP PUMP
SUPPLY
SECONDARY
FILTER
PRIMARY
FILTER

19

• Fuel system This schematic shows the fuel system components: the fuel tank, shutoff
components: valve, fuel tank drain valve, fuel return line, primary fuel filter, priming
pump and check valves, transfer pump, secondary filter, constant bleed
- Fuel tank
valve, fuel pump and governor, and one of the fuel injection nozzles.
- Shutoff valve
The fuel transfer pump moves fuel from the tank to the fuel injection
- Tank drain valve
pump. The bypass valve in the fuel transfer pump maintains the fuel
- Return line injection pump supply pressure at 170 to 280 kPa (25 to 40 psi). The
- Primary filter constant bleed valve, located in the elbow that threads into the fuel pump
body, returns approximately 34 Liters (9 gal.) per hour of fuel and air to
- Transfer pump
the tank. This flow helps keep the fuel cool and free of air.
- Priming pump and
check valves The fuel pump group meters the fuel and contains six individual fuel
injection pumps that increase the fuel pressure to 6900 kPa (1000 psi).
- Secondary filter
This high pressure fuel flows through steel lines to the direct injection fuel
- Bleed valve nozzles, which spray a fine pattern of fuel into the combustion chambers.
- Fuel pump
- Injectors
- Nozzle
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2 3
5
1

20

• Fuel tank component The D8R fuel tank (1) is located on the rear of the machine behind the
locations: operator's station. A strainer (not shown) in the fuel fill tube keeps debris
out of the fuel tank during refueling. A vented cap (2) prevents pressure
1. Fuel tank
build-up in the fuel tank and also prevents cooling fuel from creating a
2. Vented cap vacuum. The fuel tank has been designed with an indentation (5) in the
top center to improve the operator's view for rear attachment operations.
3. Fuel level sender The capacity of the tank is approximately 625 Liters (165 gal.).
4. Fast fill adaptor Located at the top center of the tank is the electrical fuel level sender (3).
The sender provides a signal to the fuel level gauge on the dash. If the
5. Indentation to
improve rearward dash gauge is not functioning, the operator can view the top of the sender
visibility to determine the tank fuel level.
The optional 2G9000 Fast-fill Fuel Adaptor (4) is located on the bottom
left of the tank which allows fuel to be pumped into the tank faster for
quicker refueling and shorter downtime while refueling. To connect to the
adaptor, use the 4C6717 Nozzle available in the Caterpillar parts system.
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3 1

21

• Fuel tank component The fuel flow from the tank to the engine fuel system can be shut off by
locations: moving the red valve handle (1) down. Return fuel from the fuel system
enters the bottom of the tank through the center hose (2). To drain
1. Shutoff valve
sediment and moisture accumulation from the bottom of the tank into a
2. Return hose suitable container, open the drain valve (3) until clear clean, diesel fuel
begins to drain.
3. Drain valve
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5
3

22

• Fuel level component As previously stated, the fuel tank (1) is equipped with an electrical fuel
locations: level sender (2) located directly behind the operator on the top of the fuel
tank. The sender uses a plastic float (3) that slides up and down and
1. Fuel tank
rotates the rod (4) as the fuel level changes. The rod rotation sends an
2. Fuel level sender electrical signal to the dash gauge (5) and, at the same time, the
mechanical indicator on the top of the sender shows the approximate level
3. Plastic float in the tank.
4. Rod

5. Dash gauge
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1 2 3
4

7
6

23

• Fuel system The fuel pump and governor group are located on the left side of the
component locations: engine (1). The primary fuel filter and priming pump (2) and the
secondary fuel filter (4) are located on the left side of the engine behind
1. Left side of engine
the access door. The fuel heater (3) is optional and uses engine coolant to
2. Primary fuel filter heat the diesel fuel. The fuel transfer pump (5 and 7) transfers fuel from
and pump the tank, through the primary fuel filter to the secondary fuel filter. After
the fuel is filtered, fuel is directed to the fuel housing. The fuel transfer
3. Fuel heater pump is located on the bottom front of the fuel housing and is driven by
the fuel injection pump camshaft.
4. Secondary fuel
filter The fuel ratio control (6) limits the amount of fuel to the cylinders during
an increase of engine speed (acceleration) to reduce exhaust smoke.
5. Fuel transfer pump Properly adjusted, it also minimizes the amount of soot in the engine. The
fuel ratio control has tamper resistant bolts that break off when they are
6. Fuel ratio control removed. Adjustment and repairs must be made by authorized personnel.
7. Fuel transfer pump
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D8R ADVANCED MODULAR COOLING SYSTEM

EXPANSION
TANK
AFTERCOOLER TEMPERATURE
REGULATOR

AIR VENT

BYPASS

SHUNT
LINE AIR
FLOW

ENGINE
OIL
COOLER

WATER PUMP
WATER OUTLET
PUMP

24

• AMOCS radiator has The Advanced Modular Cooling System (AMOCS) on the D8R utilizes
two-pass system two-pass radiator modules to increase radiator efficiency over the previous
system used on the D8N.
During normal operation, the water pump directs coolant through the
engine oil cooler and then into the cylinder block. As the coolant flows
through the block into the cylinder head, all the coolant is collected at the
temperature regulator housing. When the coolant temperature exceeds
90°C (195°F), the regulator opens and directs the coolant to the front of
the sectioned bottom tank. Hot coolant travels up the nine individual core
modules and down the back side of the modules into the rear section of
the bottom tank and then back to the water pump.
The shunt line provides positive pressure at the water pump inlet to reduce
water pump cavitation. The air vent line removes air from the cooling
system while the system is being filled and during operation. The
expansion tank is a reservoir and retains the expansion volume of the
coolant as the temperature increases.
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D8R AMOCS
RADIATOR

25

• AMOCS coolant flow This sectional view shows the coolant flow path through the AMOCS
path radiator. The top of each core module is commonly connected to the
expansion tank (not shown) that is located directly above the cores. Each
core has nine steel fins per 25 mm (1.0 in.) and uses brass tube
construction within the core. A core of six steel fins per 25 mm (1.0 in.)
is available as an option for trash applications.
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5
3

2
6

1 7

26

• AMOCS radiator The AMOCS radiator has two major advantages over earlier designs. The
advantages: first advantage is the improvement of the modular core serviceability.
The AMOCS radiator has been designed to include one divided tank (1)
- Improved core
serviceability below the modular cores (2). This design eliminates the tank which was
formerly attached to the top of the modular cores. By locating both tanks
- Increased efficiency below the modular cores, removal of a single module core is simplified.
By eliminating the top tank, the seals for all the modules need not be
• AMOCS components:
disturbed if only one must be replaced. The maintenance time to replace
1. Divided tank a single module is considerably reduced. The second advantage of the
AMOCS is increased cooling efficiency.
2. Cores
The expansion tank (3) is located directly above the cores and is used for
3. Expansion tank coolant expansion. The coolant level sight gauge (4), the radiator cap (5),
4. Sight gauge the air vent line (6), and the shunt line (7) are located on the tank.

5. Radiator cap The drain valve (8) is used to drain the coolant from both sides of the
sectioned bottom tank of the radiator and from the block and the oil
6. Air vent line cooler.
7. Shunt line The fan diameter has increased from 1016 mm (40 in.) to 1092 mm
8. Drain valve
(43 in.).
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1 4

3
2

5 7

27

• Cooling system This slide shows the components of the radiator. Each core (1) has two
components: holes (2), one for coolant in and one for coolant out. The cores are set
into the bottom tank (3) into the holes (4) using the seal (5) to seal the
1. Core module
connection.
2. Core inlet and In the bottom tank, the coolant inlet hole (6) and outlet hole (7) connect
outlet holes
the cores to the engine.
3. Bottom tank Mineral oil will damage the seal (5). If any oil is found in the cooling
system, inspect all the seals.
4. Bottom tank inlet
and outlet holes

5. Seal

6. Coolant inlet hole

7. Coolant outlet hole

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28

• Cooling system The fill tube and radiator cap (1) are located on the top of the expansion
components: tank for the cooling system. The expansion tank is located directly above
the radiator in the engine compartment. Access to the fill tube is provided
1. Fill tube and
radiator cap by lifting a spring hinged door on top of the engine compartment. The
correct coolant level can be checked two ways. A sight glass (2) in the
2. Sight glass expansion tank is visible in the left side of the engine compartment. The
sight glass should always be filled with coolant. If any air can be seen in
the sight glass, coolant needs to be added to the expansion tank. When
adding coolant to the system, maintain the level between the upper and
lower ends of the angled portion of the fill tube.
NOTE: The refill capacity of the cooling system is approximately
92 Liters (24 gal.).

WARNING

Add coolant only after the engine is stopped and the radiator cap is
cool enough to touch with the base of your hand. Remove the
radiator cap slowly to relieve system pressure. Steam can cause
personal injury.
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1
COOLANT
MONITORING

3
2

29

• Cooling system The Electronic Monitoring System (EMS) temperature switch (1) is
electrical located on the left rear of the engine cylinder head. The coolant flow
components: switch (2) is located on the inlet to the water pump. The coolant
1. EMS temperature temperature gauge sender (3) is located on the temperature regulator
switch housing on the front right of the engine.

2. EMS coolant flow

switch

3. Coolant
temperature gauge
sender
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D8R ENGINE LUBRICATION SYSTEM

CYLINDER BLOCK
OIL MANIFOLD

FROM TO TURBOCHARGER
TURBOCHARGER
OIL FILTER
OIL BYPASS VALVE
FILTER
COOLER
BYPASS
OIL PUMP VALVE

ENGINE
OIL PAN
OIL COOLER
SUCTION
BELL RELIEF VALVE

30

• Engine lube system The lubrication system consists of the following components: the oil pan,
components: the oil pump and relief valve, the oil cooler and bypass valve, the oil filter
and bypass valve, the lines to and from the turbocharger, and the suction
- Oil pan bell. During normal operation, the oil pump directs oil from the pan to the
- Oil pump and relief oil cooler, the filter, and then to the turbocharger and the cylinder block
valve manifold.
- Oil cooler and
bypass valve
When the engine is cold, both the oil cooler and the filter bypass valves
will open, permitting oil to bypass these components before going to the
- Oil filter and bypass turbocharger and the block. As the oil warms, the pressure differential in
valve
the bypass valves decreases, and the valves close to permit normal oil flow
- Turbocharger lines through the cooler and the filter. If the pressure differential of either
- Suction bell bypass valve is exceeded because of a restriction, oil will always flow
through the system to lubricate the engine.
A pressure relief valve is installed in the oil pump. The valve controls the
pressure of the oil delivered by the pump to the system. The oil pump is
capable of delivering more oil than is needed, and the excess is directed
back into the inlet of the pump.
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2 5 4

3
1

10
11
6 8

12 7
9

31

• Engine lube system On the left side of the oil pan (1) is the drain valve (2). Access is
components: provided through the lower engine guard. To drain the engine oil, install a
1. Oil pan
25 mm (1 in.) pipe with 1 - 11 NPTF threads and a flexible hose
connected, then turn the drain valve (2) with a wrench to direct the used
2. Drain valve
oil into a suitable container. The engine dipstick (3), the fill tube (4), and
3. Dipstick the optional fast oil change fitting (5) are located inside the left engine
4. Fill tube compartment access door. To use the fast fill fitting (5), a 126-7539
5. Fast oil change Nozzle Assembly must be obtained.
fitting
6. Engine oil cooler
On the right side of the engine are the following components: the oil
cooler (6), the oil filter (7), the Scheduled Oil Sampling (S•O•S) tap (8),
7. Oil filter
the lubrication pressure tap (9), the oil supply line to the turbocharger
8. S•O•S tap (10), the oil return line from the turbocharger (11), and the engine oil
9. Pressure tap pressure switch (12) for the EMS.
10. Oil supply line to
turbocharger
11. Oil return line from
turbocharger
12. Engine oil pressure
switch
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32

• Pre-lubrication The D8R can also be equipped with an optional engine pre-lubrication
starting system starting system. A pre-lubrication starting system will prime the engine
with oil before the starter begins to crank the engine. This system reduces
• Key start switch
(arrow) engine component wear that usually occurs during start-up.
To start the engine, turn the key start switch (arrow) to START and hold
the key in this position to activate the engine pre-lubrication system and
start the engine. When adequate oil pressure is available, the pre-
lubrication system will disengage and pause for three seconds. The starter
will then engage and automatically crank the engine. Release the key
after the engine starts.
NOTE: The engine pre-lubrication system is designed to be
inoperable if the engine is restarted within two minutes after being
stopped.
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LEFT LEFT BRAKE LUBE

FINAL DRIVE D8R POWER TRAIN
REGULATING MAKEUP AND
RELIEF VALVE PRIORITY VALVE

DIFFERENTIAL
STEER MOTOR
FLYWHEEL HOUSING
DIFFERENTIAL
STEER PUMP
TORQUE CONVERTER
CONVERTER
TRANSMISSION
CONTROL VALVE VENT OUTLET
RELIEF
P2
P1 3406C ENGINE
P3

LUBE
3 2 1 OIL COOLER
RATIO VALVE

TRANSFER AND
BEVEL GEAR

PARKING / SERVICE
BRAKE VALVE

1 - TRANSMISSION IMPLEMENT PUMP

RIGHT BRAKE LUBE FILTER CHARGING
2 - CONVERTER CHARGING
3 - SCAVENGE
RIGHT
FINAL DRIVE
FILTER BYPASS

37

• Power train hydraulic This schematic shows the components and oil flow in the power train
system schematic hydraulic system. Section three of the three section power train pump
removes oil from both the transmission and torque converter cases and
directs the oil to the bevel gear case. Section two draws oil from the bevel
gear case and directs oil to the regulating relief valve, then to the torque
converter as charging oil and to the flywheel housing for lubrication.
Section one draws oil from the bevel gear case and directs the oil through
the power train oil filter, to the priority valve, then to the parking and
service brake valve and the transmission control valve as supply oil for the
clutches. The ratio valve directs oil from the transmission control valve to
the torque converter as additional charging oil.

