SERV1779

February 2004

SERVICE TRAINING
TECHNICAL PRESENTATION

994D WHEEL LOADER

INTRODUCTION

Meeting Guide 779

994D WHEEL LOADER - INTRODUCTION
MEETING GUIDE 779 SLIDES AND SCRIPT
AUDIENCE
Level II - Service personnel who understand the principles of machine systems operation, diagnostic
equipment, and procedures for testing and adjusting.

CONTENT
This presentation describes the location of the basic components on the engine, and the operation of the
power train, implement, steering, and brake systems for the 994D Wheel Loader.

OBJECTIVES
After learning the information in this presentation, the serviceman will be able to:

1. locate and identify the major components in the engine, power train, implement, steering, and
brake systems;
2. explain the operation of each component in the power train, implement, steering, and brake
systems; and
3. trace the flow of oil through the power train, implement, steering, and brake systems.

REFERENCES
994D Wheel Loader Specalog AEHQ5334
994D Wheel Loader Service Manual RENR2500
994D Wheel Loader Parts Book SEBP2793
Video "994D Wheel Loader - Introduction" SEVN4643
TIM "994 Wheel Loader - Power Train" SEGV2596
TIM "994 Wheel Loader - Implement Hydraulic, Air, and Lube Systems" SEGV2601
TIM "994 Wheel Loader - Steering and Brake Systems" SEGV2602
TIM "992G Wheel Loader - Steering and Brake Systems " SERV2632-01

PREREQUISITES
Interactive Video Course "Fundamentals of Mobile Hydraulics" TEMV9001
Interactive Video Course "Fundamentals of Machine Electronics" TEMV9002

Estimated Time: 12 Hours

Visuals: 83 Illustrations
Handouts: 9 line drawings
Form: SERV1779
Date: 2/04

© 2004 Caterpillar Inc.

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TABLE OF CONTENTS

INTRODUCTION ..................................................................................................................5
Similarities and Differences..............................................................................................6
Component Location.........................................................................................................9

ENGINE................................................................................................................................13
Electrical Block Diagram................................................................................................16

COOLING SYSTEM............................................................................................................22

POWER TRAIN ...................................................................................................................27

Power Flow .....................................................................................................................27
Transmission Hydraulic System .....................................................................................32
Power Train Electrical System........................................................................................51
Component Locations and Functions..............................................................................54

IMPLEMENT HYDRAULIC SYSTEM..............................................................................71

Pilot System ....................................................................................................................73
Main Hydraulic System ..................................................................................................79
Implement Hydraulic System Schematics ......................................................................87
Implement Oil Cooling System ......................................................................................90
Autolube System.............................................................................................................91

STEERING HYDRAULIC SYSTEM..................................................................................93

Steering System Components ........................................................................................93
Steering Hydraulic System Schematics .......................................................................100

STEERING AND BRAKE OIL COOLING SYSTEM......................................................107

BRAKE HYDRAULIC SYSTEM......................................................................................108

Brake System Schematic...............................................................................................108
Brake Component Locations ........................................................................................110

CONCLUSION...................................................................................................................114

SLIDE LIST........................................................................................................................115

HANDOUTS.......................................................................................................................117
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INSTRUCTOR NOTES
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994D WHEEL LOADER

INTRODUCTION

© 2004 Caterpillar Inc.

INTRODUCTION

This presentation discusses the component locations and systems

operation of the 994D Wheel Loader. Basic engine and machine
component locations will be discussed. Also, the power train, the
implement hydraulics, the steering, and the braking system’s component
location and operation will be covered.

The 994D Wheel Loader is the largest wheel loader in the Caterpillar
product line. The loading capacity is matched with the 785 (Standard)
and the 789 (High Lift) Off-highway Trucks. The 994D can be equipped
with a 16 cubic meter to 31 cubic meter (coal application)
bucket (21 - 40 cubic yards).

The 994D operating weight is approximately 192,000 Kg (423,000 lbs).

STMG 779 -6-
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SIMILARITIES AND DIFFERENCES

FEATURES DIFFERENT SIMILAR SAME

Machine Appearance X
Operator's Station X
Engine X
Transmission X
Implement Hydraulic System X
Steering System X
Brake System X
Monitoring System X
Maintenance Items X

Similarities and Differences

This illustration compares the basic features of the 994D to the previous
994. The chart illustrates if the features are different, similar, or the same.
None of the major systems on the 994D are really different from the 994.
• Machine appearance The machine appearance and the implement hydraulic system are
• Implement hydraulic basically the same as the 994. The implement hydraulic system only has
system some additional filtration from the previous version of the 994. Many
changes were made to the 994 through the life of the machine.

• Operator station The operator station is similar, but has the new STIC power train and
steering control similar to the 992G and the 988G, as well some additional
control switches and indicators.

• Engine The 994D is equipped with a 3516B EUI as compared to the 3516 MUI in
the 994.
STMG 779 -7-
2/04

• Transmission The transmission has been updated with electronic controls and new style
impeller clutch and lockup clutch solenoid valves.

• Steering system The steering system on the 994D has been upgraded to STIC control
replacing the Hydraulic Metering Unit (HMU) and steering wheel in the
cab.

• Brake system The brake system has been changed due to the electronic transmission
update. The left pedal on the 994D modulates the impeller clutch before
applying the brakes instead of neutralizing the transmission like the 994.
As a result, the brake control valve has been simplified.

• Monitoring system The 994D is equipped with the Vital Information Management System
(VIMS), the same as the 994. The VIMS on the 994D has been updated
to include more sensors. The wiring harnesses have been updated for
better routing and efficiency.

NOTE: For more information on the VIMS refer to the VIMS

Service Manual SENR6059 and the 994D Electrical Systems Guide
RENR2531.

• Monitoring system The maintenance items on the 994D are similar to the 994. The major
changes in the maintenance are the additional filters on the 994D which
will be discussed in the next illustration.
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ADDITIONAL FILTRATION SUMMARY

SYSTEM LOCATION QTY MICRON DESCRIPTION
Implement System
Near Control Valve Canister with
High Pressure Screens 3 200
on NEEF Element
Near Main Implement Canister with
Case Drain filters 3 13
Pumps Element
Steering System
Under Left Side of Canister with
High Pressure Screens 2 200
Platform Element
Left and Right Side of Canister with
Case Drain filters 2 13
EEF Element
Brake Cooling system
Canister with
Front Brake Oil Screens Back Side of Front Axle 2 500
Element
Canister with
Rear Brake Oil Screens Front Side of Rear Axle 2 500
Element
Other
Front Pump Drive Lube Filter Top Side of NEEF 1 6 Spin on Filter

- Additional filtration The 994D has 15 additional filters when compared to the 994. This
illustration summarizes the filters that have been added, the approximate
location, the quantity of each, the micron rating, and the type of filter.

• Non-Engine End NEEF refers to the Non-Engine End Frame (front frame). EEF refers to
Frame (NEEF) the Engine End Frame (rear frame).
• Engine End Frame
NOTE: Refer to "994D Wheel Loader Operation and Maintenance
(EEF)
Manual SEBU7145" for filter change intervals.
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994D WHEEL LOADER

COMPONENT LOCATION
Auxiliary
Rear Input Drive Input
Pump Torque Drive Transfer Transmission
Shaft
Drive Converter Shaft Gear
Front
Pump
Drive Output
Transfer
Gear

3516B
Engine

Spring Final Transmission Secondary Parking Drive Final

Coupling Drive Pump Steer Pump Brake Shaft Drive
Engine Power Train Hydraulics

Component Location

This illustration shows the basic component locations on the 994D. The
component locations on the 994D are basically the same as the 994 but are
restated in this presentation as a reminder.

Power for the 994D is supplied by the 3516B engine. The engine is
connected to the rear pump drive with a spring coupling. Power flows
from the rear pump drive to the torque converter, to the input drive shaft,
and through the input transfer gear to the transmission. Power from the
transmission flows through the output transfer gears to the drive shafts, to
the bevel gears in the differentials, and then to the double reduction final
drives.

The 994D also has an auxiliary drive shaft that turns the front pump drive.

The secondary steering pump is splined to the output transfer gears and
turns whenever the machine is rolling.
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Front
of
Machine 1 1 1

994D
Front Pump Drive

2 4
3

1 Main Implement Hydraulic Pumps

2 Implement Oil Cooling Pump
3 Implement Pilot Oil Pump
4 Front Pump Drive Lubrication Pump

This illustration shows the location of the pumps on the 994D front pump
drive as viewed from above. The pump locations are the same as the 994.

The main implement hydraulic pumps (1) are fixed displacement

piston-type pumps.

The implement oil cooling pump (2), the implement pilot pump (3), and
the front pump drive lubrication pump (4) are fixed displacement gear
pumps.
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Front
of
1
Machine

2 2

994D
Rear Pump Drive

3 4

1 Service Brake Cooling Pump

2 Steering Hydraulic Oil Pumps
3 Steering and Brake Oil Cooling Pump
4 Brake Application Oil Pump

This illustration shows the location of the pumps on the 994D rear pump
drive as viewed from above. The pump locations are the same as the 994.

The service brake cooling pump (1) and the steering and brake oil cooling
pump (3) are fixed displacement gear pumps. The steering hydraulic oil
pumps (2) and the brake application oil pump (4) are variable
displacement piston-type pumps.
STMG 779 - 12 -
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1. Front pump drive This view shows the filter (1) for the front pump drive lubrication circuit
lubrication filter which is mounted to the right side of the front frame near the articulation
joint. This filter is new on the 994D.

2. Implement Also shown is the site gauge (2) for the fill level on the implement
hydraulic tank site hydraulic tank.
gauge
STMG 779 - 13 -
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8 5
3

6
2
7
1 9

• Left side of engine ENGINE

components:
This view shows the left side of the 3516B engine which can be accessed
1. SCAC water pump
from the right side of the machine.
2. Air compressor
Components which can be seen include:
3. Air conditioning
compressor - Separate Circuit After Cooler (SCAC) water pump (1)
4. Coolant regulator - Air compressor (2)
housing - Air conditioning compressor (3)
5. Engine speed - Coolant regulator housing (4)
timing sensor
- Engine speed timing sensor (5)
6. Starter - Starter (6)
7. Engine oil filters - Engine oil filters (7)
8. Secondary fuel - Secondary fuel filters (8)
filters - Engine oil fill tube (9)
9. Engine oil fill tube
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5 1

• Right side of engine This view show the right side of the engine which can be accessed from
components: the left side of the machine.
1. Engine Electronic Components which can be seen include:
Control Module
(ECM) - Engine Electronic Control Module (ECM) (1)
2. Engine coolant - Engine coolant pump (2)
pump - Power train oil coolers (3)
3. Power train oil - Engine oil cooler (4)
coolers - Alternator (5)
4. Engine oil cooler

5. Alternator
STMG 779 - 15 -
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10

• Primary fuel filter The primary fuel filter (arrow) is mounted inside the rear access door on
the left side of the machine.
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994D BASIC ENGINE BLOCK DIAGRAM

J2
16 Electronic Engine
Unit Injectors
J1 ECM
Ground Bolt
Jacket Water
Temperature Sensor
Speed / Timing
Sensor Aftercooler Coolant
Temperature Sensor
Machine Interface
Connector Permanent Timing
Calibration Sensor
Machine Interface
Connector Crankcase
Pressure Sensor
Oil Level Add Atmospheric
Switch Pressure Sensor

Coolant Flow Turbocharger Outlet

Switch Pressure Sensor

Filtered Oil
Left Exhaust Pressure Sensor
Temperature Sensor
Unfiltered Oil
Right Exhaust Pressure Sensor
Temperature Sensor
Left Turbocharger
Inlet Pressure Sensor
Cooling Fan
Speed Sensor
Right Turbocharger
Inlet Pressure Sensor
Cooling Fan
Proportional Valve Air Conditioning
ON Switch

11

Electrical Block Diagram

This block diagram of the engine electrical system shows the components
mounted on the engine which provide input signals to and receive output
signals from the Engine Electronic Control Module (ECM).