All converter outlet oil is directed through the engine mounted oil cooler
to the brakes, the transmission, and the bevel gears as cooling and
lubrication oil.
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38

Torque Divider

• Torque divider (arrow) The D8R Track-type Tractor uses a torque divider (arrow) to transfer
engine power to the transmission. The torque divider is similar to those
- Transfers power to used on other Caterpillar track-type tractors.
transmission

- Provides a hydraulic The torque divider provides both a hydraulic and a mechanical connection
and mechanical from the engine to the transmission. The torque converter provides the
connection hydraulic connection, while the planetary gear set provides the
mechanical connection. During operation, the planetary gear set and the
torque converter work together to provide an increase in torque as load on
the machine increases.
STMG 699 - 46 -
6/98

TORQUE DIVIDER
ENGINE FLYWHEEL ROTATING
HOUSING

OUTLET
PLANET GEARS PASSAGE

SUN GEAR

OUTPUT SHAFT

STATOR

PLANET CARRIER

RING GEAR INLET

PASSAGE
TURBINE

IMPELLER

39

• Torque divider This illustration shows a typical torque divider as is used in the D8R. The
operation impeller, the rotating housing, and the sun gear are shown in red. These
components are on a direct mechanical connection to the engine flywheel.
The turbine and the ring gear, shown in blue, are connected. The
planetary carrier and the output shaft, shown in yellow, are also
connected. The stator is shown in brown. The planetary gears and shafts
are green. The bearings are shown in yellow.

• During NO LOAD Because the sun gear and the impeller are connected to the flywheel, they
components rotate as will always rotate at engine speed. As the impeller rotates, it directs oil
unit against the turbine blades, causing the turbine to rotate. Turbine rotation
causes the ring gear to rotate. During NO LOAD conditions, the
components of the planetary gear set rotate as a unit at the same rpm and
the planet gears do not rotate on their shafts.

➥
STMG 699 - 47 -
6/98

• Under load, relative As the operator loads the machine, the output shaft slows down. A
motion slows turbine decrease in output shaft speed causes the rpm of the planetary carrier to
rotation
decrease. Decreasing the planetary carrier rotation causes the relative
motion between the sun gear and the planet carrier to cause the planet
gears to rotate. Rotating the planet gears decreases the rpm of the ring
gear and the turbine. At this point, the torque splits with the torque
converter multiplying the torque hydraulically, and the planetary gear set
multiplying the torque mechanically.

• During stall, turbine An extremely heavy load can stall the machine. If the machine stalls, the
and ring gear rotate in output shaft and the planetary carrier will not rotate. This condition
opposite directions causes the ring gear and turbine to rotate slowly in the opposite direction
of engine rotation. Maximum torque multiplication is achieved just as the
• Torque converter
ring gear and turbine begin to turn in the opposite direction.
provides 70% of
output During all load conditions, the torque converter provides 70% of the
output, and the planetary gear set provides the remaining 30% of the
• Planetary gear set output.
provides 30% of
output
STMG 699 - 48 -
6/98

40

Power Shift Transmission

• 3F/3R power shift The D8R transmission is located at the rear of the machine for easy
transmission (arrow) removal and installation. The three-speed forward, three-speed reverse
planetary power shift transmission (arrow) transfers power from the
- Three speed
engine to the final drives. The transmission contains three hydraulically
clutches
controlled speed clutches and two hydraulically controlled directional
- Two directional clutches. The operator manually selects the direction and the speed range.
clutches With a transmission speed and directional clutch ENGAGED, the
transmission sends power to the bevel gear and pinion, the drive planetary,
the left side steer planetary, the right side equalizing planetary, and then to
the final drives.
STMG 699 - 49 -
6/98

3
4

2
1

41

• Component locations: The speed clutch pressure (P1) test port (1) and the directional clutch
pressure (P2) test port (2) are located on the rear of the transmission
1. P1 test port
housing. The plugs are removed and replaced with test fittings to measure
2. P2 test port the P1 and P2 pressures. The P3 torque converter inlet pressure tap (3)
3. Torque converter and the transmission lube pressure tap (4) are also located on the rear of
inlet pressure tap the transmission housing.
(P3)
4. Lube pressure tap
To measure the priority valve opening pressure, exchange the priority
valve pressure plug (5) with the torque converter inlet pressure tap (3).
5. Priority valve
pressure plug
STMG 699 - 50 -
6/98

POWER SHIFT TRANSMISSION

RING RING GEARS

GEARS

INPUT SUN
GEARS INPUT SHAFT

OUTPUT SHAFT

PLANETARY OUTPUT
CARRIER SUN GEARS

1 2 3 4 5

42

• Transmission clutch This slide shows a sectional view of a typical transmission group. The
locations: planetary group has two directional and three speed clutches which are
1. Reverse
numbered in sequence (1 through 5) from the rear of the transmission to
the front. Clutches No. 1 and 2 are the reverse and forward directional
2. Forward clutches. Clutches No. 3, 4, and 5 are the third, second, and first speed
clutches. The No. 5 clutch is a rotating clutch.
3. Third
In this sectional view of the transmission, the input shaft and input sun
4. Second gears are shown in red. The output shaft and output sun gears are blue.
The ring gears are shown in green. The planetary carrier is brown. The
5. First planet gears and shafts are shown in orange. The clutch discs, clutch
plates, pistons, springs, and bearings are shown in yellow. The stationary
clutch housings are shown in gray.

The input sun gears are splined to the input shaft and drive the directional
gear trains. The output shaft is driven by sun gears No. 3 and 4 and
rotating clutch No. 5. When the No. 2, 3, or 4 clutches are engaged, their
respective ring gears are held stationary. The No. 1 planetary carrier is
held when the No. 1 clutch is engaged. When engaged, the No. 5 rotating
clutch locks the output components (for FIRST gear) to the output shaft.
STMG 699 - 51 -
6/98

43

• Component locations: The power train oil fill tube (1) and dipstick (2) are located below a
hinged cover on the right side of the machine just in front of the hydraulic
1. Power train oil fill
oil tank.
tube

2. Power train oil

NOTE: The refill capacity of the power train system is
dipstick approximately 144 Liters (38 gal.).
STMG 699 - 52 -
6/98

44

• Three section power The power train oil pump consists of the transmission control valve
train oil pump charging section (1), the torque converter charging section (2), and the
torque converter and transmission scavenge section (3).
1. Transmission
charging

2. Converter charging

3. Scavenge
STMG 699 - 53 -
6/98

3
2 5

4
1

45

• Component locations: The transmission filter (1) is located in front of the hydraulic tank on the
right side of the machine. The transmission filter housing contains the
1. Filter housing
transmission pump pressure tap (2), the S•O•S tap (3), the filter bypass
pressure switch (4), and a temperature override switch (5) that is on the
2. Transmission
pressure tap rear of the filter housing.

3. S•O•S tap The transmission filter bypass valve will open when the filter becomes
restricted or when the oil is cold and thick. Filter bypass occurs at
4. Filter bypass approximately 175 kPa (25 psi). When the oil is cold at start-up, it will
pressure switch bypass the filter; however, the temperature override switch (5) will
prevent the signal from alerting the operator when the oil temperature is
5. Temperature below 52°C (125°F). If the filter is restricted after the oil warms, the
override switch
bypass valve pressure switch (4) opens and sends a signal to the EMS to
alert the operator that the power train oil filter needs to be serviced.
STMG 699 - 54 -
6/98

46

• Transmission case The D8R utilizes an "ecology-type" drain valve to help prevent spills or
drain plug location oil loss when the transmission system is drained. To drain the oil, remove
the plug (1), install a 12.7 mm (.5 in.) pipe with 1/2 - 14 NPTF threads,
1. Plug
and connect a flexible hose to the pipe. Then, turn the drain valve (2)
with an allen wrench. The valve can be used to control the rate at which
2. Drain valve
the oil is drained into a suitable container by opening it partially or
completely. The transmission scavenge screen is located behind the drain
valve housing.
STMG 699 - 55 -
6/98

47

• Bevel gear case fast The 108-4313 Oil Change System is available for the D8R. The group
fill fitting (arrow) includes the fast fill engine and power train oil fill fitting and hose groups.
The fast fill power train oil fill fitting (arrow) for the bevel gear case is
located on the left side of the engine next to the engine oil dipstick.

To use the fast fill fitting, a 126-7538 Nozzle Assembly must be

purchased.
STMG 699 - 56 -
6/98

48

• Main sump drain The drain plug for the main power train oil sump is located on the bottom
location (arrow) of the main case and frame housing below the tractor. To drain the oil,
remove the plug (arrow) that covers the drain valve. Install a 4C8563
Hose Swivel or a 25 mm (1 in.) pipe with 1 - 11 1/2 NPTF threads to
unseat the drain valve (not shown) and start the flow of oil. To stop the
flow of oil, remove the hose swivel or pipe and a spring will close the
valve.
STMG 699 - 57 -
6/98

2
1

49

• Torque converter case The D8R utilizes an "ecology-type" drain valve to help prevent spills or
oil loss when the torque converter housing is drained. To drain the oil,
1. Plug
remove the plug (1), install a 12.7 mm (.5 in.) pipe with 1/2 - 14 NPTF
threads and connect a flexible hose to the pipe. Then, turn the drain valve
2. Drain valve
(2) with a wrench. The valve can be used to control the rate at which the
3. Hose (screen oil is drained into a suitable container by opening it partially or
location) completely. The torque converter scavenge screen (not visible) is located
just inside the flanges of the scavenge hose (3).
STMG 699 - 58 -
6/98

2
3

50

• Component locations: The torque converter outlet relief valve (1) is mounted on the torque
converter case. The torque converter outlet pressure can be checked at the
1. Converter outlet
pressure tap (2). The power train oil temperature gauge sender (3) is
relief valve
located to the left of the pressure tap. The EMS power train oil
2. Pressure tap temperature switch (4) is located next to the pressure tap.
3. Power train oil
temperature sensor
4. Power train oil
temperature switch
STMG 699 - 59 -
6/98

51

• Oil cooler (arrow) The power train oil cooler (arrow) is located at the right side of the engine
compartment. The cooler is an oil-to-water design. Oil from the torque
converter flows through the torque converter outlet relief valve and is sent
to the oil cooler. After the oil flows through the cooler, it is sent to the
lubrication circuit for the brakes and transmission.
STMG 699 - 60 -
6/98

D8R POWER TRAIN HYDRAULIC SYSTEM

TORQUE
CONVERTER TC OUTLET
PUMP RELIEF
TRANSMISSION CONTROL DRIVE VALVE
VALVE 5 4
1 2 3
COOLER

SERVICE BRAKES
WITH SHUTTLE VALVES

P3
LEFT BRAKE LUBE

R N F

P1 3
2 1 P2
RIGHT BRAKE LUBE

TRANSMISSION LUBE
BRAKE
VALVE MAIN
SUMP TRANSMISSION

TRANSMISSION
SUMP

PUMP

FILTER
MAIN MAKEUP AND
SUMP PRIORITY VALVE

52

Power Train Hydraulic System

• Power train hydraulic This schematic shows the components and the oil flow in the power train
components hydraulic system.

• Color codes for The colors used to identify the various pressures in the system are:
schematics
Red - Pump and P1 pressure

Red and White Stripes - P2 pressure and brake release pressure

Orange - Torque converter pressure

Brown - Lubrication (lube) pressure

Green - Drain or reservoir oil

➥
STMG 699 - 61 -
6/98

• Three section The system is equipped with a three-section gear-type pump. In this view,
transmission pump the scavenge section is on the left, the "split phase" torque converter
• Smoother flow to
charging section is in the center and the transmission control valve
converter from "split charging section is on the right. The term "split phase" refers to the
phase" pump alignment of two pairs of gears on the shaft that are not in
synchronization and will supply flow in alternate pulsations. This design
results in a dampened (smoother) flow to the torque converter.

• Threaded canister The transmission filter incorporates a threaded canister that uses a spring
filter to seat the filter in the base. The filter has an oil sampling port, an oil
pressure tap, a filter bypass valve set to approximately 175 kPa (25 psi), a
bypass switch, and a power train oil temperature switch.

• Filtered oil sent to After the oil is filtered, flow is sent to the brake control, makeup and
transmission and priority valve group which directs the flow to the transmission control
brake controls valve and the brake system.

• NEUTRAL position on The NEUTRAL position is on the directional spool of the transmission
directional spool control valve.

• Pump drive lube oil The source of oil for the pump drive lube has been changed from the
from converter inlet outlet side of the cooler to the inlet side of the torque converter. An
orifice is installed in the line to limit flow.

• No changes to No changes have been made to the operation of the transmission control
transmission valve, the torque converter, the torque converter outlet relief valve, and the
controls, converter,
outlet relief and
cooler.
cooler
STMG 699 - 62 -
6/98

D8R TRANSMISSION CONTROL VALVE

NEUTRAL
MODULATION
SPEED RELIEF
SELECTOR
VALVE 5 4
SPOOL
1 2 3

LOAD
PISTON

C SCREENED
ORIFICE

PRESSURE
RATIO B DIFFERENTIAL
VALVE
VALVE

DIRECTIONAL P3
SPOOL

BLOCKED
PASSAGE

R N F

P2
P1
2 1
A 3

53

• Transmission control The transmission control valve for the D8R with differential steering is
valve in NEUTRAL very similar to the transmission control valve in many other machines.
The main difference is the differential steer machine has the NEUTRAL
detent on the directional spool. The dual twist tiller control utilizes
NEUTRAL with FORWARD and REVERSE operation. The operator
selects a direction by rotating the tiller lever forward into the detent for
FORWARD or rearward into the detent for REVERSE. The tiller is
detented to hold the selected direction until the operator manually
disengages it. The NEUTRAL position is selected by the detent between
FORWARD and REVERSE.

The components of the transmission control valve are:

• Speed selector spool Speed selector spool: directs oil (P1) to the appropriate speed clutch.

• Directional spool Directional selector spool: directs oil (P2) to the appropriate
directional clutch.

• Screened orifice Screened orifice: provides a pressure drop and time delay in the oil
flow to the load piston to control clutch engagement.
➥
STMG 699 - 63 -
6/98

• Modulating relief valve Modulating relief valve and load piston: control the rate that the
and load piston pressure increases in the clutches and limit the maximum speed clutch
pressure (P1).

• Ratio valve Ratio valve: protects the torque converter during cold starts by
limiting the oil pressure (P3) that flows to the torque converter. The
converter ratio valve also ensures that clutch modulation starts at the
"initial" pressure by decreasing P3 pressure during each shift. P1 oil
works against one end of the ratio valve and P3 oil works against the
opposite end of the valve. The ratio between the P1 surface area to
the P3 surface area is approximately 4:1.

• Pressure differential Pressure differential valve: controls clutch sequencing by

valve maintaining a specified difference between the speed clutch and the
directional clutch pressures. The differential valve ensures the speed
clutch fills before the directional clutch. Most of the energy and
shock load of a shift are then carried by the directional clutch. The
differential valve also hydraulically prevents clutch engagement when
the operator starts the engine with the transmission selector lever in a
gear position and the neutral-start switch is inoperative. During a
shift, the right end of the differential valve functions as a check valve
and directs oil from the load piston chamber to drain.

• Orifice A Orifice A: controls the fill rate of the speed clutches to ensure that all
control functions are correctly sequenced. Orifice A is sometimes
called a "flow control orifice."

• Orifice B Orifice B: provides quick pump priming by allowing a low pressure

path to purge air from the lines during cold starts and also provides a
path for P1 pressure oil to drain when the engine is stopped. If the
operator turns the machine off in gear then immediately restarts the
machine, the machine could move if P1 pressure oil did not drain.

• Orifice C Orifice C: limits load piston travel. The orifice is actually three
separate holes. Depending on the application, one or two of these
holes are plugged with set screws. At least one of the three holes must
always remain open. These holes are in the valve body above the load
piston chamber when the transmission control valve body is
disassembled.

➥
STMG 699 - 64 -
6/98

• Initial pressure setting Initial pressure is created by the spring tension before the load piston
starts moving to the left to compress the load piston springs. Initial
pressure occurs just before the start of modulation. The initial pressure
setting is critical for this transmission control valve. If the initial pressure
is too low, shift hesitation may occur. A high initial pressure setting will
cause a harsh shift and will reduce transmission component life. When
checking initial pressure, remove the plug in the transmission control
valve labeled "LP." Removing this plug drains the oil going to the load
piston chamber.

• Relief valve and load The relief valve and load piston are shim adjustable. If the P1 clutch
piston are shim pressures are not within specifications, refer to the service manual for the
adjustable correct procedures. Always adjust the transmission control valve for
initial pressure rather than P1 pressure at full clutch engagement.

• Differential valve is The differential valve is not shim adjustable. If the differential pressure is
not adjustable not within specification, change the springs or replace the valve.
• Increasing P3 Since the ratio valve limits the maximum P3 pressure rather than the
pressure can cause minimum, P3 pressure should never be adjusted. Increasing the P3
converter failure
pressure can result in torque converter failures.

INSTRUCTOR NOTE: For more information about the

transmission control valve operation, refer to the Technical
Instruction Module "Manually Controlled Power Shift Transmission
Control Valve" (Form SEGV2576).
STMG 699 - 65 -
6/98

D8R TRANSMISSION CONTROL VALVE

FIRST FORWARD
SPEED MODULATION
SELECTOR RELIEF
SPOOL VALVE 5 4
1 2 3

LOAD
PISTON

C SCREENED
ORIFICE

PRESSURE
RATIO
B DIFFERENTIAL
VALVE
VALVE

DIRECTIONAL P3
SPOOL

R N F

P2
P1 2 1
A 3

54

• First Speed When the shift is made from NEUTRAL to First Speed FORWARD, the
FORWARD No. 3 clutch is opened to drain. The pressure in the system decreases, and
clutches No. 5 and 2 are modulated until they reach the appropriate
pressures.
STMG 699 - 66 -
6/98

D8R TRANSMISSION CONTROL VALVE

START IN GEAR
MODULATION
SPEED
RELIEF
SELECTOR
VALVE 5 4
SPOOL
1 2 3

LOAD
PISTON

C SCREENED
ORIFICE

PRESSURE
RATIO B DIFFERENTIAL
VALVE VALVE

DIRECTIONAL P3
SPOOL

R N F

P2
P1
2 1
A 3

VENT PASSAGE

55

• Starting in gear The D8R tiller control group incorporates a neutral-start switch in the
tiller lever. If the operator attempts to start the machine with the tiller
lever in FORWARD or REVERSE, the neutral-start switch disables the
engine starting circuit. The operator must move the tiller lever to
NEUTRAL before the engine will start.

• Neutral-start switch If the neutral-start switch does not function correctly and the engine is
started, the transmission control valve will prevent the machine from
moving because the differential valve will not move to the right to the
"set" position. By not moving to the "set" position, oil entering the
differential valve chamber is open to drain through a passage around the
valve, which keeps the oil from being directed to the directional clutch.

NOTE: The "set" position is the position of the differential valve

when the passage around the differential spool is blocked by the
directional selector spool. In the "set" position, orifices in the
differential valve spool have moved past this passage and are blocked
by the valve body.
STMG 699 - 67 -
6/98

3
2 4

56

• Brake control valve The brake control valve group contains the parking brake valve (1) and
group components: the service brake valve (2) in the upper section and the regulating relief
valve and the priority valve in the lower section. The pressure tap (3) on
1. Parking brake valve
the top of the valve body is for testing brake pressure, and the tap (4) on
the side of the valve body is for testing torque converter charging
2. Service brake valve
pressure.
3. Brake pressure tap On the left side of the valve body is a plug (5) that can be removed and a
pressure tap installed to check priority pressure before it goes to the
4. Converter charging parking brake valve.
pressure tap

5. Priority valve
pressure port

• Lower section
includes priority and
relief valves
STMG 699 - 68 -
6/98

BRAKE CONTROL VALVE GROUP

PARKING BRAKE SPOOL

LEFT
BRAKE SUPPLY BRAKE LUBE SERVICE BRAKES
PASSAGE WITH
RIGHT SHUTTLE VALVES
BRAKE LUBE

FROM CONVERTER MAIN SUMP

CHARGING PUMP
SERVICE BRAKE SPOOL
REGULATING RELIEF VALVE BRAKE VALVE

CHECK VALVE SERVICE BRAKE PLUNGER

PRIORITY VALVE PRESSURE

TO TORQUE CONVERTER

TO TRANSMISSION
PRIORITY VALVE
CONTROL VALVE

FROM TRANSMISSION MAKEUP AND

CHARGING PUMP PRIORITY VALVE

57

• Valve group The brake control valve group consolidates the parking and service brake
components: valves and the transmission priority valve into one group. The functions
- Parking brake spool
of the parking and service brake valves remain the same as the separate
valves in the single pump system. The parking brake valve is an ON/OFF
- Service brake spool valve, and the service brake valve is a pressure reducing valve. The
- Service brake priority valve function remains the same, but a regulating relief valve has
plunger been added. The regulating relief valve maintains the torque converter
- Regulating relief pump pressure so the torque converter inlet oil can help fill the
valve transmission clutches when a shift is made.
- Check valve
- Priority valve

➥
STMG 699 - 69 -
6/98

Oil from the transmission charging section of the pump flows into the
lower end of the valve body and is first sent through internal passages to
the parking brake spool. When the parking brake lever is in the
ENGAGED position, flow to the service brake spool is blocked. When
the parking brake lever is raised to the RELEASED position, oil is sent
through the brake supply passage to the service brake spool. If the service
brakes are released, the service brake plunger holds the brake spool open.
Oil flows through passages in the valve body and through passages and
tubes in the tractor main case to the brake pistons where the brakes are
held in the RELEASED position. The brake release pressure is
approximately 2950 kPa (425 psi).

When the service brake pedal is partially depressed, the service brake
spool has less force on the spring and oil pressure in the brake pistons is
reduced. This pressure reducing action allows the brake springs to
partially ENGAGE the brakes and reduces the speed of the machine.
When the service brake pedal is fully depressed, less than 70 kPa (10 psi)
is available. If the machine is in a speed and direction, the torque
converter is in a stall condition. Remember, the machine can drive
through FIRST GEAR with the brakes ENGAGED.

• Pump flow sent to After the oil from the transmission charging pump fills the brake system
transmission controls and pressure increases to 2900 kPa (420 psi), the priority valve opens and
and brakes oil flows directly to the transmission control valve.

• Converter gets flow Torque converter charging oil flows into the valve group and goes to the
from separate pump regulating relief valve. This valve is set to open at 880 kPa (130 psi) and
section
when the pressure exceeds this value, the valve moves down and oil flows
• During shift, converter to the torque converter. During a shift, the transmission supply pressure
oil sent to decreases, the check valve opens and the torque converter oil flows to the
transmission controls transmission control valve to aid in filling the clutches.

After the transmission clutches are filled and the transmission supply
pressure increases above 880 kPa (130 psi), the check valve closes and
directs the flow of oil from the torque converter charging section of the
pump to the torque converter.

As the transmission supply pressure increases, the oil continues to flow

through the priority valve to the transmission control valve and clutch
modulation occurs.

• Brake lubrication Another benefit of the three pump system is that brake lubrication oil flow
increased has been slightly increased.
STMG 699 - 70 -
6/98

FINAL DRIVE, BRAKE GROUP, AND

STEERING DIFFERENTIAL AND PLANETARY
SPROCKET
SEGMENTS

DISCS
INNER BRAKE AND PLATES SPOOL HOUSING
HOUSING STEERING PINION
PLANETARY
OUTER GEARS
PLANETARY
GEARS INNER HUB
SUN
GEAR
RING GEAR

CARRIER
PLANET GEARS
OUTER
AXLE SHAFT CENTER AXLE SHAFT

SUN GEARS
OUTER
SUN GEAR

OUTER
CARRIER
RING GEAR
CARRIER
BEVEL GEAR
HUB DUO-CONE CHAMBER
SEALS
PISTON BELVILLE SPRING

RING INNER
GEAR CARRIER

58

This left side sectional view shows the final drive, brake group, and
steering differential and planetary. Power from the transmission is
transmitted from the steering differential (purple) and the center axle shaft
(yellow) to the brakes (dark yellow) and final drive through the outer axle
shaft (yellow).

• Final drive The final drive components consist of the following:

components
- Outer planetary gears (green)
- Outer sun gear (green)
- Outer carrier (green)
- Ring gear (gray)
- Inner planetary gears (blue)
- Inner sun gear (blue) splined to the outer axle shaft
- Inner carrier (blue)
- Sprocket segments and hub (brown)
➥
STMG 699 - 71 -
6/98

The rotation of the outer axle shaft and the inner sun gear causes the inner
planet gears to turn. The ring gear is stationary. As the inner planet gears
rotate around the inside of the ring gear, the inner carrier turns. The inner
carrier is connected to the outer sun gear by splines. As the inner carrier
rotates, the outer sun gear causes the outer planet gears to turn.

The outer planet gears move around the inside of the ring gear causing the
outer carrier and hub to turn. Power is directed to the sprocket segments
and the track. All the components in the final drive are splash lubricated
from oil inside the final drive.

• Brake components The brake components (dark yellow) consist of the following:

- Brake housing
- Discs and plates
- Piston
- Belville spring
- Spool
The brakes are used only to stop the machine and do not assist in turning.
Belville springs are used to compress the plates and discs, and pressure oil
from the brake control valve is used to release the brakes by compressing
the Belville springs through the brake piston. When the service brake
pedal is depressed, the plates and discs stop the rotation of the outer axle,
the final drive components, and the hub and sprocket segments. The
machine will come to a complete stop. If the transmission is in gear, the
converter will be in a stall condition.

The parking brake lever is connected to the parking brake valve. In

PARK, the valve blocks pressure oil to the brake piston. When the lever
is moved down, the parking brake valve directs pressure oil to the brake
piston which compresses the Belville springs and releases the parking
brake.

➥
STMG 699 - 72 -
6/98

• Steering differential The steering differential and planetary components (purple) consist of the
and planetary following:
components
- Housing
- Bevel gear
- Steering pinion
- Two ring gears
- Planet gears
- Two carriers
- Two sun gears
The left side of the differential receives transmission power from the
center axle shaft. During steering, power is also supplied by the steering
motor through the steering pinion and bevel gear. The steering pinion and
bevel gear are connected to the housing which is splined to the ring gear.
The planet gears are connected to the carrier, which is splined to the outer
axle shaft.