Based on the input signals, the Engine ECM energizes the injector
solenoid valves to control fuel delivery to the engine, and the cooling fan
proportional solenoid valve to adjust pressure to the cooling fan clutch.

The two machine interface connectors provide electrical connections from

the engine to the machine including the Cat Data Link.

Some of the components connected to the Engine ECM through the

machine interface connectors are: the throttle pedal position sensor, the
throttle lock switches, the throttle lock enabled indicator, the right brake
pedal switch, the ether start control solenoid, and the ground level
shutdown switch.
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Input Components:

Speed timing sensor - The speed timing sensor sends a fixed voltage
level, patterned signal the Engine ECM uses to determine the engine
speed, direction, and timing.

Oil level switch - The oil level switch is a float type switch mounted in
the side of the engine oil sump. The Engine ECM monitors the engine oil
level switch to alert the operator when the oil level is low. When the oil
level is OK the switch contacts are CLOSED.

Coolant flow switch - The coolant flow switch mounts in the coolant
passage near the engine coolant pump. When the coolant is flowing past
the switch the paddle moves and closes the switch contacts. The Engine
ECM alerts the operator when there is no coolant flow while the engine is
running.

Exhaust temperature sensors - The exhaust temperature sensors have an

analog to digital converter that provides a Pulse Width Modulated (PWM)
signal.

Cooling fan speed sensor, permanent timing calibration sensor -

These speed sensors are passive speed sensors that provide a signal
similar to a sine wave that varies in amplitude and frequency as speed
increases. The permanent timing calibration sensor monitors the speed
and position of the flywheel.

Jacket water temperature sensor, aftercooler coolant temperature

sensor - These temperature sensors are analog temperature sensors that
provide a voltage signal to the Engine ECM.

Crankcase, atmospheric, turbocharger outlet, filtered and unfiltered

oil, left and right turbocharger inlet pressure sensors - These sensors
are analog sensors that provide a voltage signal to the Engine ECM. The
voltage varies to a level that corresponds with a calibrated pressure. The
Engine ECM calibrates the pressure sensors to the atmospheric pressure
when the key switch is moved to ON position for 3 seconds without the
engine running.

Air conditioning ON switch - The air conditioning ON switch is a

pressure switch. The contacts close when the air conditioning is ON and
the compressor is pressurizing the system.
STMG 779 - 18 -
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2
1

12

• Throttle lock switches The throttle lock switches mounted in the cab to the right of the operator's
1. Set/decelerate seat are the set/decelerate switch (1) and the resume/accelerate switch (2).
The throttle lock enable switch (not shown) is located on the dash. The
2. Resume/accelerate
throttle lock function is the same as the 992G and the 988G.
STMG 779 - 19 -
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1 4

2
3

13

• Rear left of machine The ground level shutdown switch (1) is located on the rear left of the
1. Ground level machine behind the tire. The ground level shutdown switch signals the
shutdown Engine ECM to stop the injection of fuel into the engine but will allow the
engine starter to turn the engine.
2. VIMS ground level
download port Also shown are the VIMS ground level download port (2), the VIMS
ground level download key switch (3), and the stair light switch (4).
3. VIMS ground level
download key

4. Stair light switch

STMG 779 - 20 -
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3516B ENGINE DERATES

• Exhaust Temperature
• Altitude Compensation
• Air Inlet Restriction

14

The 994D engine derates for the 3516B are as follows:

Exhaust Temperature Derate: The engine power will be derated when

the turbine inlet temperatures reach a critical level that may cause engine
damage. The Engine ECM measures the turbine inlet temperatures using
the signals from the left and right exhaust temperature sensors.

When the highest of the right or left turbine inlet temperatures is above
750º C (1382º F) for 30 seconds, the torque map is reduced by 2%. If the
measured temperature does not return to below 750º C within 30 seconds,
the torque map will be reduced an additional 2%. This will continue in
2% steps with each step lasting 30 seconds until the temperature drops
below 750º C or the maximum derate of 20% is reached. The last derate
level reached will remain active until the Engine ECM is powered down.

If the condition reoccurs and the Engine ECM has not been powered
down, the fuel will be limited in the same manner starting from the
previous derate level instead of the beginning level of 2%.

If there is a detected failure in the left or right exhaust temperature sensor

circuits, the Engine ECM will default to the maximum derate value of
20%.

An exhaust temperature derate occurrence will log an Engine Event in the

Engine ECM that requires a Level 3 password to clear.
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Altitude Compensation Derate: The Engine ECM derates engine power

according to operating altitude to reduce exhaust temperatures. The
engine ECM calculates the operating altitude of the machine based on the
signal received from the atmospheric pressuresensor.

The Engine ECM derates the engine power approximately 3% per 305 m
(1000 ft) when the machine is operated above 3050 m (10,000 ft), with a
maximum engine derate of 24% at 5180m (17,000 ft).

Altitude compensation derate does not log an event in the Engine ECM.

Air Inlet Restriction Derate: The Engine ECM derates engine power
when the air inlet or filter becomes plugged and restricts air available for
combustion resulting in elevated exhaust temperatures.

The Engine ECM determines inlet air restriction by subtracting the

turbocharger inlet air pressure, as measured by the turbocharger inlet air
pressure sensors, from the atmospheric air pressure.

The Engine ECM limits fuel by 1% when the inlet air restriction reaches
6.5kPa (25 inches of water). The engine will default to a maximum
derate of 20% if the Engine ECM detects a fault in the circuits for the left
or right turbocharger inlet pressure sensors or the atmospheric pressure
sensor.

An air inlet restriction event will be logged in the Engine ECM when the
engine starts derating. A password is not required to clear an air inlet
restriction event.

NOTE: Multiple engine derate percentages can add up and result in

a total engine power derate greater than 20%.
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994D ENGINE COOLING SYSTEM

ADVANCED MODULAR COOLING SYSTEM (AMOCS)

Separate Circuit Engine Coolant

Aftercooler (SCAC) Radiator
Radiator

Regulator
Housing
Radiator
Aux. Coolant Bypass
Direction of Pump Direction of
Air Flow Main Air Flow
Coolant
Pump Brake
Oil Cooler
Aftercoolers

Hottest
Engine
Oil
Increasing Cooler
Coolant
Temperature Power Train
Oil Cooler
Coldest

Hot SCAC Coolant

15

COOLING SYSTEM

This illustration shows the flow of the engine coolant through the radiator,
the engine, the oil coolers, and the Separate Circuit After Cooler (SCAC)
coolant through the after coolers.

The 994D has been updated with Advanced Modular Cooling System
(AMOCS) radiator cores for the engine coolant and the SCAC.

Hot engine coolant from the engine enters the half of the bottom tank that
is closest to the rear of the machine. The coolant flows up through the
dual pass radiator cores, then down through the same cores and enters the
half of the bottom tank nearest the engine after it has been cooled.
STMG 779 - 23 -
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The main coolant pump draws the cooled engine coolant from the
radiator, or the regulator housing when the regulators are in bypass, and
sends it through the engine oil cooler, the brake oil cooler, the power train
oil cooler, and then into the engine block. The engine coolant flows
through the engine coolant passages and exits the engine block through
the regulator housing.

The engine coolant regulators open and allow the engine coolant to
bypass the engine radiators and flow to the main coolant pump inlet when
the engine is cold. The regulators close and direct engine coolant to the
radiator when the engine reaches operating temperature. The engine
coolant regulators and radiator bypass circuit allow coolant flow through
the engine and coolers when the engine is below operating temperature.

The auxiliary coolant pump pulls the coldest coolant from the SCAC
radiator cores and sends it to the aftercoolers. The coolant flows through
the aftercoolers in series and then returns to the SCAC radiator cores.

The SCAC radiator cores are AMOCS radiator cores. The hot coolant
enters the split bottom tank and flows up through the tube in the dual pass
radiator cores nearest the back of the machine. The coolant then flows
down through the same cores to the half of the bottom tank nearest the
engine after it has been cooled.

The brake oil cooler has not changed and is mounted below the engine on
the inside of the left rear frame rail. The brake oil cooler is an oil to water
cooler and cools the oil from the brake cooling circuit not the brake
application hydraulic oil.
STMG 779 - 24 -
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16

This view shows the engine coolant radiator cores (1) and the SCAC
radiator cores (2) as viewed from the rear of the machine. The engine
coolant and the SCAC coolant radiator cores are two-pass AMOCS
radiator cores that allow the individual cores to be serviced separately.

The engine radiator has a total of thirteen cores and the SCAC radiator
has five cores.
STMG 779 - 25 -
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2
1

17

Shown is the hydraulic oil cooler (1) located between the AMOCS engine
coolant , the SCAC radiator groups (not shown), and the cooling fan (not
shown) at the rear of the engine. This view is from the engine side of the
coolers.

The pump sends hot hydraulic oil through the filter (not shown) and the
inner passages of the oil cooler and modules. The cooling fan sends
cooler air flow through the outer fins of the oil cooler and modules. Heat
from the hot oil transfers to the cooler air. The warmer air is discharged
to the atmosphere. The cooler oil is returned to the hydraulic tank.

Also seen here are the steering and brake hydraulic oil cooler (2) and the
air conditioning condenser (3).
STMG 779 - 26 -
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4
2 5 1

18

This view shows the expansion tanks for the engine coolant (1) and the
SCAC (2) as viewed from standing on the rear of the machine above the
engine and looking over the bumper. The coolant for the systems are
separated and have individual fill caps (3) and sight gauges (4) to check
coolant level.

The welded seam (5) between the engine radiator expansion tank and the
SCAC expansion tank can also be seen in this view.
STMG 779 - 27 -
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994D WHEEL LOADER

POWER TRAIN POWER FLOW
Rear Input Input
Pump Torque Drive Transfer Transmission
Drive Converter Shaft Gear

Front
Pump
Drive

3516B
Engine

Spring Final Transmission Secondary Parking Drive Final

Coupling Drive Pump Steer Pump Brake Shaft Drive

19

POWER TRAIN
Explain power flow
Power Flow
- Engine

- Spring coupling Power from the diesel engine is sent from the flywheel through the spring
coupling to the rear pump drive. The rear pump drive is splined to the
- Rear pump drive
torque converter. Other components (not shown on this illustration) that
- Torque converter are driven by the rear pump drive are: the two steering pumps, the brake
actuation pump, the brake cooling pump, and the steering cooling pump.
- Universal joints and Two universal joints and the input drive shaft connect the torque converter
input drive shaft
to the transmission input transfer gear.
- Input transfer gear The input transfer gear is splined to the transmission input shaft. The
transmission output shaft is splined to the output transfer gear. Power
from the output transfer gear is sent through the front drive shaft and it’s
respective pinion, bevel gear, differential carrier, and axles to the front
final drives and similarly to the rear final drives.
STMG 779 - 28 -
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5 3
4

2
1

7
6

20

Identify Impeller Clutch The impeller clutch torque converter is bolted on the rear pump drive.
Torque Converter
Impeller clutch pressure is monitored by the Power Train ECM via the
• Components: impeller clutch pressure sensor (1). The priority valve (2) maintains
1. Impeller clutch pressure to the impeller clutch and the lockup clutch during a shift. The
pressure sensor
lockup clutch pressure tap (3) and the impeller clutch pressure tap (4) can
2. Priority valve
3. Lockup clutch be used to measure the pressure in the respective clutches.
pressure tap
The torque converter output speed and direction are monitored by the
4. Impeller clutch
pressure tap Power Train ECM via the torque converter output speed sensor (5)
5. Torque converter mounted near the torque converter output shaft (6).
output speed
sensor The torque converter outlet relief valve (7) limits the maximum pressure
6. Torque converter in the torque converter during normal operating temperatures.
output speed
sensor
7. Torque converter
outlet relief valve
STMG 779 - 29 -
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Turbine IMPELLER CLUTCH

TORQUE CONVERTER
Stator

Housing

Lockup Clutch Impeller clutch

Impeller

21

Shown is a sectional view of the torque converter. The major components

include the rotating housing, the impeller, the turbine, the stator, the
impeller clutch, and the lockup clutch.

The rotating housing is splined to the engine flywheel and turns with the
flywheel. The impeller is connected to the rotating housing through the
• Impeller clutch impeller clutch. The clutch discs are splined to the impeller. The clutch
plates are splined to the rotating housing. Pressure oil moves the clutch
piston to engage the discs and plates. When the clutch is engaged, the
impeller rotates with the housing.
STMG 779 - 30 -
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The turbine is splined to the output shaft. In torque converter drive, the
turbine is turned by oil from the impeller. In direct drive, the lockup
• Lockup clutch
clutch connects the turbine to the rotating housing. The lockup clutch
discs are splined to the turbine. The lockup clutch plates are splined to
the rotating housing. Pressure oil moves the clutch piston to engage the
discs and plates. When the clutch is engaged, the turbine, the housing, the
impeller, and the output shaft rotate as a unit at engine rpm. The stator,
• Freewheel stator
which is mounted on a freewheel assembly (also referred to as a "sprag
clutch"), is driven by the force of the oil in the housing and will freewheel
at approximately the same rpm.
STMG 779 - 31 -
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3
1
5
4
6
7

22

Identify transmission The input transfer gear (not shown) and the basic transmission remains
and output transfer unchanged.
gear components

• Components:
The planetary power shift transmission (1) has three FORWARD and
1. Transmission three REVERSE speeds. Electronic solenoids located in the hydraulic
control valve (2) shift the transmission. The solenoids are actuated by the
2. Hydraulic control
valve
Power Train Electronic Control Module (ECM) located on the rear of the
cab.
3. Transmission
output speed The transmission output speed sensor (3) monitors the transmission
sensor output shaft. The signal is sent to the Power Train ECM. The
transmission output speed signal indicates when the clutches have
engaged.

4. Oil screen covers The two transmission oil screens located in the front of the output transfer
gear housing can be accessed by removing the covers (4).

5. Secondary steering Also shown here are the secondary steering pump and diverter valve (5)
pump and diverter and the output shaft (6) for the rear drive shaft.
valve
The sump for the power train oil is located in the bottom of the output
6. Transmission
transfer gear case (7).
output shaft

7. Output transfer
gear case
STMG 779 - 32 -
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994D POWER TRAIN

SCHEMATIC Priority
Valve

Transmission
Control Valve
Lube Input
Transfer
T/C Trans.
LUC Solenoid IC Solenoid Filter Filter

Lockup Impeller Transmission

Clutch Clutch
PUMP
Torque
Converter

Output Transfer Gear

T/C Outlet
Relief Valve
Lube Output
Lube Rear Lube Output
Transfer
Pump Drive Transfer Gear
Bearings
Cooler

23

Transmission Hydraulic System

- Sump This schematic shows the components and the oil flow in the power train
- Two section gear
hydraulic system. Oil from the sump (located in the bottom of the
pump
- Magnetic screens transmission transfer case) flows through two magnetic screens located in
the oil sump to a two section gear pump.
• Right (small) section Oil from the right (small) section of the pump flows through the
of pump transmission filter to the priority valve. When a shift is made, the priority
valve prevents oil pressure in the lockup clutch and impeller clutch from
- Filter dropping below 2205 kPa (320 psi). From the priority valve, oil flows to
- Priority valve the lockup clutch solenoid and the impeller clutch solenoid. When the
lockup clutch solenoid is energized, oil flow pressurizes the lockup clutch
- Lockup clutch
solenoid and places the converter in direct drive. When the impeller clutch
solenoid is energized, oil flow to the impeller clutch is stopped. The
- Impeller clutch
impeller clutch releases allowing the impeller to slip.
solenoid
STMG 779 - 33 -
2/04

- Transmission control When the priority valve opens, oil flows to the transmission control valve.
valve The transmission control valve controls transmission clutch engagement,
provides modulation and sequencing of the directional and speed clutches,
limits the maximum clutch pressure, and limits the maximum inlet oil
pressure to the torque converter. From the transmission control valve,
surplus oil is sent to the torque converter inlet.

• Left (large) section of Oil from the left (large) section of the pump flows through the filter and
pump joins with the surplus oil from the transmission control valve. The
combined oil flows to the torque converter inlet and to the input transfer
- Filter
gear for lubrication. Oil pressure in the torque converter is maintained by
- Input transfer gear the outlet relief valve. An orifice in the outlet relief valve permits some
lubrication oil to flow to the power train components at all times. From
- Torque converter the outlet relief valve, oil is sent through the oil cooler to the rear pump
drive, to the transmission, to the output transfer bearing, and to the output
- Outlet relief valve
transfer gear lubrication circuits.
STMG 779 - 34 -
2/04

1
3

24

Identify components This view shows the transmission and torque converter pump mounted
• Components: below the output shaft (1) on the rear pump drive. The pump is a
1. Transmission two-section gear pump. The small section (2) in the rear of the pump
output shaft supplies oil for the torque converter, and the large section (3) in the front
2. Torque converter of the pump supplies oil to the transmission, the impeller clutch, and the
pump lockup clutch.
3. Transmission The two sections of the pump have a common suction line (4) from the
pump
sump located in the bottom of the output transfer gears.
4. Pump suction
A drain hose connects to the opening (5) on the bottom of the torque
5. Sump drain to
converter sump to allow oil to drain back to the sump in the bottom of the
output transfer
gear housing
output transfer gear housing.
STMG 779 - 35 -
2/04

5
3
1

2
4

25

1. Transmission Oil The transmission oil filter (1) and the torque converter oil filter (2) are
filter bolted to the inside of the engine housing on the left side of the machine.
2. Torque Converter
Oil filter
The two filters are identical.

3. S•O•S Tap The power train oil S•O•S tap (3) is located on the inlet manifold for the
transmission filter facing the engine.
4. Power train oil
coolers Also shown here are the power train oil coolers (4) and the engine oil
5. Engine oil cooler cooler (5).
STMG 779 - 36 -
2/04

5 6
3 4

1
8

2
7

26

1. Priority valve The priority valve (1) is located on the upper rear of the torque converter.
2. Transmission Transmission pump output pressure to the transmission control valve can
pressure be measured at the pressure tap (2) located on the side of the priority
3. Lockup clutch
valve. When the engine is running, the priority valve maintains a
solenoid valve minimum of 2205 kPa (320 psi) oil pressure to the lockup clutch solenoid
valve (3) and impeller clutch solenoid valve (4).
4. Impeller clutch
solenoid valve Also shown are the lockup clutch pressure tap (5), the impeller clutch
5. Lockup clutch pressure tap (6), the impeller clutch pressure sensor (7), and the analog to
pressure tap digital converter (8) for the impeller clutch pressure sensor.
6. Impeller clutch
pressure tap

7. Impeller clutch
pressure sensor

8. Analog to digital
converter
STMG 779 - 37 -
2/04

Test Port

IMPELLER CLUTCH Valve

Spring Ball Orifice Spring
SOLENOID VALVE Spool

IMPELLER CLUTCH
SOLENOID DE-ENERGIZED
TEST PORT

Solenoid Armature
From
Assembly
Pump
To Impeller
Clutch

Test Port

Valve
Spring Ball Orifice Spool Spring

IMPELLER CLUTCH
SOLENOID ENERGIZED

Solenoid Armature
From
Assembly
Pump
To Impeller
Clutch

27

Shown is a sectional view of the impeller clutch solenoid valve.

• Solenoid ENERGIZED When the impeller clutch solenoid is energized, the solenoid moves the
armature assembly against the spring and away from the ball. Pump oil
flows through the center of the valve spool, through the orifice, and past
the ball to drain. The spring on the right end of the spool moves the valve
spool to the left. The valve spool blocks the passage between the impeller
clutch and the pump and opens the passage between the impeller clutch
and drain. Pump flow to the impeller clutch is blocked. The oil in the
impeller clutch flows past the valve spool to drain.

• Solenoid When the impeller clutch solenoid is de-energized, the spring moves the
DE-ENERGIZED armature assembly against the ball. The ball blocks the pump flow
through the orifice to drain. The oil pressure increases at the left end of
the valve spool and moves the valve spool to the right against the spring.
The valve spool blocks the passage between the impeller clutch and drain
and opens the passage between the impeller clutch and the pump. Pump
oil flows past the valve spool to the impeller clutch.
STMG 779 - 38 -
2/04

Test Port

LOCKUP CLUTCH Ball Orifice

Valve
spool Spring
SOLENOID VALVE

SOLENOID DE-ENERGIZED

Solenoid Pin From

Pump
To Clutch

Test Port

Valve
Ball Orifice Spool Spring

SOLENOID ENERGIZED

Solenoid Pin From

Pump
To Clutch

28

Shown is a sectional view of the impeller clutch solenoid valve.

• Solenoid ENERGIZED When the lockup clutch solenoid is energized, the solenoid moves the pin
against the ball. The ball blocks pump oil flow through the orifice to
drain. The oil pressure increases at the left end of the valve spool and
moves the valve spool to the right against the spring. The valve spool
blocks the passage between the lockup clutch and drain and opens the
passage between the lockup clutch and the pump. Pump oil flows past the
valve spool to the lockup clutch.

• Solenoid When the lockup clutch solenoid is de-energized, the force that held the
DE-ENERGIZED pin against the ball is removed. The pump oil flows through the orifice
and past the ball to drain. The spring moves the valve spool to the left.
The valve spool opens the passage between the lockup clutch and drain
and blocks the passage between the lockup clutch and the pump. Pump
flow to the lockup clutch is blocked. The oil in the lockup clutch flows
past the valve spool to drain.
STMG 779 - 39 -
2/04

4
1
3

29

1. Torque converter This view shows the torque converter outlet relief valve (1). The torque
outlet relief valve converter outlet pressure can be measured at the pressure tap (2).
2. Torque converter
outlet relief
The torque converter outlet oil temperature is monitored by the torque
pressure tap converter outlet oil temperature sensor (3) and the torque converter outlet
oil temperature sender (4).
3. Torque converter
outlet oil The Power Train ECM monitors the torque converter outlet oil
temperature sensor
temperature sensor to display the torque converter oil temperature on the
4. Torque converter VIMS display.
outlet oil
temperature sender The torque converter oil temperature gauge in the dash of the cab uses the
signal from the power train oil temperature sender to display the torque
converter oil temperature on the gauge.
STMG 779 - 40 -
2/04

7 1

5
2
6 4

30

•Transmission The transmission hydraulic control valve is bolted to the top of the
hydraulic control planetary group inside the transmission case (1). The control valve
valve
consists of a top manifold, a pressure control group, a separator plate, a
1. Control valve bottom manifold and five shift solenoids.