The hub (gray) is stationary and uses a Duo-cone seal to keep

contaminants out of the housing.
STMG 699 - 73 -
6/98

59

• Brake housing plugs: Two plugs are installed in both the right (shown) and left brake housings:

1. Right side brake - Right side brake lube pressure (1)

lube pressure
- Right side brake release pressure (2)
2. Right side brake
release pressure
At HIGH IDLE with the brakes RELEASED, the brake lubrication
pressure is approximately 205 kPa (30 psi). At HIGH IDLE with the
brakes ENGAGED, the brake lubrication is approximately 150 kPa
(22 psi).

NOTE: On the left brake housing the plugs are reversed.

STMG 699 - 74 -
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BRAKE SHUTTLE SPOOL

SHUTTLE SHUTTLE
SPOOL TO SPOOL TO
STEERING STEERING
PINION PINION

PISTON
BRAKE ENGAGED BRAKE RELEASED

60

• Brake shuttle spool This illustration shows the shuttle spool operation during brake
conditions: engagement and release. The brake piston and the shuttle spool are shown
in yellow. Pressure oil from the parking brake valve releases the brake by
- Engaged
moving the piston to the right. Movement of the piston causes the shuttle
- Released spool to move to the right and restrict the flow of lube oil to the brake.
This restriction has an effect on lube flow similar to that of an orifice.
Maximum lube flow to the brake is not required while the brake is
released. During brake engagement, the Belville spring moves the piston
to the left. Lube oil can then move the shuttle spool to the left and
completely open the passage to the brake. Lube flow is now maximum.
STMG 699 - 75 -
6/98

61

• Pump drive lube On the left front of the pump drive is the pressure tap (arrow) to test the
pressure tap (arrow) lubrication pressure to the pump drive gears.

At LOW IDLE with the brakes ENGAGED, the pressure should be

approximately 310 kPa (45 psi). At HIGH IDLE with the brakes
ENGAGED, the pressure should be approximately 620 kPa (90 psi).
STMG 699 - 76 -
6/98

DIFFERENTIAL STEER COMPONENTS

HYDRAULIC TRANSMISSION
MOTOR INPUT INPUT

TO LEFT TO RIGHT
FINAL DRIVE FINAL DRIVE

STEER DRIVE EQUALIZING

PLANETARY PLANETARY PLANETARY

62

DIFFERENTIAL STEER MECHANICAL OPERATION

Differential steer tractors are not equipped with steering clutches but have
a steering differential, a hydraulic pump, a hydraulic steering motor, and
steering controls.

• Steering differential The steering differential has two power inputs: a speed and direction
has two power inputs: (FORWARD and REVERSE) input from the transmission and a steering
(LEFT and RIGHT) input from the hydraulic motor. The steering
- Transmission differential uses hydraulic motor power input to increase the speed of one
track and equally decrease the speed of the other track. The resulting
- Hydraulic motor
track speed difference turns the tractor.

➥
STMG 699 - 77 -
6/98

• Steering differential: The steering differential consists of the steer planetary, the drive
planetary, and the equalizing planetary.
- Steer planetary
Color codes in this illustration designate the various components.
- Drive planetary
The pinion, the bevel gear shaft, and the drive planetary carrier are red.
- Equalizing planetary The bevel gear shaft is splined to the drive planetary carrier. During turns,
the pinion for the hydraulic motor drives the steer planetary ring gear.
• Schematic color
codes The hydraulic motor pinion and the steer planetary ring gear are orange.
The center shaft connects the sun gears for all three planetaries.

The sun gears and the center shaft are blue.

The planet gears for all three planetaries are yellow.

The center axle shaft is splined to the steer planetary and the equalizing
planetary. Also, the steer planetary carrier is directly connected to the
drive planetary ring gear. These components are green.

The equalizing planetary ring gear is bolted to the right brake housing and
remains stationary. The equalizing planetary is gray.
STMG 699 - 78 -
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DIFFERENTIAL STEER COMPONENTS

STRAIGHT LINE OPERATION

HYDRAULIC TRANSMISSION
MOTOR INPUT INPUT

TO LEFT TO RIGHT
FINAL DRIVE FINAL DRIVE

STEER DRIVE EQUALIZING

PLANETARY PLANETARY PLANETARY

63

• Straight line operation This illustration shows the power flow through the differential steer
system during straight line operation (FORWARD or REVERSE). In this
- Steering motor does condition, the hydraulic steering motor does not turn. Since the hydraulic
not turn
steering motor does not turn, the steering pinion and steer planetary ring
- Transmission gear are stationary (gray) and the transmission provides all power flow
provides all power through the system.

- Arrows show power The transmission sends power through the transfer gears, the pinion, the
flow bevel gear, and the bevel gear shaft to the drive planetary carrier. At this
point, the power divides causing a torque split.
- Outer axles rotate in
same direction
Most of the torque goes through the drive planetary ring gear to the steer
planetary carrier. From the steer planetary carrier, the resulting power is
transmitted to the left final drive through the left outer axle.

The remaining torque from the drive planetary carrier is transmitted to the
equalizing planetary sun gear through the drive planetary sun gear and the
center axle.

➥
STMG 699 - 79 -
6/98

The equalizing planetary planet gears multiply the torque from the sun
gear and send the resulting power through the right outer axle to the right
final drive.

The effect of this operation is that the left and right outer axles rotate in
the same direction with the same power magnitude and the machine,
therefore, tracks in a straight line.
STMG 699 - 80 -
6/98

DIFFERENTIAL STEER COMPONENTS

LEFT TURN - FORWARD
HYDRAULIC
MOTOR INPUT TRANSMISSION
INPUT

TO LEFT TO RIGHT
FINAL DRIVE FINAL DRIVE

STEER DRIVE EQUALIZING

PLANETARY PLANETARY PLANETARY

64

• LEFT TURN During a turn, both the transmission and the hydraulic motor provide
FORWARD inputs to the differential steer system with the transmission supplying
most of the power to the system.
• Transmission input
shown with black The transmission input power is sent to the outer axles in the same manner
arrows
as during straight line operation.
• Steering motor input The hydraulic motor input determines the turn direction and turn radius.
shown with white
arrows
The rpm of the hydraulic motor controls the turn radius (the higher the
rpm, the smaller the turn radius) and the direction of rotation establishes
the turn direction.

➥
STMG 699 - 81 -
6/98

During a LEFT TURN in the FORWARD direction, the hydraulic motor

sends power through the steering planetary ring gear and the planet gears
to the sun gear.

• Steering motor input The input from the hydraulic motor has two effects on the system:
causes:
- Right outer axle 1. The first effect is that the speed of all three sun gears and the
speed to increase speed of the center axle increases causing the speed of the right
- Left outer axle speed outer axle to increase.
to decrease
2. The second effect is that the relative motion of the sun gear and
planet gears in the steer and the drive planetaries cause the drive
planetary ring gear, the steer planetary carrier, and the left outer
axle to slow down. (This relative motion is due to the fact that the
drive planetary carrier is turning at a constant rpm.) The speed
decrease of the left outer axle is equal to the speed increase of the
right outer axle.

• Reversing steering To make a RIGHT TURN, the direction of the hydraulic motor is opposite
motor causes of the direction for a LEFT TURN. The motor now sends power to the
opposite turn steering planetary carrier causing an increase in the speed of the steering
planetary carrier, the drive planetary ring gear, and the left outer axle.
Simultaneously, all three sun gears, the center axle, and the right outer
axle slow down. The speed decrease of the right outer axle is equal to the
speed increase of the left outer axle.

NOTE: During normal operation, this system does not provide a

"pivot turn" capability.

INSTRUCTOR NOTE: For more information about differential

steering operation, see STMG 547 "D8N Track-type Tractor—Power
Train and Implements" (Form SESV1547).
STMG 699 - 83 -
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D8R STEERING AND IMPLEMENT HYDRAULIC SYSTEM

STEERING
STEERING PUMP
MOTOR CHARGE STEERING
PUMP PILOT
STEERING SYSTEM VALVE

BYPASS AND
PRESSURE
CONTROL
GROUP

RIPPER
RIPPER STEERING CHARGE
CYLINDERS
DIVERTER CIRCUIT FILTER
VALVE

TANK

IMPLEMENT SYSTEM
IMPLEMENT
PUMP

INLET MANIFOLD

RIPPER
LIFT
QUICK-DROP CYLINDERS
LIFT
VALVE

TILT TILT
CYLINDER/S
END COVER

66

Differential Steering System

• Steering and This block diagram shows the steering and implement hydraulic system.
implement systems
connected at two
The two systems are functionally separate, but they are connected at two
points: points. Charge pressure is used to move the ripper diverter valve, and the
implement pump output is sent to the bypass and pressure control group to
- Ripper diverter valve supplement charge flow if the pressure decreases below a specified value.
- Bypass and A single quick-drop valve is used for both lift cylinders.
pressure control
group The various color codes which will be used in this section of the
presentation to identify oil flow and pressures are:
• Single quick-drop
valve Red - Drive loop or high pressure
• Color codes for
schematics
Red and White Stripes - First reduction of supply pressure

Red Dots - Second reduction of supply pressure

Blue - Blocked oil

➥
STMG 699 - 84 -
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Orange - Charge pressure

Orange and White Stripes - Pilot pressure

Orange Dots - Pump control pressure

Green - Tank or case drain oil

Yellow - Activated valve envelopes or

moving parts
STMG 699 - 85 -
6/98

3
4

67

• Pilot valve location The steering pump is controlled by a pilot valve connected to the bottom
of the steering tiller. The valve contains two pressure reducing valves that
• Contains two pressure convert charge pressure to pilot pressure. The two pressure taps are for
reducing valves
the right (1) and left (2) steer pilot pressures. The hoses (3 and 4) are for
1. Pilot pressure tap the return oil and charge pressure, respectively.
(right turn)

2. Pilot pressure tap

(left turn)

3. Return hose

4. Charge pressure
hose
STMG 699 - 86 -
6/98

6
7

1
3

68

• Steering pump The steering pump circuit is a closed loop hydraulic system which
components: includes an axial piston pump with over-center capability. The steering
pump contains the charge pump (1), the pressure compensator (cutoff)
1. Charge pump
valve (2), the charge pressure relief valve (3), and the right and left
2. Pressure crossover relief and makeup valves (4 and 5).
compensator valve
The charge pump (1) is contained in the end of the steering pump. The
3. Charge pressure pump control spool (6) and the pump control piston (7) use charge
relief pressure oil to move the swashplate for left and right turns.

4. Right crossover The pressure taps on the top of the pump control piston are for
relief valve troubleshooting steering problems and adjusting the neutral setting of the
steering pump.
5. Left crossover
relief valve

6. Pump control
spool

7. Pump control
piston and
pressure taps
STMG 699 - 87 -
6/98

69

• Steering motor has The steering motor (1) is a bent axis design with a self-contained flushing
flushing valve in port valve in the port plate. The addition of this valve allows a controlled
plate
amount of oil from the low pressure side of the steering motor to flow into
• Charge and case drain the motor case to cool, lubricate, and flush all components of the motor.
oil cool motor Oil from the steering pump case (case drain oil) is sent to the bypass and
pressure control valve and then to the steering motor to provide additional
1. Steering motor cooling. The combined oil flow in the motor is directed to the bypass and
2. Right steer
pressure control group and then sent to the tank.
pressure tap
The top pressure tap (2) on the motor is for the right steer pressure and the
3. Left steer pressure bottom tap (3) is for the left steer pressure. When the operator moves the
tap tiller lever to the right or left during a stall condition, system pressure will
be approximately 40000 kPa (5800 psi) with the brakes applied.
STMG 699 - 88 -
6/98

70

• Bypass and pressure The bypass and pressure control group (arrow) is a collection manifold for
control group (arrow) the charge pressure and cooling circuit of the steering system. The
control valve group is mounted on the transmission case directly to the
right of the steering motor.
STMG 699 - 89 -
6/98

2
3

71

• Bypass and pressure The bypass and pressure control group directs oil from the charge pump
control group: to the filter, the pressure control valve and then to the cooler. The oil then
returns to the pressure control valve and enters the steering closed loop.
- Directs charge oil
through filter and The valve group also contains the cooler and cold oil bypass valves and
cooler controls the makeup functions for the steering charge pump oil circuit.

The three pressure taps are:

- Protects circuit from
high pressures
- Steering pump case drain (1)
- Directs implement
oil to charge circuit - Charge pump discharge pressure (2)
for back-up
- Charge pump relief pressure (3)
- Contains three
pressure taps:
The pilot and ripper diverter valve use charge oil from the upstream side
of the cooler. This pressure is lower than the charge pump discharge
1. Steering pump
pressure but higher than the charge pump relief pressure.
case drain
The check valve (4) is used to block oil flow from the steering circuit into
2. Charge pump
discharge pressure
the implement circuit. If the steering system charge pressure decreases
below 2000 kPa (290 psi), oil from the implement pump flows through
3. Charge pump relief the check valve to replenish the charge circuit.
pressure

4. Check valve
STMG 699 - 90 -
6/98

2
1
7
5
6

72

• Hydraulic tank for The hydraulic tank (1) serves as a reservoir for the steering and
steering and implement hydraulic oil. The hydraulic tank contains a 165 micron
implement systems: screen filter for the implement circuit, while the steering system return oil
1. Hydraulic tank
is filtered by the case drain reverse flow element (2). The return filter and
the bypass valve, the fill strainer, the Electronic Monitoring System
2. Case drain filter temperature switch, the oil level sight glass, the vacuum breaker relief
valve, and the ecology drain are additional features of the tank. The tank
3. Filter holds 70 L (18.5 gal.) of oil which represents a 21% increase in oil
capacity from the former model.
4. Temperature
switch The steering charge circuit oil filter (3) is located behind a hinged access
door on the right side of the machine and in front of the tank. The filter
5. Bypass switch
has a spin-on canister, an oil pressure tap (6), an oil sampling port (7), a
6. Pressure tap
bypass switch (5), and a temperature override switch (4). If either system
overheats, the Electronic Monitoring System will register a Category 3
7. S•O•S tap Warning.
STMG 699 - 91 -
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73

• Hydraulic tank drain The drain plug for the hydraulic tank is located below the tank directly
valve (arrow) above the right track. To drain the oil, remove the cover (arrow) to access
the ecology drain valve. Install a 25.4 mm (1 in.) pipe with
1 - 11 1/2 NPTF threads to unseat the valve (not shown) to start the flow
of oil. To stop the flow of oil, remove the pipe and a spring will close the
valve.
STMG 633 - 92 -
12/92

74

1. Steering and The hydraulic oil cooler (1) is an air-to-oil design located on the front left
implement cooler side of the engine directly behind the radiator guard. The charge pump oil
2. Cooler pressure tap
is filtered and sent through the cooler to the steering pump. The cooler
has a heat rejection rating of 14 kW (13 Btu/sec.) at 57 Lpm (14.8 gpm)
• Charge oil is filtered and dissipates the heat from the steering and implement systems.
then cooled
Located at the bottom of the cooler is the cooler pressure tap (2). The
bypass and pressure control valve contains the cooler bypass relief valve.
This valve is set to open and bypass charge pump oil around the cooler if
the pressure differential is higher than 345 kPa (50 psi).