2. Transmission inlet Also shown are the transmission inlet pressure tap (2), the torque
pressure tap converter inlet pressure tap (3), the P1 pressure tap location (4), the P2
3. Torque converter pressure (5), the P3 pressure tap location (6), and the electrical harness
inlet pressure tap connector for the transmission shift solenoids (7)
4. P1 pressure NOTE: The P2 pressure passes through the tube (5) to the P2
5. P2 pressure to pressure sensor mounted to the cover of the transmission. The tube
cover can be removed and a pressure tap installed for testing purposes
6. P3 pressure when the cover is removed.
7. Connector for
transmission shift
solenoids
STMG 779 - 41 -
2/04

TRANSMISSION
HYDRAULIC CONTROL VALVE
Modulation
Relief Valve

Converter Inlet First And Third Speed

Ratio Valve Selector Spool

Load Piston
Directional
Selector Spool

Pressure
Differential Valve

Second Speed
Selector Spool

31

Identify components Also included in the transmission hydraulic controls are:

and explain function
Modulation relief valve: Limits the maximum clutch pressure.

First and third speed selection spool: Directs oil flow to the No. 5 and
No. 3 clutches.

Load piston: Works with the modulation relief valve to control the rate
of pressure increase in the clutches.

Second speed selector spool: Directs oil flow to the No. 4 clutch.

Pressure differential valve: Controls speed and directional clutch

sequencing.

Directional selection spool: Directs oil to the FORWARD and

REVERSE directional clutches.

Converter inlet ratio valve: Limits the pressure to the torque converter.
STMG 779 - 42 -
2/04

TRANSMISSION HYDRAULIC SYSTEM

FIRST SPEED FORWARD
CONVERTER DRIVE

2 3

Priority Impeller Clutch

Valve Solenoid Valve
1 5 4

Lockup Clutch
Solenoid Valve

Torque
Transmission Converter
3 Filter Filter
2
5 Torque
1 Converter
Torque Outlet
Converter Relief
4 Valve
Pump

Transmission
Control Valve

Sump To
Transmission

32

POWER TRAIN HYDRAULIC SCHEMATICS

In this schematic, the engine is running and the transmission is in

• Transmission in
NEUTRAL NEUTRAL.

- No. 3 clutch solenoid When the operator moves the directional switch to the NEUTRAL
energized position, the Power Train ECM energizes the No. 3 clutch solenoid and
the impeller clutch solenoid. The Power Train ECM also de-energizes the
- Impeller clutch
lockup clutch solenoid.
solenoid energized
Flow from the power train pump is sent through the transmission filter to
- Lockup clutch
the priority valve, the impeller clutch solenoid valve, and the lockup
solenoid
de-energized clutch solenoid valve. The priority valve maintains a minimum oil
pressure to the impeller clutch solenoid valve and the lockup clutch
solenoid valve during transmission shifts.
STMG 779 - 43 -
2/04

When the power train pump supply pressure increases above the priority
valve setting, the priority valve opens and sends oil flow to the manifold
for clutch solenoid valves No. 2 and 3, the manifold for clutch solenoid
valves No. 1, 5, and 4, and the inlet passage for the selector and pressure
control valves.

• Oil to solenoid valves The clutch solenoid valve manifolds directs oil to move the transmission
is used as oil to move speed and directional selector spools.
selector spools
When the No. 3 clutch solenoid is ENERGIZED, the No. 3 clutch
solenoid valve sends oil to one end of the selector spool for speed
clutches No. 3 and 5. The oil pressure overcomes the force of the selector
valve spring and moves the spool from its center position. Oil from the
inlet passage flows through the orifice, past the selector spool for speed
clutches No. 3 and 5, and into the No. 3 speed clutch.

When directional solenoids No. 1 and 2 are DE-ENERGIZED, oil is

blocked at the directional solenoid valves. The directional clutch selector
spool spring centers the valve. Oil flow from the differential valve to the
directional clutches is blocked.

When the oil requirements of the selector and pressure control valve have
been satisfied, the remaining power train pump oil flows to the torque
converter.

• Excess power train Flow from the power train pump is sent to the torque converter filter. Oil
pump oil flows from flows from the filter and joins with the oil from the selector and pressure
the transmission to control valve. The combined oil flows to the torque converter. Flow
the torque converter continues through the torque converter to the torque converter outlet relief
valve. The torque converter outlet relief valve maintains the pressure in
the torque converter. From the outlet relief valve, flow continues through
the cooler to the transmission lubrication circuit.

• ECM pressurizes When the transmission is in NEUTRAL, the Power Train ECM
impeller clutch in pressurizes the impeller clutch in response to the engine speed. When the
response to engine engine speed is less than 1100 rpm, the impeller clutch pressure is
speed
maintained at a holding pressure of 550 ± 207 kPa (80 ± 30 psi). When
the engine rpm increases from 1100 to 1300 rpm, the Power Train ECM
increases the impeller clutch pressure from 550 ± 207 kPa (80 ± 30 psi) to
2580 ± 207 kPa (375 ± 30 psi) for one second. The Power TrainECM
then reduces the impeller clutch pressure to
2274 ± 207 kPa (330 ± 30 psi). The impeller clutch pressure remains at
2274 ± 207 kPa (330 ± 30 psi) for all engine speeds above 1300 rpm. The
torque converter housing and impeller rotate at engine speed.
STMG 779 - 44 -
2/04

When the engine rpm decreases from 1300 to 1100 rpm, the Power Train
ECM decreases the impeller clutch pressure from
2274 ± 207 kPa (330 ± 30 psi) to 550 ± 207 kPa (80 ± 30 psi). The
impeller clutch pressure remains at a holding pressure of
550 ± 207 kPa (80 ± 30 psi) for all engine speeds below 1100 rpm. The
low pressure allows the impeller clutch to remain filled without engaging.
The torque converter housing rotates with the engine while the torque
converter impeller is only partially engaged without transmitting torque.

When the transmission is in NEUTRAL, the Power Train ECM de-

energizes the optional lockup clutch solenoid. When the lockup clutch
solenoid is de-energized, the lockup clutch solenoid valve closes. The
closed valve blocks pump flow to the lockup clutch and allows the lockup
clutch oil to flow to the tank. The lockup clutch releases and disconnects
the turbine from the rotating housing. No power is transmitted through
the turbine from the housing.

NOTE: The impeller clutch pressure is reduced because the pressure

to the impeller clutch is reduced after the first second (1/60 of a
minute) of engagement to extend the life of the seals and pistons in
the impeller clutch. This can be demonstrated by connecting a
pressure gauge to the impeller clutch pressure tap and viewing the
gauge during a directional shift. Caterpillar Electronic Technician
(ET) can also be used to view the impeller clutch pressure and the
impeller clutch solenoid valve current during a directional shift.

INSTRUCTOR NOTE: Operation of the modulating relief valve, the

torque converter inlet ratio valve, and the pressure differential valve
is the same as explained in STMG 421 "966D Wheel Loader Part 2--
Power Train" (Form SERV1421).
STMG 779 - 45 -
2/04

TRANSMISSION HYDRAULIC SYSTEM

SECOND SPEED FORWARD
SPEED SHIFT

2 3

Test Port

Priority
Impeller Clutch
Valve
Solenoid Valve
1 5 4

Lockup Clutch
Solenoid Valve

Torque
Transmission Converter
3 Filter Filter
2

5 Torque
1 Converter
Torque Outlet
Converter Relief
4 Valve
Pump

Transmission
Control Valve

Sump To
Transmission

33

• Speed Shift When the operator shifts from FIRST SPEED FORWARD to SECOND
SPEED FORWARD (speed shift), the Power Train ECM de-energizes the
No. 5 clutch solenoid, and energizes the No. 4 clutch solenoid. The
Power Train ECM also continues to de-energize the impeller clutch
solenoid and the lockup clutch solenoid.

When de-energized, the No. 5 clutch solenoid valve blocks the oil flow
and sends the oil at the end of the selector spool for speed clutches No. 3
and 5 to drain.

When energized, the No. 4 clutch solenoid valve sends oil to the end of
the selector spool for speed clutch No. 4. The oil pressure overcomes the
force of the selector valve spring and moves the spool from its center
position.
STMG 779 - 46 -
2/04

Oil from the inlet passage flows through the orifice, past the selector
spool for speed clutches No. 3 and 5, past the selector spool for speed
clutches No. 4, and into the No. 4 speed clutch.

The empty No. 4 clutch causes the P1 and P2 pressures to decrease to less
than 375 kPa (55 psi). The decrease in P1 oil pressure allows the
differential valve spring to move the differential valve up. When the
differential valve moves up, the differential valve opens a passage for oil
in the differential valve spring chamber and the load piston cavity to flow
to drain.

The transmission control valve then repeats the fill and modulation cycle.

During a speed shift, the Power Train ECM maintains maximum pressure
in the impeller clutch. The transmission directional clutch picks up the
load after a shift.

The impeller clutch pressure is not reduced during a speed shift. Once
again this can be demonstrated with a pressure gauge connected to the
impeller clutch solenoid valve pressure tap or by using Caterpillar ET.
STMG 779 - 47 -
2/04

TRANSMISSION HYDRAULIC SYSTEM

SECOND SPEED FORWARD
DIRECT DRIVE

2 3

Priority Impeller Clutch

Valve Solenoid Valve
1 5 4

Lockup Clutch
Solenoid Valve

Torque
Transmission Converter
3 Filter Filter
2

5 Torque
1 Converter
Torque Outlet
Converter Relief
4 Valve
Pump

Transmission
Control Valve

Sump To
Transmission

34

• Conditions for When the machine is operating in torque converter drive, six conditions
DIRECT DRIVE must be present before the Power Train ECM will energize the lockup
operation
clutch solenoid and shift the torque converter to direct drive.

1. The transmission is in second or third gear.

2. The lockup clutch enable switch is in the ON position.

3. The torque converter output speed is above 1375 ± 50 rpm.

4. The machine has been in the present speed and direction for more
than two seconds.

5. Neither brake pedal is depressed.

6. The lockup clutch has been released by the Power Train ECM for
at least four seconds.
STMG 779 - 48 -
2/04

• Lockup clutch When the lockup clutch solenoid is energized, the lockup clutch solenoid
operation valve opens. The transmission pump oil flows past the lockup clutch
solenoid valve and fills the lockup clutch. The lockup clutch engages and
connects the turbine to the rotating housing.

• Impeller clutch
In DIRECT DRIVE, both the impeller clutch and the lockup clutch are
engaged during engaged. The torque converter rotating housing, the impeller, and the
lockup clutch turbine turn as a unit. The stator, which is mounted on a freewheel
engagement assembly, is driven by the force of the oil in the housing and will
freewheel at approximately the same rpm.
STMG 779 - 49 -
2/04

TRANSMISSION HYDRAULIC SYSTEM

SECOND SPEED REVERSE
DIRECTIONAL SHIFT

2 3

TEST PORT

Priority Impeller Clutch

Valve Solenoid Valve
1 5 4

Lockup Clutch
Solenoid Valve

Transmission Torque
Filter Converter
2 3 Filter
5 Torque
1 Converter
Torque Outlet
Converter Relief
4 Valve
Pump

Transmission
Control Valve

Sump To
Transmission

35

• Directional shift When the operator shifts from FIRST SPEED FORWARD to SECOND
SPEED REVERSE (directional shift), the Power Train ECM de-energizes
clutch solenoids No. 2 and 5 and energizes clutch solenoids No. 1 and 4.
The Power Train ECM also energizes the impeller clutch solenoid and de-
energizes the lockup clutch solenoid.

When the Power Train ECM de-energizes the No. 2 clutch solenoid, the
No. 2 clutch solenoid valve blocks the oil flow and sends the oil at the end
of the selector spool to drain. The force of the selector valve spring
moves the spool to its center position. When the selector spool moves to
the center position, oil in the No. 2 clutch flows to drain.
STMG 779 - 50 -
2/04

When the Power Train ECM energizes the No.1 clutch solenoid, the No. 1
clutch solenoid valve sends oil to one end of the directional clutch
selector spool for directional clutches No. 1 and 2. The oil pressure
overcomes the force of the selector valve spring and moves the spool
from its center position. Directional clutch oil flows from the pressure
differential valve, past the directional clutch selector spool, and into the
REVERSE directional clutch (No. 1).

When the Power Train ECM de-energizes the No. 5 clutch solenoid, the
No. 5 clutch solenoid valve blocks the oil flow and sends the oil at the end
of the selector spool for speed clutches No. 3 and 5 to drain. The oil
pressure overcomes the force of the selector valve spring and moves the
spool from its center position. When the selector spool moves to the
center position, oil in the No. 5 clutch flows to drain.
When the Power Train ECM energizes the No. 4 clutch solenoid, the
No. 4 clutch solenoid valve sends oil to one end of the speed clutch
selector spool for speed clutch No. 4. The oil pressure overcomes the
force of the selector valve spring and moves the spool from its center
position. Oil from the inlet passage flows through the orifice, past the
selector spool for speed clutches No. 3 and 5, past the selector spool for
speed clutches No. 4, and into the No. 4 speed clutch.
As the empty No. 1 and 4 clutches fill, they cause the P1 and P2 pressures
to decrease to less than 375 kPa (55 psi) momentarily. The momentary
decrease in P1 oil pressure allows the differential valve spring to move
the differential valve up. When the differential valve moves up, the
differential valve opens a passage for oil in the differential valve spring
chamber and the load piston cavity to flow to drain.
The transmission control valve then repeats the fill and modulation cycle.

During a directional shift, the Power Train ECM reduces the pressure in
the impeller clutch (fully energized) allowing the impeller clutch to slip.
The Power Train ECM monitors the torque converter output speed sensor
and the transmission output speed sensor to determine when the
transmission clutches are engaged. When the transmission clutches are
engaged, the Power Train ECM engages the impeller clutch in the torque
converter. The impeller clutch in the torque converter absorbs the energy
of a directional shift.

The pressure to the impeller clutch is reduced to approximately

2275 kPa (330 psi) one second after the clutch is engaged. The strategy
for shifting is the same as the 992G, with the exception of the lockup
clutch engagement. Lockup clutch engagement is explained later in the
text.
STMG 779 - 51 -
2/04

POWER TRAIN Cat Data Link Control and Monitor

Power
ELECTRICAL SYSTEM Train
Systems
ECM
INPUT COMPONENTS OUTPUT COMPONENTS

STIC
Air Start
Upshift, Downshift,
Solenoid
Forward, Neutral,
Reverse
Reduced Rimpull
Ignition Keyswitch Indicator Lamp
Reduced Rimpull
Clutch 1
Enable Switch
Reverse Solenoid
Reduced Rimpull
Selection Switch Clutch 2
Forward Solenoid
Torque Converter Oil
Outlet Temperature Clutch 3
3rd Gear Solenoid
Parking Brake
Pressure Switch
Clutch 4
Lockup Clutch 2nd Gear Solenoid
Enable Lamp
Lockup Clutch Clutch 5
Enable Switch 1st Gear Solenoid
Steering / Transmission
Impeller Clutch
Lock Switch
Solenoid
Torque Converter Lockup Clutch
Pedal Position Sensor Solenoid
Torque Converter
Output Speed Sensor Gear Indicator
Transmission Output
Speed Sensor
Impeller Clutch Back-up Alarm
Pressure Sensor Relay

36

Power Train Electrical System

This diagram of the Power Train Electrical System shows the components
which provide input signals to the Power Train ECM.

Based on the input signals, the Power Train ECM energizes the
appropriate transmission control valve solenoids for speed and directional
clutch engagement. The Power Train ECM also energizes the starter relay
when starting the machine and the back-up alarm when the operator
selects a reverse gear.

When required, the Power Train ECM energizes the impeller clutch
control valve solenoid, the lockup clutch control valve solenoid, and the
reduced rimpull indicator lamp.

The CAT Data Link connects the Power Train ECM to the Engine ECM.
The data link also connects the ECMs to the Vital Information
Management System (VIMS) and electronic service tools such as
Caterpillar Electronic Technician (ET).
STMG 779 - 52 -
2/04

• Input components The input components to the Power Train ECM are:

STIC: Combines control of the vehicle steering system and the

transmission shifting system in a single input device.

Ignition key switch: Provides a signal to the Power Train ECM when the
operator wants to start the engine. The STIC directional switch must be
in the NEUTRAL position before the Power Train ECM will permit
engine starting.

Reduced rimpull enable switch: When in the CLOSED position, causes

the Power Train ECM to use the position of the reduced rimpull selector
switch to determine rimpull torque range (through the impeller clutch).

Reduced rimpull selection switch: When enabled by the reduced

rimpull enable switch, this rotary switch determines the maximum rimpull
torque.

Torque converter oil outlet temperature sensor: Provides a pulse

width modulated signal the Power Train ECM uses to determine the
temperature of the oil flowing out of the torque converter through the
outlet relief valve.

Park brake pressure switch: Monitors the park brake hydraulic

pressure and the Power Train ECM can determine when pressure is
applied to release the park brake.

Lockup clutch enable switch: When in the ON position, enables the

lockup clutch to ENGAGE when the machine operating conditions are
correct. The lockup clutch enable lamp is turned on by electrical contacts
in the switch whenever the lockup clutch is enabled..

Steering and transmission lock switch: When in the LOCK position,

causes the Power Train ECM to shift the transmission to NEUTRAL.

Torque converter pedal position sensor: Signals the position of the

torque converter pedal to the Power Train ECM. The Power Train ECM
uses the position information to vary torque to the drive train through the
impeller clutch. The actual value of torque reduction is determined by a
combination of different input signals.

Torque converter speed sensor: Provides a signal the Power Train ECM
uses to determine the output speed and direction of the torque converter.

Transmission speed sensor: Provides a signal the Power Train ECM

uses to determine the output speed of the transmission.
STMG 779 - 53 -
2/04

Impeller clutch pressure sensor: Provides a pulse width modulated

signal the Power Train ECM uses to determine the impeller clutch
hydraulic pressure.

• Output components The output components which receive signals from the Power Train ECM
are:

Starter relay: The Power Train ECM energizes the air start solenoid
valve when the appropriate conditions to start the machine have been met.

Reduced rimpull indicator lamp: The Power Train ECM illuminates

the reduced rimpull indicator lamp when the appropriate machine
operating conditions are met and the Power Train ECM is providing
reduced rimpull.

Clutch solenoids: The solenoids control oil flow to the speed and
directional control spools.

Impeller clutch solenoid: The Power Train ECM energizes the impeller
clutch solenoid with different levels of current to control hydraulic
pressure to the impeller clutch.

Lockup clutch solenoid: The Power Train ECM energizes the lockup
clutch solenoid to ENGAGE the lockup clutch when the correct machine
conditions have been met.

Back-up alarm relay: The Power Train ECM energizes the back-up
alarm when the operator selects the REVERSE direction with the STIC.
The backup alarm relay energizes the two backup alarms.
STMG 779 - 54 -
2/04

37

Component Locations and Functions

• Power Train ECM The Power Train ECM (arrow) is mounted on the exterior of the back
(arrow) wall of the operator cab.

The Power Train ECM makes decisions based on control program

information in memory and switch and sensor input signals.

The Power Train ECM responds to machine control decisions by sending

output voltage to the appropriate circuit which creates an action. For
example, the operator selects an upshift using the STIC. The Power Train
ECM interprets the input signals from the STIC, evaluates the current
machine operating status and energizes the appropriate clutch solenoids.

The Power Train ECM receives three different types of input signals:

1. Switch input: Provides the signal line to battery, ground, or open.

2. PWM input: Provides the signal line with a square wave of a
specific frequency and a varying positive duty cycle.
3. Speed signal: Provides the signal line with either a repeating, fixed
voltage level pattern signal or a sine wave of varying level and
frequency.
STMG 779 - 55 -
2/04

• Output signals The Power Train ECM has three types of output drivers:

1. ON/OFF driver: Provides the output device with a signal level of

+Battery voltage (ON) or less than one Volt (OFF).
2. PWM solenoid driver: Provides the output device with a square
wave of fixed frequency and a varying positive duty cycle.
3. Controlled current output driver: The ECM will energize the
solenoid with 1.25 amps for approximately one half second and
then decrease the level to 0.8 amps for the duration of the on time.
The initial higher amperage gives the actuator rapid response and
the decreased level is sufficient to hold the solenoid in the correct
position. An added benefit is an increase in the life of the solenoid.
• ECM controls: The Power Train ECM controls the transmission speed and directional
- Speed and direction clutches and the operation of the impeller clutch and lockup clutch. The
Power Train ECM interprets signals from the STIC, the torque converter
- Impeller and lockup
pedal position sensor, the lockup clutch enable switch, and the current
clutches
machine operating status to determine the appropriate output signals to
the systems. Different conditions of the inputs affect the output
conditions. These conditions will be discussed later.

The Power Train ECM communicates through the CAT Data Link. The
CAT Data Link allows high speed proprietary serial communications over
a twisted pair of wires. The CAT Data Link allows different systems on
the machine to communicate with each other and also with service tools
such as Caterpillar Electronic Technician (ET).

The Power Train ECM has built-in diagnostic capabilities. As the Power
Train ECM detects fault conditions in the power train system, it logs the
faults in memory and displays them on the VIMS. The fault codes can
also be accessed using the ET service tool. VIMS software can be used to
view faults logged by the VIMS.

INSTRUCTOR NOTE: Power Train ECM faults displayed on the

VIMS relating to the Power Train ECM will have a Module Identifier
(MID) of "81." For more information, refer to the Service Manual
module "Power Train Electronic Control System, Systems Operation
Testing and Adjusting" (Form RENR2522).
STMG 779 - 56 -
2/04

1
4

38

1. STIC The STIC (1) is bolted to the seat at the front of the left armrest. The
2. Transmission transmission directional control switch (2) is a three position rocker
directional control switch that the operator uses to select NEUTRAL, FORWARD, or
switch REVERSE. The transmission speed upshift switch (3) and the
3. Speed upshift transmission speed downshift switch (4) are momentary contact switches
switch that the operator uses to select the desired speed.
4. Speed downshift When the operator selects REVERSE by depressing the top of the
switch
directional control switch, the Power Train ECM energizes the reverse
directional solenoid. The Power Train ECM also activates the back-up
alarm. When the operator selects FORWARD by depressing the bottom
of the directional control switch, the Power Train ECM energizes the
forward directional solenoid.

When the operator selects NEUTRAL by placing the directional control

switch in the center position, the Power Train ECM de-energizes both the
forward and the reverse directional solenoids. After two seconds, the
Power Train ECM energizes speed solenoid No. 3 and the transmission is
in NEUTRAL until the operator selects a different gear.
STMG 779 - 57 -
2/04

When the operator presses the upshift switch, the Power Train ECM
energizes the appropriate speed clutch solenoid to select the next higher
gear, and the transmission upshifts. When the operator presses the
downshift switch, the Power Train ECM energizes the appropriate speed
clutch solenoid to select the next lower gear, and the transmission
downshifts.

The switches must be released and pressed again to continue shifting. If

the operator presses and holds the upshift or the downshift switch, the
transmission will shift once and remain in that speed until the switch is
released and pressed again.
STMG 779 - 58 -
2/04

39

• Gear indicator Three indicator lamps (arrows) located in the front dash panel of the cab
are used to identify the active speed of the transmission. The active
direction is determined by the position of the directional control switch on
the STIC controller.
STMG 779 - 59 -
2/04

40

• Steering and When the steering and transmission lock lever (arrow) is moved to the
transmission lock LOCK position (not shown), the STIC is held in the center position and
lever (arrow)
steering is disabled. In the LOCK position, the steering lock lever
• Steering and
depresses the steering and transmission lock switch (not visible). The
transmission lock steering and transmission lock switch signals the Power Train ECM to
switch shift the transmission to NEUTRAL.