The cooler is the same part number used in the previous model, but the
system pressure is higher, approximately 2500 kPa (365 psi).
STMG 699 - 93 -
6/98

STEERING MOTOR STEERING PUMP

PILOT VALVE

BYPASS
AND
PRESSURE
CONTROL TO RIPPER
GROUP COOLER DIVERTER VALVE

TO CASE DRAIN
FILTER IN TANK
FROM IMPLEMENT
PUMP CASE D8R
FROM IMPLEMENT STEERING CHARGE STEERING SYSTEM
PUMP SUPPLY CIRCUIT FILTER

75

Steering System Operation

• Steering system with This schematic shows the components and conditions of the steering
engine running and system with the engine started and the dual twist tiller in NEUTRAL (no
tiller in NEUTRAL
turn).

The major components of this system are: the steering pump, the steering
motor, the pilot valve, the bypass and pressure control group, the steering
charge circuit filter, and the cooler.

Flow to the closed loop steering system is supplied by an axial piston

pump with over-center capability.

Components included in the pump are:

• Steering system
components Charge pump: Fills the system with oil during start-up and provides
cool oil for the drive loops and steering pilot valve. This oil is called
"charge pump discharge pressure" and is 345 to 550 kPa (50 to 80 psi)
higher than charge pressure.

➥
STMG 699 - 94 -
6/98

Pressure compensator (cutoff) valve: When the pressure in either

side of the loop reaches 40000 kPa (5800 psi), this valve destrokes the
pump by draining the charge pressure sent to the pump control spool
to move the pump control piston.

Charge pressure relief valve: This valve limits the charge pressure
to 2500 kPa (365 psi) after the charge pump oil is filtered and cooled.
Charge oil is then sent to the drive loop, pilot valve, ripper diverter
valve, and pump control piston.

Crossover relief and makeup valves: Each side of the drive loop has
a valve that limits the pressure spikes and also directs the charge
pressure through the internal check valve to fill the low pressure side
of the loop.

Pump control spool and pump control piston: Oil pressure from
the pilot valve moves the spool a small distance and directs charge
pressure to either end of the pump control piston. As the pump
control piston moves and changes the angle of the swashplate, the
feedback link of the pump control spool follows up and maintains the
correct pressure to the pump control piston for the amount of steering
flow requested. In the NEUTRAL (or no turn) position, a small
amount of pressure is present on both ends of the pump control piston.

Other components included in the steering system are:

Pilot valve: Contains two pressure reducing valves which control the
displacement of the steering pump. Pressure from the pilot valve is
set to begin upstroking the pump at 600 kPa (87 psi) and provide
maximum displacement at 1800 kPa (261 psi).

Steering motor with flushing valve: Uses flow from the steering
pump to turn the motor clockwise or counterclockwise for either left
or right turns. A flushing valve that meters oil from the low pressure
side of the loop is contained in the port plate to help keep the motor
cool during operation.

Steering charge circuit filter: Spin-on filter housing with a bypass

valve rated at 175 kPa (25 psi). The normally open bypass switch is
installed in the inlet passage and held closed by the bypass valve.
When the bypass valve opens to bypass oil flow around the filter
element, the switch opens and the Electronic Monitoring System
provides a Category 3 Warning. The alert indicator and action lamp
flash and the action alarm sounds.

➥
STMG 699 - 95 -
6/98

Bypass and pressure control group: This valve group serves as a

collection manifold for the charge pressure and cooling circuit of the
steering system. The valve group directs oil from the charge pump,
through the filter and the cooler, and then back to the steering pump.
The bypass and pressure control group also provides the oil cooler
bypass and makeup functions for the charge circuit.

Components included in the bypass and pressure control group are:

Cold oil bypass valve: This valve protects the charge circuit and
opens at 3200 kPa (460 psi) when the oil is cold.

Cooler bypass valve: This valve protects the cooler from differential
pressures higher than 345 kPa (50 psi).

Pressure reducing and check valve: If the charge pump pressure

decreases below 2000 kPa (290 psi), oil from the implement pump
flows through the check valve and the pressure reducing valve to
replenish the charge circuit.

Cooling orifice: As the case drain oil from the steering pump flows
into the pressure control group, the flow is restricted and directed into
the steering motor for additional cooling.
STMG 699 - 96 -
6/98

D8R STEERING PILOT VALVE

NO TURN

LEFT STEER
RIGHT STEER LOOP PRESSURE
LOOP PRESSURE

TO PUMP FROM BYPASS TO PUMP

CONTROL AND PRESSURE CONTROL
SPOOL CONTROL GROUP SPOOL

76

• Pilot control valve The pilot signal to the pump control spool originates at the pilot control
sends signal to move valve. This valve contains two pressure reducing valves that use charge
swashplate
pressure for the source of oil. The pump swashplate angle is directly
related to the amount of oil pressure sent from the pilot valve to the pump
• Feedback lever
maintains desired
control spool. The pump control spool acts as a servo valve to direct
pump flow charge pressure oil in and out of the pump control piston to mechanically
move the swashplate. A feedback lever which connects the pump control
piston to the pump control spool helps maintain pump flow for any given
pilot signal.
STMG 699 - 97 -
6/98

D8R STEERING PUMP

RIGHT STEER
NO TURN PUMP CONTROL PISTON PRESSURE
LOOP LEFT LOOP RIGHT LOOP

TO STEER
MOTOR
(RIGHT)

CHARGE
PRESSURE PRESSURE
CHARGE COMPENSATOR RELIEF STEERING
PUMP VALVE PUMP

PUMP
CROSSOVER CONTROL
RELIEF VALVE PISTON

TO STEER TO
MOTOR BYPASS
(LEFT) AND
PRESSURE
CONTROL
GROUP

LEFT STEER TO STEERING

LOOP PILOT VALVE
(RIGHT)
PUMP CONTROL
FROM BYPASS SPOOL
AND PRESSURE
CONTROL VALVE
TO BYPASS TO STEERING
AND PRESSURE PILOT VALVE
CONTROL VALVE (LEFT)

77

• Charge pressure oil in This slide shows a close view of the steering pump in the NEUTRAL (no
pump goes to: turn) condition. Charge pressure oil from the bypass and pressure control
group enters the steering pump and flows to the charge pressure relief
- Charge pressure
valve, the right and left crossover relief valves, and the pressure
relief valve
compensator valve. Charge oil also flows through the orifice to the pump
- Right and left control spool and pressurizes both ends of the pump control piston. After
crossover reliefs
the pump control piston is pressurized, a drain passage in the pump
- Pressure control spool constantly bleeds a small amount of charge pressure oil to
compensator valve the tank. Most of the charge pressure oil flows to the tank through the
- Pump control spool charge pressure relief valve.
- Pump control piston
STMG 699 - 98 -
6/98

D8R STEER MOTOR

NO TURN

RIGHT STEER FROM

LOOP PRESSURE STEER
PUMP

TO BYPASS FLUSHING
AND PRESSURE VALVE
CONTROL GROUP

FROM
STEER
PUMP
FROM BYPASS
AND PRESSURE
CONTROL GROUP LEFT STEER
LOOP PRESSURE

78

• In NEUTRAL, flushing In the NEUTRAL (no turn) condition, charge pressure is prevented from
valve blocks charge flowing through the steering motor because the flushing valve is centered.
oil
The flushing valve permits oil to flow from the low pressure side of the
• When turning, loop through the valve when a right or left turn is initiated.
flushing valve lets
return oil flow through Two sources of cooling oil are provided to the motor: The first is the
motor internal flushing valve in the port plate and the second is the steering
pump case drain oil that is routed through the bypass and pressure control
• Additional source of
cooling oil from pump group to the motor. On many systems, a case drain pressure test for the
case drain steering motor is a good diagnostic check, but with the two sources of
flow going through the motor, a pressure tap is not provided.
STMG 699 - 99 -
6/98

STEERING MOTOR
FLUSHING VALVE

PIN

79

• Flushing valve lets oil The flushing valve is contained within the port plate of the steering motor.
flow through motor This valve is designed to bleed off approximately 4 Lpm (1 gpm) of flow
during turns
from the motor when the steer pressure increases to 2500 kPa (262 psi).
• Pump case drain oil
sent to motor for more When the pressure in the drive side of the loop is 2500 kPa (262 psi)
cooling higher than the return side, the higher pressure moves the pin and allows
oil to flow through the port plate into the motor case. This oil combines
with the steering pump case drain oil for more cooling. The combined
flow is directed through the bypass and pressure control group to the tank.

NOTE: Longitudinal slots are machined into the round pin in the
port plate.
STMG 699 - 100 -
6/98

D8R STEERING PILOT VALVE

LEFT TURN

RIGHT STEER LEFT STEER

LOOP PRESSURE LOOP PRESSURE

FROM PUMP FROM BYPASS TO PUMP

CONTROL AND PRESSURE CONTROL
SPOOL CONTROL GROUP SPOOL

80

• LEFT TURN operation This schematic shows the operation of the steering system when the
operator moves the dual twist tiller for a LEFT TURN. The dual twist
• Tiller moves right tiller moves linkage that causes the left steering plunger to retract and
steering plunger send pilot oil to the pump control spool in the steering pump. Pressure
from the pilot valve is set to begin upstroking the pump at 600 kPa
• Pilot pressure moves (87 psi) and provide maximum displacement at 1800 kPa (261 psi).
pump control spool
STMG 699 - 101 -
6/98

D8R STEERING PUMP

LEFT TURN
PUMP CONTROL PISTON PRESSURE
LEFT LOOP RIGHT LOOP
TO STEER
MOTOR
(RIGHT)

CHARGE
PRESSURE PRESSURE
CHARGE COMPENSATOR RELIEF STEERING
PUMP VALVE PUMP

PUMP
CROSSOVER CONTROL
RELIEF VALVE PISTON

TO
BYPASS
AND
PRESSURE
TO STEER CONTROL
MOTOR GROUP
(LEFT)
TO STEERING
PILOT VALVE
(RIGHT)
PUMP CONTROL
FROM BYPASS SPOOL
AND PRESSURE
CONTROL VALVE
TO BYPASS FROM STEERING
AND PRESSURE PILOT VALVE
CONTROL VALVE (LEFT)

81

• Oil from control spool This slide shows the steering pump during a LEFT TURN. The pilot
to piston moves valve sends oil to the left end of the pump control spool which directs
swashplate charge pressure oil to the pump control piston. The control piston
mechanically moves the swashplate to the desired pump angle. Steering
pump flow is then sent to the steering motor which provides a mechanical
input to steer the machine.
• Crossover relief
valves: As the pressure increases in the drive side of the steer loop, the left
crossover relief valve closes. The right crossover relief valve opens and
- Limit pressure lets charge pressure oil flow into the return side to provide makeup oil to
spikes replenish leakage in the loop.
- Provide makeup oil
During a stall condition, the pressure spike which occurs in the drive side
• Pressure of the loop is relieved by the crossover relief valve and sent to the return
compensator
destrokes pump in
side of the loop. If the operator continues to hold the tiller in the same
stall condition position, the pressure compensator valve, which is set at 40000 kPa
(5800 psi), opens and drains the oil sent by the pump control spool to the
pump control piston. The piston causes the swashplate to move toward a
minimum angle and maintain maximum pressure.

➥
STMG 699 - 102 -
6/98

NOTE: Charge pressure (orange) and low pressure return oil (red
and white stripes) are equal. The respective flows are shown this way
to help keep the circuits separate.
STMG 699 - 103 -
6/98

STEERING PUMP
END VIEW RIGHT CROSSOVER
RELIEF VALVE
TOP

CHARGE PRESSURE
RELIEF VALVE

ORIFICE PLUG

LEFT CROSSOVER
PRESSURE RELIEF VALVE
COMPENSATOR VALVE

82

• Pump shown in LEFT This slide shows the steering pump during a LEFT TURN. The left
TURN condition crossover relief valve is closed, and the drive loop pressure (red) is sent to
the steering motor. The right crossover relief valve is in the makeup
• Four relief valves
mode, allowing charge pressure oil (orange) to replenish the return side of
the loop. If a pressure spike occurs in the drive side of the loop, the left
crossover relief valve opens and directs excess oil into the return side of
the loop.

The pressure compensator valve destrokes the pump if the pressure

exceeds the valve setting by draining the charge pressure oil (orange) that
is sent to the pump control spool and the pump control piston. The orifice
plug just above the left crossover relief valve helps maintain the charge
pressure when the pressure compensator drains the charge oil to the pump
control spool and the pump control piston.