When the steering and transmission lock lever is moved to the UNLOCK
position, the steering and transmission functions are enabled.

The power train portion of the STIC sends input signals to the Power
Train ECM. Certain machine operating conditions will override the
operator desired function of the STIC. If the directional switch is in the
FORWARD or REVERSE position when the steering and transmission
lock lever is moved to the UNLOCK position, the Power Train ECM will
not shift from NEUTRAL. The directional switch must first be moved to
the NEUTRAL position, then to the direction desired before the Power
Train ECM will engage a directional clutch.

If the steering and transmission lock lever is in the UNLOCK position

when the machine is started, the lever must be moved to the LOCK
position and then to the UNLOCK position before the Power Train ECM
will shift the transmission out of NEUTRAL.
STMG 779 - 60 -
2/04

41

• Key start switch The operator turns the key start switch (arrow) clockwise to signal the
Power Train ECM to start the engine. The key start switch supplies a
signal of +Battery to the Power Train ECM. The Power Train ECM
energizes the air start solenoid and the air start solenoid supplies air to the
starting motor and begin engine cranking. Three conditions must be
present before the Power Train ECM will energize the air start solenoid:

1. The key switch is turned to the start position.

2. The transmission directional control switch is in neutral.
3. The system voltage is below +32 Volts.
If the machine is equipped with the engine prelubrication attachment the
Power Train ECM will request prelubrication status from the Engine
ECM via the datalink. If the Engine ECM determines the need for
prelubrication, the Engine ECM will perform the prelubrication function
and signal the Power Train ECM when prelubrication has been
completed. The Power Train ECM will make sure the three conditions
stated above have been met and energize the air start solenoid.
STMG 779 - 61 -
2/04

42

• Reduced rimpull The reduced rimpull enable switch (arrow) is a two position rocker switch
enable switch mounted on the implement lift lever.

In the OPEN position, this switch signals the Power Train ECM that the
operator requests maximum rimpull. The Power Train ECM provides
maximum rimpull when the torque converter pedal is fully released by
keeping the impeller clutch fully engaged. In this position, maximum
rimpull will be provided regardless of the rimpull selection switch.

In the CLOSED position (shown), this switch provides a +Battery signal

to the Power Train ECM. The Power Train ECM monitors the reduced
rimpull selection switch to determine the rimpull setting with the torque
converter pedal fully released. This condition occurs only when the
machine is in FIRST GEAR. If the machine is not in FIRST GEAR, the
rimpull will stay at maximum.
STMG 779 - 62 -
2/04

8
2 3
4

6 7

43

Identify components: When the reduced rimpull enable switch is in the CLOSED position, the
reduced rimpull selection switch (1) indicates the desired maximum
1. Reduced rimpull
rimpull setting to the Power Train ECM. The desired maximum rimpull
selection switch
setting will be active when the torque converter pedal is fully released,
2. 85% rimpull and the machine is in FIRST GEAR.
3. 70% rimpull
The Power Train ECM reduces rimpull by increasing the current to the
4. 55% rimpull impeller clutch solenoid, which reduces the hydraulic pressure to the
5. 45% rimpull impeller clutch and allows slippage between the impeller and the torque
6. Torque converter
converter housing. By additionally decreasing the impeller clutch
pedal pressure, the impeller will slip more resulting in lower torque to the
power train. The resulting additional engine horsepower can be used for
7. Torque converter
pedal position
the implements.
sensor
The reduced rimpull selection switch has four positions. Each position
8. Reduced rimpull corresponds to a maximum allowable percentage of maximum rimpull.
indicator light The default values for each position are indicated as follows:

- 85% Rimpull (2)

- 70% Rimpull (3)
- 55% Rimpull (4)
- 45% Rimpull (5)
STMG 779 - 63 -
2/04

The Power Train ECM monitors the position of the torque converter
pedal (6) with the torque converter pedal position sensor (7) located
behind the panel at the pivot for the pedal. As the operator depresses the
pedal, the Power Train ECM increases the current to the impeller clutch
solenoid and reduces the hydraulic pressure to the impeller clutch. The
rimpull will decrease with pedal travel from the reduced maximum setting
to the minimum setting. When the operator releases the left pedal, the
rimpull will return to the maximum percentage as set by the selector
switch.

When the maximum allowable percentage is in the lower values, the total
change of rimpull from maximum to minimum is decreased. This
condition results in a more gradual change of rimpull over the travel of
the torque converter pedal.

If the machine is not in FIRST GEAR, the impeller clutch pressure will
remain at the maximum level until the transmission is shifted into FIRST
GEAR.

The torque converter pedal functions similarly when the maximum

rimpull enable switch is in the OPEN position, except the maximum
allowable percentage is now 100%.

The reduced rimpull indicator light (8) will be illuminated when the
Power Train ECM detects the reduced rimpull enable switch is in the
ENABLE (closed) position, and the system is providing reduced rimpull.
The light will be off when the switch is in the DISABLED (open)
position.

NOTE: An increase in current to the impeller clutch solenoid from

the Power Train ECM results in a decrease in pressure to the impeller
clutch.

INSTRUCTOR NOTE: To change the setting for each position of the

reduced rimpull selection switch, refer to the Service Manual module
"Power Train Electronic Control System, Systems Operation Testing
and Adjusting" (Form RENR2522).
STMG 779 - 64 -
2/04

1 2

5
7
4

6 3

44

1. Impeller clutch The impeller clutch solenoid (1) is mounted on the impeller clutch
solenoid valve (2). The impeller clutch valve is located on the left side of the
2. Impeller clutch torque converter housing.
valve
The Power Train ECM monitors the status of the impeller clutch solenoid
and can determine certain faults that may affect operation of the impeller
clutch. These faults include: a short to +Battery, a short to ground, an
open circuit, or the impeller clutch not responding properly.

3. Impeller clutch The Power Train ECM receives a signal from the impeller clutch pressure
pressure sensor sensor (3) to monitor the impeller clutch pressure. The Power Train ECM
can compare the control of the impeller clutch solenoid with the response
of the impeller clutch pressure to determine if the impeller clutch is
responding properly.

When the Power Train ECM detects a fault in the impeller clutch solenoid
circuit, a fault will be displayed on the VIMS message center.

When a fault is detected, controlled throttle shifting is used. When a

directional shift is made above 1100 rpm, the Power Train ECM will
request a desired engine speed of 1100 rpm from the Engine ECM for 1.9
seconds if shifting into forward and a desired engine speed of 1100 rpm
for 2.5 seconds if shifting into reverse. This feature helps decrease the
energies absorbed in the transmission.
STMG 779 - 65 -
2/04

The torque converter pedal position sensor and the impeller clutch
solenoid must be calibrated through the VIMS to ensure proper operation.

Also shown are the lockup clutch solenoid (4) and the lockup clutch
4. Lockup clutch
solenoid
valve (5). The lockup clutch solenoid and lockup clutch valve look
similar to the impeller clutch solenoid and impeller clutch valve but are
5. Lockup clutch
different and should not be interchanged.
valve
The lockup clutch solenoid is mounted on the lockup clutch valve. The
lockup clutch valve is located on the left side of the torque converter
housing between the impeller clutch solenoid valve and the torque
converter housing.

The Power Train ECM energizes the lockup clutch solenoid to allow oil to
flow through the lockup clutch valve to the lockup clutch. The pressure
increases in the lockup clutch, causing it to engage and the machine
operates in DIRECT DRIVE.
The lockup clutch solenoid is a proportional solenoid and is energized by
a modulated signal from the Power Train ECM. The Power Train ECM
varies the amount of current to control the amount of oil flow through the
lockup clutch valve to the lockup clutch.

6. Torque converter
The Power Train ECM receives a signal from the torque converter output
output speed speed sensor (6). The speed sensor is mounted on the front of the torque
sensor converter housing above the output shaft. The signal is a fixed voltage
level, patterned waveform which the Power Train ECM uses to determine
the speed and direction of the torque converter output.
If the machine is allowed to roll backwards on an incline when a forward
gear is selected the toque converter output can turn in reverse . This
condition is called reverse turbine and can result in high temperatures
inside the torque converter. If the Power Train ECM determines the
output of the torque converter is turning in the reverse direction greater
than 500 rpm, the Power Train ECM will ignore the left pedal position
input and increase the impeller clutch pressure to prevent this condition.
The Power Train ECM will also override the reduced rimpull setting if
necessary to try to eliminate the reverse turbine.

7. Torque converter The Power Train ECM monitors the temperature of oil exiting the torque
outlet oil converter with the torque converter outlet oil temperature sensor (7)
temperature sensor which is mounted on the front right of the torque converter housing, just
above the torque converter outlet relief valve.

INSTRUCTOR NOTE: An increase in current to the lockup clutch

solenoid from the Power Train ECM results in an increase in pressure
to the lockup clutch.
STMG 779 - 66 -
2/04

45

1. Lockup clutch The lockup clutch enable switch (1) is located on the front dash in the
enable switch cab. When the switch is in the ON (closed) position and the proper
conditions have been met, the Power Train ECM will engage the lockup
2. Lockup clutch
enabled lamp
clutch to improve the efficiency of the power train.

The Power Train ECM first energizes the lockup clutch to a hold level for
.75 seconds to allow time for the clutch to fill. The current is then ramped
up to full on in .65 seconds.

During normal operation, the Power Train ECM will ENERGIZE the
torque converter lockup clutch solenoid based on the following
conditions:

1. Lockup clutch enable switch state: ON (closed).

2. Torque converter output speed: When the torque converter
output speed is greater than 1125 ± 50 rpm.
3. Time in gear: The transmission must be in the present speed and
direction for at least two seconds.
4. Time since lockup clutch solenoid was de-energized: At least
four seconds must have passed since the Power Train ECM de-
energized the lockup clutch solenoid.
5. Left pedal and right brake pedal status: Both pedals must be
fully released.
STMG 779 - 67 -
2/04

6. Current gear: The current gear is not first forward. All gears
other than first forward will allow the Power Train ECM to engage
the lockup clutch when the appropriate conditions are met.
An indicator lamp (2) on the dash lights when the lockup clutch is
enabled.

During normal operation, the following conditions will cause the Power
Train ECM to DE-ENERGIZE the torque converter lockup clutch
solenoid valve and release the lockup clutch:

1. Lockup clutch enable switch: OFF (open).

2. Torque converter output speed: When the torque converter
output speed is less than 975 ± 50 rpm.
3. A shift is made.
4. Left pedal and right brake pedal status: If either pedal is
depressed.
5. Rapid vehicle deceleration: If the machine slows down rapidly,
such as when engaging the pile, the Power Train ECM will
disengage the lockup clutch to prevent engine lugging.
6. Lockup clutch system fault: If the ECM detects a fault in any of
the lockup clutch control inputs such as: the enable switch, the
torque converter output speed sensor, the left pedal position, or the
service brake pedal status.
The Power Train ECM will enable the lockup clutch four seconds after:
the service brake is released, the left pedal is released, a lockup clutch
enable switch off-to-on transition, a rapid vehicle deceleration, or a fault
in the lockup clutch control inputs has been cleared.

NOTE: To prevent engine overspeed, the Power Train ECM will not
engage the lockup clutch when the torque converter output speed is
higher than 1750 rpm.

During lockup clutch engagement, the impeller clutch pressure is

maintained at the system pressure 2275 ± 207 kPa (330 ± 30 psi).
STMG 779 - 68 -
2/04

3 2

4 5 1

46

• Transmission The Power Train ECM shifts the transmission by energizing the solenoid
solenoid valves valves (arrows) that are located in the transmission control valve group on
top of the transmission.

1. Reverse (No. 1 Two solenoid valves are used to control REVERSE (1) or FORWARD (2)
clutch) directional shifts and three solenoid valves are used to control speed
2. Forward (No. 2 shifts: FIRST (3), SECOND (4), and THIRD (5).
clutch)
The solenoid valves are two-position, three-way solenoid valves. The
3. First (No. 5 clutch) solenoid valves are normally open to drain. When energized, the solenoid
4. Second (No. 4 valve spool moves to direct pressure oil to one end of the transmission
clutch) control valve spool. The transmission control valve spool then directs oil
5. Third (No. 3 clutch)
to the appropriate clutch.

The solenoids are operated by 12VDC max. The Power Train ECM first
energizes the solenoids with 12VDC for one second and then decreases
the voltage to approximately 8.25VDC for the remainder of the time that
the solenoid is energized. The decreased voltage level is enough to keep
pressure oil to the control valve spool to maintain position while
extending the service life of the solenoid.
STMG 779 - 69 -
2/04

2 1

47

1. Park brake knob The park brake lever has been changed to a park brake knob (1) located
(arrow) on the dash in the cab. The park brake control knob is connected by a
push pull cable to the park brake control valve. The park brake control
valve supplies hydraulic oil to the park brake to release the park brake
when the park brake control knob is pushed in.

• Park brake pressure The park brake pressure pressure switch (not shown) provides a signal to
switch (not shown) the Power Train ECM indicating if the park brake is applied. When
hydraulic pressure is present the Power Train ECM determines the park
brake is released.

If the transmission is in FIRST FORWARD or REVERSE when the park

brake is engaged, the Power Train ECM will shift the transmission to
neutral. If the transmission is in SECOND or THIRD, FORWARD, or
REVERSE, when the park brake is engaged the transmission will remain
in the current speed and direction.

The transmission can be shifted to SECOND or THIRD, FORWARD or

REVERSE, when the parking brake is applied for normal service
operations.
STMG 779 - 70 -
2/04

To shift the transmission to FIRST FORWARD or REVERSE while the

park brake is applied, with the directional switch in neutral, the downshift
switch must be pressed until FIRST speed is indicated by the gear
indicator on the dash. The directional switch must then be toggled to the
desired direction, to neutral, and then back to the same desired direction a
second time. The transmission will shift into FIRST gear of the desired
direction when the directional switch is moved to the desired direction the
second time.

This operation can be performed for service procedures requiring FIRST

SPEED or for emergencies requiring the machine to be moved when the
park brake can not be released due to hydraulic failure in the brake circuit.

2. Service tool Also shown is the service tool connecting port (2) for connecting the
connecting port diagnostic tool Electronic Technician. The connecting port gives access
to the Cat Data Link for the machine and engine as well as the ATA Link
for the engine.
STMG 779 - 71 -
2/04

994D WHEEL LOADER

IMPLEMENT HYDRAULIC SYSTEM
Implement Oil Cooling Pump Pilot Control Valve
Pilot Relief Valve
Pilot Pump
Pilot Filter
Implement Pump Case Drain Filters
Implement Pumps
Main Relief Valves
Main Control Valves

Tilt Cylinders

Implement Cooling High Pressure Lift Implement

Oil Coolers Oil Filter Oil Screens Cylinders Oil Tank

Pilot System Main System Cooling System

48

IMPLEMENT HYDRAULIC SYSTEM

The 994D Wheel Loader implement hydraulic system consists of two

basic systems plus a common cooling system.

The color codes for the components in each system are:

Orange -Pilot hydraulic system

Red -Main hydraulic system
Green -Cooling system (common)
STMG 779 - 72 -
2/04

HYDRAULIC SCHEMATIC COLOR CODE

Black - Mechanical connection or Seal Red - High pressure oil

Dark Gray - Cutaway section Red/White Stripes - 1st pressure reduction

Light Gray - Surface color Red Crosshatch - 2nd reduction in pressure

White - Atmosphere or Pink - 3rd reduction in pressure

Air (no pressure)

Purple - Pneumatic pressure Red/Pink Stripes - Secondary source oil pressure

Yellow - Moving or activated components Orange - Pilot, signal, or Torque Converter oil

Cat Yellow - (restricted usage) Orange/White Stripes -

Identification of components Reduced pilot, signal, or TC oil pressure
within a moving group
Orange Crosshatch - 2nd reduction in
Brown - Lubricating oil pilot, signal, or TC oil pressure.

Green - Tank, sump, or return oil Blue - Trapped oil

Green/White Stripes -
Scavenge Oil or Hydraulic Void

49

This illustration identifies the meanings of the colors used in the hydraulic
schematics and cross-sectional views shown in this presentation.
STMG 779 - 73 -
2/04

994D IMPLEMENT PILOT SYSTEM

To Coolant Selector Valves
Circuit

Pilot
Pilot Control
Relief Valves
Check
Valve
Valve Tilt Lift
Pilot
Filter To Lift
Control
Valves

To Tilt
Control
Valves
From
Lift Cylinders
Check
Pilot Selector Valve
Pump And Pressure
Control Valve

Implement
Hydraulic Tank

50

Pilot System

Shown is a block diagram of the pilot system. The pilot system is a

• Tank pressure oil closed center design. Oil (green) is drawn from the tank by the pump.
(green) Pump oil (orange) flows through the filter, past the relief valve and
• Pilot oil (orange) through the inlet (left) check valve to the pilot control valves. The oil is
blocked at the pilot control valves until either the tilt or lift control lever is
moved.

Moving the tilt or lift control lever sends pilot oil to the respective spool
in the main control valve. When system pressure reaches the relief valve
setting, the relief valve opens and allows pump oil to return to the tank.
The pilot system will constantly operate at the relief valve pressure
setting.
STMG 779 - 74 -
2/04

When the engine is running, the pilot pump oil is at a higher pressure than
the oil from the selector and pressure control valve. Pilot oil opens the
inlet check valve in the pilot oil line and seats the check valve (right),
blocking the flow of oil from the lift cylinders to the selector and pressure
control valve.

Pressurized oil from the lift cylinders flows to the selector and pressure
control valve. The selector and pressure control valve reduces the
pressure and makes the low pressure oil available for emergency use in
the pilot system. When the engine is not running, no pump flow is
available. Oil from the selector and pressure control valve flows through
the check valve (right) to the pilot control valves. The inlet check valve
blocks flow to the pilot relief valve, pilot filter and pump.
STMG 779 - 75 -
2/04

3 1

51

1. Implement pilot oil This illustration shows the implement hydraulic pilot oil pump (1) and the
pump pilot oil filter (2). Notice the bypass switch monitored by VIMS on the
2. Implement pilot oil pilot oil filter base.
filter
Also shown is the cooling pump for the hydraulic system (3).
3. Hydraulic system
cooling pump
STMG 779 - 76 -
2/04

2
1

52

1. Pilot relief valve The pilot relief valve (1) is located in the right side of the manifold on the
2. Selector and front frame above the main control valve and to the left of the selector and
pressure control
pressure control valve (2). The pilot relief valve maintains the pilot
valve
3. S•O•S tap pressure at 2400 kPa (350 psi). Pilot system oil samples are taken at the
oil sampling tap (3).
4.-Selector valves
Two selector valves (4) are mounted near the main control valve. The
selector valves provide a path for the pilot oil to circulate when the pilot
control valve is in the hold position. Constantly circulating the pilot oil
through the selector valves helps keep the oil warm during cold weather
conditions.
STMG 779 - 77 -
2/04

Lift Tilt
Raise Tilt Back
Hold Hold
Lower Dump
Float

PILOT CONTROL
To Makeup VALVE
And Vent Valve
LOWER

To Tank

To Lift
Control Valve

From Lift
Control Valve

From
Pump

53

• Explain operation This sectional view shows the lift pilot control spool in the LOWER
position. The lift pilot spool directs pilot oil to the LOWER end of the lift
main control valve spool and allows oil from the RAISE end of the lift
main control valve spool to flow to tank.

The oil pressure in the implement control valve is the same as the pilot
supply pressure.

Movement of the lift pilot control spool is the same as the tilt pilot control
spool.

In the LOWER position, the lift pilot control spool is not pushed into the
valve body far enough to open the passage from the makeup and vent
valve and the oil is blocked.
STMG 779 - 78 -
2/04

Lift Tilt
Raise Tilt Back
Hold Hold
Lower Dump
Float

To Makeup
And Vent Valve
PILOT CONTROL
VALVE
To Tank FLOAT

To Lift
Control Valve

From Lift
Control Valve

From
Pump

54

• Explain operation This sectional view shows that the lift pilot control spool has been moved
farther into the valve body to the FLOAT position. The flow of pilot oil to
the lift control valve is the same as when the control spool was in the
LOWER position. However, in the FLOAT position, the passage from the
makeup valve is open to the tank.

When the passage from the makeup valve is open to the tank, oil behind
the makeup valve flows to the tank. The decrease in pressure behind the
makeup valve allows the makeup valve to open.

When the makeup valve opens, oil normally used to lower the implement
goes through the makeup valve to the tank. The implement is allowed to
move with the contour of the ground. (The main control valve FLOAT
position is discussed in visual No. 60.)
STMG 779 - 79 -
2/04

994D
IMPLEMENT Relief
Valve
HYDRAULIC SYSTEM

Right
Implement
Pump

Center
Implement
Pump Relief
Valve

Left Main Right Main

Control Valve Control Valve
Group Group
Relief
Valve
Left
Implement
Pump

Implement
Hydraulic Tank

55

Main Hydraulic System

The implement hydraulic circuit has three fixed displacement, piston-type

pumps, three main relief valves, two main control valves, two lift
cylinders, and two tilt cylinders.

Oil flow from the three implement pumps operates the lift and tilt
cylinders. Oil flow is metered to the cylinders by the main control valve
spools. The main control valve spools are controlled by the pilot circuit
(not shown).

Oil flow from the right pump enters the top of the right control valve. Oil
flow from the left pump enters the top of the left control valve. Oil flow
from the center pump enters both control valves between the auxiliary and
tilt spools.
STMG 779 - 80 -
2/04

The operator controls the pilot oil flow and pressure that moves the main
control valve spools. The movement of the main valve spools opens
passages for pump oil flow to one end of the tilt and lift cylinders.
Movement of the valve spools also opens a passage for oil in the opposite
end of the cylinders to return to the tank.
STMG 779 - 81 -
2/04

56

1. Main implement The three main implement hydraulic pumps (1) are located on the front
hydraulic pumps pump drive between the lift arms. The pumps are fixed displacement,
piston-type pumps, each with the same output.
2. Case drain filters Three case drain filters (2), one for each pump, are mounted nearby.

The case drain filters are canister type filters with replaceable 177 micron
elements. The case drain filters are additional filtration that has been
added to the 994D.

INSTRUCTOR NOTE: See the 994D Wheel Loader Hydraulic

Systems Operation, Testing and Adjusting module (Form
RENR2513) for the relief valve testing and adjusting procedure.
STMG 779 - 82 -
2/04

4
2
2

57

1. Manifold A manifold (1) with a pressure tap (2) is bolted to the underside of each
2. Pressure tap main implement pump. Each manifold contains a pressure relief valve (3)
and a check valve (4). The check valves isolate each pump and its
3. Relief valve
pressure relief valve from reverse flow from the other pumps. The check
4. Check valve valves allow the pressure relief valves for each pump to be set on the
machine.