The charge pressure relief valve limits the charge pressure (orange) used
in the steering system and continually drains the excess oil that is not
required in any of the circuits.
STMG 699 - 104 -
6/98

D8R STEER MOTOR

LEFT TURN
RIGHT STEER TO
LOOP PRESSURE STEER
PUMP

TO BYPASS FLUSHING
AND PRESSURE VALVE
CONTROL GROUP

FROM
STEER
PUMP
FROM BYPASS
AND PRESSURE
CONTROL GROUP LEFT STEER
LOOP PRESSURE

83

• High pressure oil The high pressure oil sent to the steering motor causes the motor to rotate
rotates motor during and provide a mechanical input to the steer planetary in the bevel gear
turns
case. The high pressure oil moves the flushing valve. When the valve
• Flushing valve sends moves, low pressure (return) oil flows into the motor housing and then to
return oil through the bypass and pressure control group.
motor
STMG 699 - 105 -
6/98

STEERING PUMP (TOP VIEW)

LEFT TURN
FROM
RETAINER PILOT VALVE
SPRING COLLAR

RIGHT SIDE
PUMP CONTROL
ROD SPOOL
RIGHT
LEVER
ARM

STOP

COMPRESSION
SPRING

LEFT
PUMP CONTROL LEVER
PISTON ARM

LEFT SIDE
FEEDBACK SPOOL
LEVER CONTROL
TO PILOT VALVE
ARM
PIVOT POINT

84

• Feedback lever The pump control spool uses pilot oil (orange and white stripes) to control
connects control the amount of charge pressure oil (orange dots) that is sent to the pump
spool to piston
control piston.
• Control spool moves
control piston The movement of the pump control spool is approximately 2.00 mm
(.078 in.) in each direction. This spool constantly meters the charge oil to
• Control spool maintain the correct pressure at the pump control piston and the correct
movement very small swashplate angle.
• Pilot valve sends oil to This slide shows the pilot valve moved to the LEFT TURN position. Pilot
control spool
pressure is proportional to the amount of lever movement that is directed
• Control spool directs to the upper end of the pump control spool. As the spool moves down, a
charge oil to piston passage opens and sends charge pressure oil to the upper end of the pump
control piston. At the same time, the spool control arm shifts the left lever
arm. This movement increases the tension of the compression spring
proportional to the force created by the pilot oil from the pilot valve. The
lever arms and the feedback lever pivot on the eccentric screw (pivot
point). An adjustment screw can be used to adjust the center position of
the spool.

➥
STMG 699 - 106 -
6/98

• Charge oil moves Charge pressure oil directed to the upper end of the pump control piston
control piston compresses the large springs and moves the pump control piston down.
As the pump control piston moves, the feedback lever pivots at the pivot
point and the stop on the feedback lever opens the right lever arm to cause
• Feedback lever moves
control spool toward more compression on the spring. The spring force moves the spool back
neutral toward the neutral position. As the spool moves toward neutral, the
opening to the passage for charge pressure oil to the pump control piston
is reduced. The charge pressure at the upper end of the piston is
decreased and the large springs move the swashplate toward minimum
angle to maintain the turn.

• During stall, If the operator stalls the steer motor, the pressure compensator valve
swashplate moves destrokes the pump by bleeding off charge pressure oil at the pump
toward minimum control piston. The large springs in the pump control piston move the
angle
swashplate toward minimum angle to reduce pump output. This condition
• System pressure occurs automatically and prevents the operator from stalling the steering
maintained by system at maximum flow. If the operator holds the tiller at the full left
compensator valve turn position with the brakes engaged, the crossover relief valve will limit
the pressure spike and the pressure compensator will bleed the charge
pressure oil from the upper end of the control piston to decrease the angle
of the swashplate. Pump flow is at minimum, but the system pressure is
at the setting of the pressure compensator.

NOTE: In NEUTRAL (no turn), the pump control spool (if centered)
will send equal pressure to each end of the pump control piston.
Since 600 kPa (87 psi) is needed to move the pump control piston, a
difference of more that 600 kPa (87 psi) will cause the machine to
move when the parking brake lever is moved to the released position.

The adjustment screws on the pump control spool are used to adjust
the pressures on each end of the pump control piston. If the spool
needs adjustment, follow the procedure in the Service Module
Supplement (Form SENR4983). Loosening and tightening the
locknuts will change the center position of the pump control spool.

The small adjustment screws on both ends of the pump control piston
are not adjustable and have factory installed tamper proof caps. Any
attempt to adjust the screws will cause the valve to react differently to
pilot oil directed from the tiller lever. Replace the valve if the caps
have been tampered with and if the valve is damaged.
STMG 699 - 107 -
6/98

PUMP FEEDBACK LEVER PUMP CONTROL VALVE

CONTROL
PISTON

RIGHT
STEERING PUMP CROSSOVER
RELIEF VALVE
SIDE VIEW
CHARGE
PUMP

LEFT
SWASHPLATE PISTONS CROSSOVER
RELIEF VALVE

85

• Steering pump This slide shows a side view of the steering pump. The following
components: components are visible: the right and left crossover relief valves, the
charge pump, the pump control valve, the feedback lever, the pump
- Right and left
crossover relief control piston, the swashplate, and the pistons.
valves
- Charge pump
- Pump control valve
- Feedback lever
- Pump control piston
- Swashplate
- Pistons
STMG 699 - 108 -
6/98

BYPASS AND PRESSURE CONTROL GROUP

TO COOLER, PILOT, AND
FROM RIPPER DIVERTER VALVE
STEER PUMP CHARGE
TO STEER MOTOR FROM FROM
CASE DRAIN PUMP CHARGE
CASE DRAIN DISCHARGE CHARGE PUMP PUMP RELIEF COOLER
PRESSURE PRESSURE

CASE RETURN
PRESSURE
COLD OIL
COOLING ORIFICE BYPASS

FROM STEER CHARGE

COOLER
MOTOR CIRCUIT
BYPASS
CASE DRAIN MAKEUP
VALVE

TO CASE DRAIN
FILTER IN TANK

TO FROM TO CHARGE
FILTER FILTER PRESSURE RELIEF
FROM VALVE
IMPLEMENT PUMP
CASE DRAIN FROM IMPLEMENT
PUMP SUPPLY

86

• Bypass and pressure The main purpose of the bypass and pressure control group is to direct
control group directs charge pump discharge oil through the steering charge circuit filter and
oil through filter and
cooler. After the oil has been filtered and before it is cooled, the charge
cooler
pressure oil is available for the ripper diverter valve and steering pilot
valve. After the oil goes through the cooler, the flow is directed to the
steering pump control spool and to the drive loop for makeup oil.

The cold oil bypass valve protects the filter and charge pump during start-
• Filter and cooler have
cold oil bypass valves up and the cooler bypass valve protects the cooler. The cold oil bypass
valve is set to open at 3200 kPa (460 psi). The cooler bypass valve will
open when the pressure differential is 345 kPa (50 psi).

The charge circuit makeup valve is a pressure reducing valve that directs
• Implement pump implement pump oil to the charge circuit if the charge pressure decreases
assists steering
charge circuit
below 2000 kPa (290 psi).

The cooling orifice restricts the flow of steering pump case drain oil. This
• Cooling orifice sends restriction forces some of the steering pump case drain oil through a line
steering pump case to the steering motor case. This oil adds to the oil from the steering motor
drain oil to motor
flushing valve for cooling and lubrication of the steering motor.
STMG 699 - 113 -
6/98

D8R STEERING AND IMPLEMENT HYDRAULIC SYSTEM

STEERING
STEERING PUMP
MOTOR CHARGE STEERING
PUMP PILOT
STEERING SYSTEM VALVE

BYPASS AND
PRESSURE
CONTROL
GROUP

RIPPER
CYLINDERS RIPPER STEERING CHARGE
DIVERTER CIRCUIT FILTER
VALVE

TANK

IMPLEMENT SYSTEM
IMPLEMENT
PUMP

INLET MANIFOLD

RIPPER
LIFT
QUICK-DROP
LIFT CYLINDERS
VALVE

TILT TILT
CYLINDER
END COVER

90

IMPLEMENT HYDRAULIC SYSTEM

• Implement hydraulic The implement hydraulic system consists of a variable displacement

system components:
pump, three control valves with an inlet manifold, and a single quick-drop
- Pump valve.

- Three control valves The steering and implement systems are connected at one point. The
with inlet manifold implement pump output is sent to the bypass and pressure control group to
supplement the steering charge pump if the discharge pressure decreases
- Single quick-drop below a specified value.
valve

- Ripper diverter valve

• Two systems share

two functions
STMG 699 - 114 -
6/98

2 1

91

• Hydraulic tank The hydraulic tank serves as a reservoir for the implement and steering
components: hydraulic oil and is located on the right fender. The oil cap and fill tube
(1) are located on the top of the tank. Inside the fill tube is a fine mesh
1. Cap and fill tube
screen which removes large particles of dirt or foreign material from the
2. Vacuum oil as the tank is filled. A vacuum breaker/relief valve (2) is also located
breaker/relief valve on the top of the tank. The oil level sight gauge (3) on the front of the
tank permits an easy check of the hydraulic system oil level. Always
3. Sight gauge
clean the sight gauge to be sure the oil level is visible. Dirt and stains on
the glass frequently give the appearance of a full tank.
STMG 699 - 115 -
6/98

92

• Implement hydraulic The implement hydraulic system is load sensing and pressure
system components: compensated with a variable displacement slipper-type pump (1). The
pump is very similar to many other models in the Caterpillar equipment
1. Implement pump
line.
2. Flow compensator
valve Mounted on the left side of the pump are the flow compensator valve (2)
and the pressure compensator valve (3).
3. Pressure
compensator valve The implement hydraulic pump maintains a low standby pressure between
2100 kPa (305 psi) and 3600 kPa (520 psi). Margin pressure is 2100 kPa
(305 psi) and high pressure cutoff is 26200 kPa (3800 psi). Contained in
the inlet manifold of the implement valve stack are the main relief valve
and the charging valve. The main relief valve protects the system from
pressure spikes over 27000 kPa (3900 psi). The charging valve restricts
return flow to the tank that helps prevent cavitation in the cylinders.
STMG 699 - 116 -
6/98

1
2
5

93

• Implement stack The implement control valve consists of three parallel valve sections:
includes: ripper, lift, ripper, dozer lift, and dozer tilt. The ripper control valve is standard on all
and tilt valves
machines even if the machine is purchased without a ripper. The ripper
• Ripper control valve hardware may be added in the future along with the ripper diverter valve.
standard
Pressure taps are provided on the inlet manifold for signal oil (1) and
1. Signal oil pressure pump discharge (2). By using these pressure taps, margin pressure, low
tap pressure standby and high pressure stall can be tested.

2. Pump discharge The inlet manifold includes the main relief valve (3) and the return oil
pressure tap charging valve (4). The main relief valve is set at 27000 kPa (3900 psi),
which is 2750 kPa (400 psi) higher than the pressure compensator (cutoff)
3. Main relief valve
valve. The main purpose of the main relief in the system is to eliminate
4. Charging valve pressure spikes. If the system is in a stall condition, the pressure cutoff
valve will cause the implement pump to destroke toward a minimum
5. Supply line to steer angle.
circuit
The charging valve restricts the cylinder return oil flow to the tank. This
• Charging valve valve keeps oil pressure in the cylinder return oil passage of the
assists makeup and implement control valves and is used with the makeup valves to prevent
quick-drop valve
cavitation in the cylinders. A typical function when the charging valve
• Supply line sends assists the makeup valve and the quick-drop valve for the lift cylinders is
implement oil to when the dozer control lever is moved to the full lower position (quick-
steering charge circuit drop) and the dozer is lowered rapidly.

➥
STMG 699 - 117 -
6/98

The supply line (5) goes to the bypass and pressure control group and
connects to the external check valve. This line supplies implement pump
oil to the internal pressure reducing valve in the valve group. The
pressure reducing valve in the bypass and pressure control valve provides
implement pump oil to supplement the steering charge pump discharge oil
if the charge pressure decreases below 2000 kPa (290 psi). Implement
system pressure will be felt in this line at all times.
NOTE: To connect the 1U5796 Differential Pressure Gauge Group to
these two pressure taps, remove only the floor plate in the operator's
station. Both hydraulic hose couplings can be connected to the
pressure taps by laying down on the outside of the right side of the
operator's station and using the right hand to secure them. The
operator's seat and seat plate need not be removed for this test.
STMG 699 - 118 -
6/98

94

• Implement hydraulic The threaded gland lift cylinders (1) have built-in bypass plungers that
system components: prevent high pressure loads at either end of the stroke.
1. Threaded gland The single quick-drop valve (2) replaces the former quick-drop valves
cylinders which were located on the head end of each lift cylinder.
2. Quick-drop valve
STMG 699 - 119 -
6/98

95

• Blade tilt cylinders The D8R is available with single or dual tilt cylinders (arrows). If the
(arrows) machine is equipped with a single cylinder, the left cylinder is replaced
with a brace. The tilt control valve will operate either the single or dual
tilt cylinder arrangement. The dozer tilt cylinders have the conventional
bolt-on head design.
STMG 699 - 120 -
6/98

1
5

96

• Ripper component This view of the rear of the machine shows the main components of a
locations: single shank ripper. The visible components include: the carriage (1), the
shank and tooth assembly (2), the ripper frame (3), the lift cylinders (4),
1. Carriage
the tip cylinders (5), and the diverter valve (6).
2. Shank and tooth
NOTE: The ripper shank and tooth assembly is mounted in the
3. Ripper frame machine travel position. For the ripper to be used, the shank must be
mounted in the carriage with the tooth pointing toward the ground.
4. Lift cylinders
The machine can also be equipped with a multi-shank ripper for
5. Tip cylinders
other ripping applications.
6. Diverter valve
STMG 699 - 121 -
6/98

3
1
2

4
5

97

• Ripper component The optional ripper diverter valve group (2) is mounted on the rear of the
locations: machine. The control lever has been redesigned to accommodate the
diverter valve switch (1). When the switch is depressed and the lever is
1. Diverter valve
switch moved left and right, the ripper tip will move in or out. Releasing the
switch and moving the lever left and right will raise or lower the ripper.
2. Ripper diverter
valve The switch activates a solenoid on the ripper diverter valve that sends
steering system charge pressure oil to move the spool in the diverter valve.
3. Pin puller toggle The diverter valve permits a single ripper control valve to be used for both
switch operations.
4. Pin puller control The single shank ripper can be equipped with an optional hydraulically
valve
operated pin puller (5). The pin puller control valve (4) allows the
5. Pin puller cylinder operator to release and engage the pin for the ripper shank with the toggle
switch (3) without leaving the operator's station. Oil for operation of the
pin puller circuit is supplied by the power train hydraulic system.
STMG 699 - 122 -
6/98

D8R IMPLEMENT HYDRAULIC SYSTEM

BYPASS AND PRESSURE
STEERING CONTROL GROUP
CHARGE PRESSURE

INLET
MANIFOLD

DIVERTER RIPPER PUMP

LIFT

TILT

END
COVER

QUICK-DROP

98

Implement System Operation

This diagram shows the hydraulic system with all the implements in
HOLD. Oil is sent from the common steering and implement hydraulic
tank to the variable displacement, piston-type pump. Supply oil is
directed to the closed-center control valves. Return oil and pump case
drain oil are sent to the tank.
When a control lever is moved, oil from the implement control valve is
directed to double acting implement cylinders.
• Signal network in The signal network line (orange) is in series with each control valve and
series passes through each valve body. The signal network terminates at the
pump control valve. When an implement is activated, a signal is
• Highest signal
pressure sent to pump generated by the work port load. This signal is sent through the signal
control valve network. A resolver network inside the implement valves consists of a
series of check valves which compare the signals from the implements
and send the highest signal to the pump control valve.
STMG 699 - 123 -
6/98

STEERING
CHARGE D8R IMPLEMENT HYDRAULIC SYSTEM
PRESSURE
TO BYPASS TO STEERING
AND PRESSURE CHARGE PUMP
CONTROL VALVE
RIPPER
DIVERTER

TO BYPASS X
AND PRESSURE
CONTROL VALVE

INLET
MANIFOLD IMPLEMENT PUMP

QUICK-DROP
VALVE

RIPPER

LIFT

TILT

TANK

99

• Schematic shows This schematic shows the components and conditions in the implement
components in HOLD system with the engine started and the implements in HOLD.
The major components in this system are: the implement pump, the inlet
manifold, the ripper, lift and tilt control valves, the quick-drop valve, and
the ripper diverter valve.
Three changes have occurred from the D8N: a single quick-drop valve, an
• Three changes electrically actuated ripper diverter valve, and the flow control spool in the
include: control valve is solid rather than hollow.
- Single quick-drop In addition to the implement oil being used to move the cylinders, it also
is sent to the bypass and pressure control group to supplement the charge
- Ripper diverter valve
pump if the discharge pressure decreases below 2000 kPa (290 psi). The
- Flow control spools ripper diverter valve uses steering charge pressure to move the ripper
are solid diverter spool when the operator selects the RIPPER TIP or LIFT
functions.
STMG 699 - 124 -
6/98

PRESSURE AND FLOW COMPENSATOR VALVE

ADJUSTMENT SCREWS

PRESSURE COMPENSATOR
(CUTOFF) SPRING

FLOW COMPENSATOR
(MARGIN) SPRING TO TANK

TO
ACTUATOR PISTON
FLOW COMPENSATOR
FROM
(MARGIN) SPOOL
OUTPUT PORT

PRESSURE COMPENSATOR
(CUTOFF) SPOOL

100

Implement Pump

• Two spools in pump Shown here is the pressure compensator valve used on the implement
control valve:
pump. Two spools are installed in the valve:
1. Flow compensator
1. The flow compensator (or margin) spool is on the left. This valve
2. Pressure controls margin pressure and low pressure standby. Margin
compensator pressure is set at 2100 kPa (305 psi) above the signal pressure.
Low pressure standby is approximately 3000 kPa (435 psi). If this
pressure is below 2100 kPa (305 psi) or above 3600 kPa (520 psi),
margin pressure should be checked. Adjusting the margin pressure
to specification allows the standby pressure to be maintained
within specification.

2. The pressure compensator (or cutoff) spool (on the right) controls
the stall pressure. The valve is set at 24100 kPa (3500 psi).

NOTE: Each spring has an individual adjustment screw.

STMG 699 - 125 -
6/98

PUMP AND COMPENSATOR OPERATION

ENGINE OFF

NO SIGNAL PUMP OUTPUT

LARGE ACTUATOR YOKE PAD

SWASHPLATE

DRIVE
SHAFT

FLOW COMPENSATOR PRESSURE

( MARGIN) SPOOL COMPENSATOR SMALL ACTUATOR
(CUTOFF) SPOOL AND BIAS SPRING
PISTON AND
BARREL ASSEMBLY

101

• Identify all labeled When the engine is OFF, the bias spring holds the swashplate at maximum
components angle.
• Bias spring holds When the engine is started, the pump drive shaft starts to rotate. Oil is
swashplate at drawn into the piston bores. As the piston and barrel assembly rotates, the
maximum angle
oil is forced out into the system.
STMG 699 - 126 -
6/98

PUMP AND COMPENSATOR OPERATION

LOW PRESSURE STANDBY

NO SIGNAL PUMP OUTPUT

102

• Pump produces flow: When no flow is demanded from the implements, no signal pressure is
generated. System pressure (red and white stripes) generated by the pump
- Flow blocked at
implement valves
is called "low pressure standby." The pump produces enough flow to
compensate for system leakage at sufficient pressure to provide for
- Pressure increases instantaneous implement response when an implement is actuated.

- Margin spool moves At machine start-up, the bias spring holds the swashplate at maximum
up angle. As the pump produces flow, system pressure begins to increase
because the flow is blocked at the implement control valves. This
- Flow directed to
pressure is felt under both the margin spool and the pressure cutoff spool.
large actuator
The margin spool moves up against the low spring force and permits
- Pump is destroked system oil to go to the large actuator piston in the pump.

• Low pressure
standby:

- No flow demand

- Minimum flow
produced

➥
STMG 699 - 127 -
6/98

• Low pressure standby As pressure in the large actuator piston increases, the large actuator piston
higher than margin overcomes the force of the bias spring and the pressure in the small
actuator piston and moves the swashplate to a reduced angle. The large
actuator piston moves to the right until the cross-drilled passage in the
stem is uncovered. Oil in the large actuator piston then bleeds off to the
pump case. At this minimum angle, the pump will produce just enough
flow to make up for system leakage. The system pressure at this time is
called "low pressure standby" and is approximately 3000 kPa (435 psi).
Low pressure standby is higher than margin pressure. This characteristic
is due to a higher back pressure created by the oil which is blocked at the
closed-center valves when all the valves are in HOLD. Pump supply oil
pushes the margin spool up and further compresses the margin spring.
More supply oil then goes to the large control piston and flows through
the cross-drilled hole in the stem to the pump case.
STMG 699 - 128 -
6/98

PUMP AND COMPENSATOR OPERATION

UPSTROKING

SIGNAL PUMP OUTPUT

REDUCED PRESSURE

103

• Upstroking: When an implement requires flow, a signal is sent to the pump control
valve. This signal causes the force (margin spring plus signal pressure) at
- When flow is
the top of the margin spool to become higher than the supply pressure at
required
the bottom of the spool. The spool then moves down, blocks oil to the
- Signal is sent large actuator piston and opens a passage to drain. Pressure at the large
actuator piston is reduced or eliminated, which allows the bias spring to
- Margin spool moves move the swashplate to an increased angle. The pump will now produce
down
more flow. This condition is called "upstroking."
- Drains large actuator The following conditions can result in upstroking the pump:
- Bias spring and 1. An implement control valve is activated when the system is at
small actuator
low pressure standby.
increase swashplate
angle
2. The control valve directional spool is moved for additional flow.

3. An additional circuit is activated.

➥
STMG 699 - 129 -
6/98

4. Engine rpm decreases. In this case, pump speed decreases which

causes a decrease in flow and pump supply pressure. The pump
must then upstroke to maintain the system flow requirements.

NOTE: Signal pressure does not necessarily have to increase for the
pump to upstroke. For example, if one implement is activated and is
operating at 13800 kPa (2000 psi), the system supply pressure is
15900 kPa (2305 psi) due to the maximum signal pressure of
13800 kPa (2000 psi) plus the margin spring force. Now, if the
operator activates another implement at an initial operating pressure
of 6900 kPa (1000 psi), the maximum signal pressure is still 13800
kPa (2000 psi), but the supply pressure decreases momentarily to
provide the increased flow now needed at the implements. The force
at the top of the margin spool (now higher than the force at the
bottom of the margin spool) pushes the spool down and allows oil in
the pump control to drain. The swashplate angle increases and the
pump provides more flow.
STMG 699 - 130 -
6/98

PUMP AND COMPENSATOR OPERATION

CONSTANT FLOW

SIGNAL PUMP OUTPUT

REDUCED PRESSURE

104

• Constant flow: As pump flow increases, pump supply pressure also increases. When the
pump supply pressure (red) increases and equals the sum of the load
- Signal pressure plus
pressure plus the margin spring pressure, the margin spool moves to a
spring equals
system pressure metering position and the system becomes stabilized.

- Swashplate at
The difference between the signal pressure and the pump supply pressure
constant angle is the value of the margin spring, which is 2100 kPa (305 psi).
STMG 699 - 131 -
6/98

PUMP AND COMPENSATOR OPERATION

DESTROKING

SIGNAL PUMP OUTPUT

INCREASED PRESSURE

105

• Destroking: When less flow is needed, the pump is destroked. The pump destrokes
when the force at the bottom of the margin spool becomes higher than at
- Less flow required
the top. The margin spool then moves up and allows more flow to the
- System pressure large actuator piston. Pressure in the large actuator piston then overcomes
moves margin spool the combined force of the small actuator piston and bias spring and moves
up the swashplate to a reduced angle. The pump will now produce less flow.
- Oil flows to large The following conditions can result in destroking the pump:
actuator
1. All implement control valves are moved to the HOLD position.
- Swashplate angle is The pump returns to low pressure standby.
reduced
2. The control valve directional stem is moved to reduce flow.
• Four conditions for
destroking
3. An additional circuit is deactivated.

4. Engine rpm increases. In this case, pump speed increases causing

an increase in flow. The pump destrokes to maintain system flow
requirements.

➥
STMG 699 - 132 -
6/98

• Pump flow stabilizes As pump flow decreases, pump supply pressure also decreases. When the
when margin spool pump supply pressure (red) decreases and becomes the sum of load
moves to metering
pressure plus margin pressure, the margin spool moves to a metering
position
position and the system stabilizes.
NOTE: Signal pressure does not necessarily have to decrease for the
pump to destroke. For example, if two implements are activated with
one at 13800 kPa (2000 psi) and the other at 6900 kPa (1000 psi), the
system supply pressure is 15900 kPa (2305 psi) due to the maximum
signal pressure of 13800 kPa (2000 psi) plus the margin spring force.
Now, if the operator returns the implement at 6900 kPa (1000 psi) to
HOLD, maximum signal pressure is still 13800 kPa (2000 psi), but the
supply pressure increases due to reduced flow needed at the
implements. The supply pressure will push the margin spring up and
allow more oil to go to the pump control which causes the pump to
destroke.
STMG 699 - 133 -
6/98

PUMP AND COMPENSATOR OPERATION

HIGH PRESSURE STALL

SIGNAL AT PUMP OUTPUT

MAX. PRESSURE AT MAX.
PRESSURE

106

• High pressure stall: The pressure compensator (or cutoff) spool is in parallel with the flow
compensator (or margin) spool. The pressure compensator limits the
- Margin spool is
maximum system pressure at any given pump displacement. The spool is
down
held down during normal operation by the pressure compensator spring.
- Pressure
compensator is up During stall or when system pressure is maximum, signal pressure is
equal to pump supply pressure. The combination of the signal pressure
- Flow directed to
and the margin spring forces the margin spool down. This movement of
large actuator
the margin spool normally opens a passage in the pump control valve for
- Pump is destroked the oil in the large actuator piston to drain and causes the pump to
upstroke. However, if the supply pressure is high enough, the pressure
- System pressure at cutoff spool is forced up against the spring. This movement of the
maximum
pressure cutoff spool blocks the oil in the large actuator piston from going
to drain and allows supply oil to go to the large actuator piston. The
increase in pressure allows the large actuator piston to overcome the
combined force of the small actuator piston and bias spring to destroke the
pump. The pump is now at minimum flow and pump supply pressure is at
maximum. This condition is maintained for a single implement in a stall
condition.

➥
STMG 699 - 134 -
6/98

• Main relief valve limits This system also incorporates a main relief valve located in the inlet
pressure spikes manifold. The pressure cutoff spool can be adjusted in the machine to
destroke the pump at 24100 kPa (3500 psi). The main relief valve must
• Pressure cutoff spool
destrokes pump be removed from the machine and adjusted to 27000 kPa (3900 psi) using
the 1U5216 Test Block Manifold. This valve is set higher to limit
pressure spikes in the system.
When operating two or more implements with one in stall, the pump will
• Pump can still
produce flow for other produce flow to meet the needs of the other implements operating at a
implements lower work port pressure. In this case, the pump could be producing up to
maximum flow while the supply pressure is at the maximum of
• Upstrokes to meet 24100 kPa (3500 psi).
flow requirements
NOTE: Contained within the pump is a case drain relief valve. If the
internal pressure exceeds 170 kPa (25 psi), excess flow will be directed
to the inlet of the pump. The relief valve is designed to protect the
pump shaft seals.
STMG 699 - 135 -
6/98

D8R IMPLEMENT VALVES IMPLEMENT

PUMP SIGNAL
PRESSURE
TO BYPASS AND SIGNAL LINE TO
PRESSURE CONTROL GROUP IMPLEMENT PUMP
TO
IMPLEMENT
INLET MANIFOLD CHARGE PUMP SUPPLY
IMPLEMENT PUMP RELIEF
PRESSURE VALVE
SYSTEM
RELIEF
VALVE
.
FLOW
CONTROL
VALVE

RIPPER

TO RIPPER
LIFT CYLINDERS

FLOW
CONTROL
VALVE
LIFT

TO LIFT
CYLINDERS

FLOW
CONTROL
VALVE
TILT

TO TILT
CYLINDERS

107

Implement Control Valves

• Signal line to pump The implement valve group consists of an inlet manifold, the ripper, lift,
compensator valve and tilt control valves and an end cover. All machines are equipped with
this valve group. Even though the customer may not order the ripper, the
valve is included in the stack.
• Oil sent to bypass and The signal line is connected from the inlet manifold to the pump
pressure control compensator valve. Another line from the inlet manifold sends implement
group
pump oil to the bypass and pressure control group in the steering system.

• Inlet manifold The inlet manifold contains a system relief valve and a charge relief valve.
components: The system relief valve limits pressure spikes and is set higher than the
pressure compensator spool. The charge relief valve restricts return oil
- System relief valve
going to the tank when the pump is not upstroked. This restriction keeps
- Charge relief valve oil pressure in the cylinder return oil passage of the implement control
valves. This oil pressure can be used with the makeup valves to prevent
cylinder cavitation.
STMG 699 - 136 -
6/98

DOZER LIFT VALVE

HOLD
PLUG ROD END HEAD END RETURN
TO TANK FROM
PREVIOUS
VALVE

MAKEUP
VALVE

MAIN CONTROL RESOLVER

SPOOL

LOAD CHECK
VALVE
TO
FLOW CONTROL SPOOL COMPENSATOR
FROM PUMP VALVE

108

• Control valve The dozer lift valve is the second valve in the stack. The lift control valve
operation is a closed-center, manually operated valve controlled through mechanical
- Dozer lift valve
linkage. The lift valve has four positions: RAISE, HOLD, LOWER, and
FLOAT. A centering spring keeps the spool in the HOLD position when
the blade lift cylinders are not in use. To operate in the FLOAT condition,
the operator must move the control lever forward until the detent balls
hold the valve spool. The operator must manually release the lift control
lever from the FLOAT position.

• Lift valve in HOLD This slide shows the lift control valve in HOLD. In HOLD, the center
axial passage is open to the tank through a drain passage in the valve
• Axial passage open to body.
tank
With the engine not running, the spring behind the flow control spool
• Flow control valve is holds the flow control spool to the left. When the operator starts the
initially to left machine, the pump sends oil through the inlet manifold to the flow
• Flow blocked at control spool, out the throttling slots on the left side of the spool, through
control spool and the load check valve, and to the main control spool. With the control
pressure increases spool in the HOLD position, oil cannot flow to the cylinders, and oil
pressure will begin to increase.
➥
STMG 699 - 137 -
6/98

• Flow control spool The increasing pressure in the chamber to the right of the load check
moves to the right valve pushes the flow control spool to the right against the force of the
spring. Moving the flow control spool to the right closes the throttling
• Throttling slot on right
closes slot on the left side of the spool. Oil can continue to flow to the
remaining control valves in the system. In HOLD, pressure at the main
• Throttling slot on left control spool is equal to the flow control spool spring.
opens
Flow control spool: Receives all the oil flow from the inlet valve group.
• Flow control spool The flow control spool provides the "pressure compensating" feature of
maintains maximum the lift circuit by controlling the maximum pressure drop across the lift
pressure differential
control spool. This operation results in a constant implement speed for a
given lever displacement.

• Load check valve Load check valve: Prevents reverse implement flow when the operator
moves a valve from HOLD and system pressure is lower than the cylinder
or work pressure. Without the load check valve, the implement would
drift down. The load check valve will open to allow supply oil to flow
through the control valve when the system pressure is higher than the
work port pressure.

• Resolver Resolver: Also called a double check valve. The resolver compares the
signal between the valves and sends the highest resolved working pressure
to the implement pump flow compensator. Although this slide shows the
resolver and signal lines as external components, the resolver is actually
inside the control valve, and the signal lines are internally drilled
passages.
• Main control spool Main control spool: Controls oil flow to the implement and contains
three cross-drilled holes that connect to an axial drilled passage in the
center of the control spool. The cross-drilled holes sense work port
pressure in both the head and rod ends of the cylinders.
• Makeup valve Makeup valve: Allows pressure in the tank to fill voids in the head end
of the cylinders during times when cylinder supply pressure decreases
below the tank pressure.
• Orifice Orifice: Provides smoother implement operation by delaying the rate that
the signal pressure in the flow control spring cavity decreases when the
operator changes implement directions.

➥
STMG 699 - 138 -
6/98

NOTE: The throttling slot near the left end of the flow control spool
spool is never completely closed, and the check valve does not
completely block oil from reaching the main control spool. A small
amount of oil meters through the flow control spool and past the load
check valve to maintain a pressure at the main control spool that is
equal to the flow control spool spring force. Maintaining pressure at
the main control spool improves implement response.

If the flow control spool is explained as a pressure reducing valve

with a variable spring rate due to changes in signal pressure, the
operation of the spool is easier to understand. The spool will limit the
maximum pressure difference across the control spool to the value of
the flow control spool spring and cylinder pressure to provide
constant flow for a given lever displacement.

INSTRUCTOR NOTE: For more information about the valve

components and operation, refer to STMG 591 "446 Backhoe
Loader--Steering and Implement Hydraulic System" (Form
SESV1591).
STMG 699 - 139 -
6/98

DOZER LIFT VALVE

RAISE
ROD END HEAD END
RETURN
TO TANK FROM
PREVIOUS
VALVE

PLUG MAKEUP
VALVE

MAIN CONTROL RESOLVER

SPOOL

LOAD CHECK
VALVE
TO
FLOW CONTROL SPOOL COMPENSATOR
FROM PUMP VALVE

109

• Lift valve in RAISE As the operator moves the lift control lever to the RAISE position, the
control valve spool shifts to the left allowing pump supply to go through
• Spool shifts left the quick-drop valve to the rod end of the cylinders and opens the head
end of the cylinders to the tank. The oil will begin filling the rod end of
the lift cylinders and begin raising the blade.

• Supply passage Shifting the spool also opens the supply passage drilled in the center of
opened to rod end the control valve spool to the rod end port. Pump pressure going to the lift
cylinder or pressure from the rod end goes through the drilled passage in
• Signal pressure
sensed in flow control
the control spool and this signal oil goes to two places. First, the oil
valve spring chamber travels through the orifice and fills the spring chamber of the flow control
spool moving the spool to the left. As the control valve spool shifts to the
left, the opening at the throttling slots near the left end of the spool
increases so more oil can flow to the work port, while the throttling slots
toward the right end of the control valve spool are open to the head end of
the cylinder and to the tank. The amount of flow from the pump,
combined with the amount of flow the lift work port needs, determines the
distance that the flow control valve shifts.

➥
STMG 699 - 140 -
6/98

• Signal oil sent to Also, the signal oil is sent to the resolver valve. If the lift circuit is
resolver to upstroke producing the highest signal pressure, oil is sent through the manifold to
pump
the implement pump flow compensator valve. The pump will then
upstroke to maintain the margin pressure, approximately 2100 kPa (305
psi) above the pressure of the signal oil.
STMG 699 - 141 -
6/98

DOZER LIFT VALVE

LOWER
ROD END HEAD END
RETURN
TO TANK FROM
PREVIOUS
VALVE

MAKEUP
PLUG VALVE

MAIN CONTROL RESOLVER

SPOOL

LOAD CHECK
VALVE
TO
FLOW CONTROL SPOOL COMPENSATOR
FROM PUMP VALVE

110

• Lift valve in LOWER This slide shows the operation of the lift control valve when the operator
has selected the dozer LOWER position. The main control spool has
• Spool shifts right shifted to the right opening a passage for supply oil to flow through the
• Supply oil goes to quick-drop valve to the head end of the cylinders and a passage for oil
head end from the rod end of the cylinders to return to the tank. The main control
spool movement allows the cylinder pressure to become signal oil that is
• Rod end opens to directed to the resolver and the flow control spool spring chamber through
drain the drilled passages in the main control spool. System pressure controls
• Signal oil sent to the upstroking of the pump by means of the resolver signal pressure and is
resolver and flow the same as described in the dozer RAISE operation.
control spring
chamber The passage to the head end of the lift cylinders contains a makeup valve
for the lift circuit. When the pressure in the cylinder supply passage
• Head end passage decreases below the pressure in the tank, the makeup valve opens and
contains makeup allows return oil from the tank to fill voids in the head end of the
valve
cylinders. The makeup valve is needed because the weight of the blade
tends to force oil out of the rod end of the cylinders faster than the pump
can fill the head end of the cylinders. By including a makeup valve in the
head end passage, the possibility of cavitation is greatly reduced.

➥
STMG 699 - 142 -
6/98

• Dozer LOWER has two The dozer lower operation can function in two conditions. If the control
conditions: lever is moved up to 75% of its maximum non-float travel, the valve
operates as previously described. However, if the operator continues to
- Normal lower
move the lever past this position, the quick-drop mode is activated.
- Quick-drop
operation In FLOAT, detents are used to hold the control valve spool in the FLOAT
position. No signal pressure is generated, which keeps the pump
destroked. Both the rod and head ends of the lift cylinders are open to the
tank, which allows the cylinder rods to move freely in either direction
according to the amount and direction of the force on the blade.
NOTE: Quick-drop valve operation will be discussed in greater
detail later in this presentation.
STMG 699 - 143 -
6/98

DOZER TILT VALVE

TILT LEFT
ROD END HEAD END
RETURN
TO TANK

PLUG PLUG

MAIN CONTROL RESOLVER

SPOOL

LOAD CHECK
VALVE
TO
FLOW CONTROL SPOOL COMPENSATOR
FROM PUMP VALVE

111

• Blade tilt valve in TILT The blade tilt valve is the third valve in the stack. The blade tilt control
LEFT valve is a closed-center, manually operated valve controlled through
mechanical linkage. The valve has three positions: TILT LEFT, HOLD,
and TILT RIGHT. The valve has a centering spring to return the spool to
the HOLD position when the operator releases the dozer control lever.
This slide shows the position of the blade tilt control valve components
during TILT LEFT operation. This valve functions basically the same as
the blade lift control valve with several differences. When the valve spool
shifts to the right, pump supply oil is directed to the head end of the
cylinder, and the rod end of the cylinder is opened to drain. The load
check valve and the resolver valve operate the same as the dozer lift valve.

• No makeup valves One major difference in the blade tilt valve is that no makeup valves are
included in either the head end or rod end circuit. Since the pump can
supply the necessary amount of oil to fill cylinder without cavitation,
makeup valves are not necessary.
STMG 699 - 144 -
6/98

RIPPER CONTROL VALVE

HOLD
ROD END HEAD END
RETURN
TO TANK FROM
PREVIOUS
VALVE

MAKEUP
PLUG VALVE

MAIN CONTROL RESOLVER

SPOOL

LOAD CHECK
VALVE
TO
FLOW CONTROL SPOOL COMPENSATOR
FROM PUMP VALVE

112

• Ripper control valve The ripper control valve is the first valve in the stack. The ripper control
in HOLD valve is a closed-center, manually operated valve controlled through
mechanical linkage. The valve is used to raise and lower the ripper or to
• Switch controls two
functions move the ripper shank IN and OUT. Both functions are controlled by the
operator through the use of a switch located in the ripper control handle.
• Diverter valve for The switch shifts a spool in the ripper diverter valve which directs oil to
ripper lift and tip the correct circuit.
The ripper control valve has a centering spring to return the valve spool to
the HOLD position when the operator releases the lever. This slide shows
the position of the ripper control valve components in HOLD. This valve
functions basically the same as the blade lift control valve with several
differences. When the main control spool shifts to the right, pump supply
oil is directed to the head end of the ripper lift cylinders or to the head end
of the ripper tip cylinders, and the rod end of the cylinders are opened to
drain. The load check valve and the resolver valve operate the same as the
dozer lift valve.

➥
STMG 699 - 145 -
6/98

• Head end passage The passage to the head ends of either the ripper lift or tip cylinders in the
contains makeup control valve contains a makeup valve. When the pressure in the cylinder
valve
supply passage decreases below the pressure in the tank, the makeup
valve opens and allows return oil from the tank to fill voids in the head
end of the cylinders. The makeup valve is needed because the weight of
the ripper tends to force oil out of the rod end of the cylinders faster than
the pump can fill the head end of the cylinders. By including a makeup
valve in the head end, the possibility of cavitation is greatly reduced.