The third main implement pump and manifold (not shown) is mounted
between the two pumps shown.
STMG 779 - 83 -
2/04

58

• Implement oil high Three high pressure screens (arrows) are mounted near the implement
pressure screens control valve. Oil flow from each pump passes through a screen before
(arrows)
entering the implement control valve. A bypass sensor is mounted in
each screen base. The VIMS will alert the operator if any of the screens
have a bypass condition during normal operating temperatures.

The high pressure screens are canister type filters with replaceable 200
micron elements. The high pressure screens are new to the 994D.
STMG 779 - 84 -
2/04

1
1

59

1. Makeup and vent This visual shows the back side of the implement hydraulic control valve.
valves Two makeup and vent valves (1) that are connected to the pilot lines (2)
2. Pilot lines from the pilot control valve to allow the implements to FLOAT when the
lift pilot control valve is moved to the FLOAT position.

The operation of the makeup and vent valves are described in the next
illustration.
STMG 779 - 85 -
2/04

From Pump MAKEUP AND VENT VALVE

CUTAWAY
To Pilot Control To Pilot Control
Valve Valve

From Pump

Rod End Rack Dump Head End

From To
Rod End Rod End
Cylinder Cylinder
Rod End
Head End

Lower Lift To Tank From Tank

994D WHEEL LOADER FLOAT MAKEUP

From Rod End To Head End
MAIN CONTROL VALVE Cylinder To Tank Cylinder
FLOAT
To Line Relief
To Makeup And Load Check and
Vent Valve Valve Makeup Valve
Lift Cylinder Lift cylinder
Rod End Head End

LIFT CONTROL VALVE

CUTAWAY

To Tank
through
From Pilot
Pilot
Control Valve
Control Valve

60

Identify components The makeup and vent valve functions as a makeup valve when the
pressure in the cylinder decreases below the pressure in the hydraulic
Explain makeup and
tank.
vent valve functions
When lowering the bucket with the engine OFF or when lowering the
bucket faster than the pump can fill the rod end of the cylinder, the piston
- Engine OFF
displacement causes a vacuum (cavitation) in the rod end of the lift
cylinders. When the oil pressure in the hydraulic tank exceeds the
pressure in the cylinders, the higher tank pressure opens the makeup valve
and tank oil flows into the cylinders.

- FLOAT position The makeup and vent valve functions as a vent valve when the hydraulic
control valve is moved to the FLOAT position. When the lift pilot control
valve is moved to the float position, the oil behind the makeup valve is
opened to the tank. The small orifices in the base of the makeup valve
restrict oil flow to the chamber behind the valve.
STMG 779 - 86 -
2/04

With oil flowing from behind the makeup valve faster than oil flowing in,
the pressure difference between the oil around the makeup valve and the
oil behind the makeup valve increases enough to lift the makeup valve off
its seat. When the makeup valve moves off its seat, the oil from the
implement pump flows past the makeup valve to the tank. Both ends of
the lift cylinders are opened to the tank allowing the bucket to float along
the contour of the ground.
STMG 779 - 87 -
2/04

Pilot Control Valve

994D WHEEL LOADER
Tilt Lift IMPLEMENT HYDRAULIC SYSTEM
BUCKET LOWER
Tilt Lift

Selector And Main Control

Fluid Pressure Main Control Valve Group Valve Group
Sampling Reducing Valve
Pilot Aux. Aux.
Relief Valve
Cooler Bypass
Relief Valve

Main
Relief
Valves Tilt Tilt
Filter Cooling/
Pilot Pump
Main
Pumps
Oil Breaker
Cooler And
Relief Valve
Lift Lift

Cooler
Bypass
Valve

Makeup and
Makeup and
Vent Valve
T T Vent valve

Selector Valves

61

• Explain schematic Implement Hydraulic System Schematics

This schematic shows the hydraulic flow when the control lever is moved
to the LOWER position.

• Pilot oil to left end of When the pilot control valve is in the LOWER position, pilot oil is sent to
control valve spools the left ends of the lift control valve spools. The pilot control valve opens
a passage for the oil in the right end of the control valve spool to flow to
the tank. The pilot oil on the left end of the control valve spools moves
the spools to the right against the centering springs to the LOWER
position.
• Implement pump oil to
rod end of lift The control valve spools open passages for oil flow from the implement
cylinders pumps, through the load check valves, the lift control spools and the rod
end of the lift cylinders to lower the bucket.

The position of the main control valve spool also opens a passage for the
oil in the head end of the lift cylinders to flow to the tank.
STMG 779 - 88 -
2/04

Pilot Control Valve

994D WHEEL LOADER
Tilt Lift IMPLEMENT HYDRAULIC SYSTEM
BUCKET FLOAT
Tilt Lift

Fluid Selector And Main Control

Sampling Pressure Main Control Valve Group Valve Group
Reducing Valve
Pilot Aux. Aux.
Relief Valve

Cooler Bypass
Relief Valve

Main
Relief
Valves Tilt Tilt
Filter Cooling/
Pilot Pump
Main
Pumps
Oil Breaker
Cooler And
Relief Valve
Lift Lift

Cooler
Bypass
Valve

Makeup and
Makeup and
Vent Valve
T T
Vent valve

Selector Valves

62

Explain schematic This schematic shows the hydraulic flow when the control lever is moved
to the FLOAT position.

• Pilot oil to left end of When the pilot control valve is in the FLOAT position, pilot oil is sent to
control valve spools the left ends of the lift control spools which causes the spools to move
against the centering springs to the LOWER position. The control spools
open passages for oil flow from the implement pumps, through the load
check valves, the lift control spools and the rod end of the lift cylinders to
lower the bucket.

• Makeup valves vented Also, when the pilot control valve is in the FLOAT position, oil behind the
rod end makeup valves is vented through the pilot control valve to the
tank. The small orifices in the makeup valves cause a restriction to the
implement pump oil when filling the cavities behind the makeup valves.

• Makeup valves open With oil flowing from behind the makeup valves faster than oil flows in,
the pressure difference between the oil around the makeup valves and the
oil behind the makeup valves becomes high enough to lift the makeup
valves off their seats.
STMG 779 - 89 -
2/04

• Rod and head end of When the makeup valves move off their seats, oil from the implement
lift cylinders open to pumps flows past the makeup valves to the tank. Both ends of the lift
tank
cylinders are open to the tank allowing the bucket to float along the
ground.
STMG 779 - 90 -
2/04

From Pilot
Bypass Relief Valve
Relief Valve

Cooling
Pump

Filter
IMPLEMENT
Oil Cooler OIL COOLING SYSTEM

Cooler Bypass
Valve

63

Identify components Implement Oil Cooling System

- Cooling pump The hydraulic oil cooling system consists of a pump, a bypass relief valve,
- Bypass relief valve two filters, and an oil-to-air cooler with a bypass valve.
- Filter
- Oil-to-air cooling core Hydraulic oil from the cooling system pump joins with oil from the pilot
- Oil-air-cooler relief valve and flows to the bypass relief valve, the filters, and the
coolers. When the oil is cold, the high resistance to flow through the
filters and coolers causes the pressure to increase. The high pressure
opens the bypass relief valve allowing cold oil to bypass the coolers and
flow to the tank. [The bypass relief valve is set to open at 1680 kPa (245
psi).] As the temperature of the oil increases, the resistance to flow
through the filters and the cooler decreases. When the higher temperature
causes pressure to decrease below the bypass relief valve setting, the
bypass relief valve closes. The hot oil flows through the filters to the
cooler. When the hot oil flows through the cooler, air from the cooling
fan removes the heat from the oil. The cooler hydraulic oil returns to the
tank.
STMG 779 - 91 -
2/04

3
5

2 7

1
6

64

• Identify components: Autolube System

1. Autolube control
valve Shown are the components of the automatic lubrication system.
2. Regulator When the autolube control valve (1) solenoid energizes, air from the
3. Vent valve reservoir flows through the autolube control valve and the regulator (2) to
the vent valve (3). Air flow continues from the vent valve to the
4. Pump
pump (4). The pump draws lubricant from the tank and sends pressurized
5. Pressure sensor lubricant to the injector banks (not shown).
6. Lube level sensor
The lube pressure and level is monitored by VIMS by the lube pressure
7. Reservoir sensor (5) and the lube level sensor. Low lube level will signal a Level 3
warning on the VIMS.

When the autolube control valve solenoid de-energizes, the control valve
blocks air flow to the vent valve and pump. The vent valve opens the
passage to the vent hose. The pressurized lubricant seats the check valve
and flows through the vent hose to the reservoir (7).

NOTE: The basic operation of the lube system is the same as the 994.
STMG 779 - 92 -
2/04

65

• Identify components: This illustration shows the bottom of the autolube reservoir (1) as seen
1. Autolube reservoir from standing on the ground near the articulation joint on the left side of
the machine.
2. Removable access
panel The autolube reservoir has been modified. These modifications include:
increased capacity, sloped sides, a removable access panel (2), and a
ground level fill tube (not shown) on the left side of the machine near the
articulation joint.
STMG 779 - 93 -
2/04

994D WHEEL LOADER

STEERING SYSTEM

Steering Steering Steering Compensator High Pressure

Cooling Cooling Hydraulic Valve Group Screens Steering Control Valve
Pump Filter Tank
Pilot Valve

Pressure Reducing Valves

Neutralizer Valves

Steering Steering Case Diverter Secondary Steering

Coolers Pumps Drain Valve Steering Cylinder
Filters Pump

66

• Identify components: STEERING HYDRAULIC SYSTEM

- Steering pilot
Steering System Components
system

- Main steering Shown are the components of the steering hydraulic system on the 994D
system
Wheel Loader. The color codes for the components in the steering
- Steering cooling hydraulic system are:
system

Orange - Steering pilot system

Red - Main steering system

Green - Steering cooling system

STMG 779 - 94 -
2/04

67

• Pilot control valve The pilot control valve (arrow) for the steering system is mounted below
(arrow) the STIC on the left side of the operator's seat. The pilot control valve
directs pilot oil through the neutralizer valves to the ends of the
directional control valve stem in the steering valve body.
STMG 779 - 95 -
2/04

68

• Steering neutralizer Two neutralizer valves (arrows) located on the rear frame near the
valves (arrows) articulation joint block pilot oil to the steering control valve when the
machine is fully articulated.
STMG 779 - 96 -
2/04

1 2
3

69

• Identify components: The steering control valve (1) is located behind the cab on the inside of
1. Steering control the right frame below the deck access panel. The steering control valve
valve houses the steering system relief valve (2) and the directional control
2. Steering system
valve spool (not shown).
relief valve
Pilot oil from the pilot control valve flows to the ends of the control valve
3. Pilot hoses through two pilot hoses (3) to move the directional control valve spool
4. Oil manifold inside the control valve.

The control valve sends system oil to the steering cylinders through the
large manifold (4) mounted to the bottom of the control valve.

The steering control valve also sends signal oil to the margin spool spring
chamber in the compensator valve groups on the pumps.
STMG 779 - 97 -
2/04

70

• Identify components: Steering oil is supplied by the two steering pumps (1) mounted to the
1. Steering pumps front of the rear pump drive. Each pump has a compensator valve (2). A
check valve (3) mounted to each pump output prevents oil from one pump
2. Compensator valve
entering the other pump.
3. Check valve
The two pumps are mounted with 180º rotation from each other. The
pump output from the right pump is on the bottom and the compensator
valve is on the right side of the pump.
STMG 779 - 98 -
2/04

71

• Steering high This view shows the high pressure screens (arrows) for the steering
pressure screens pumps. There is one screen for each pump output. The screens are
mounted on the left side of the rear frame below the access doors.

The high pressure screens are canister type screens with replaceable 200
micron elements.

The high pressure screens have bypass valves that are monitored by
VIMS.
STMG 779 - 99 -
2/04

72

• Identify components: This view shows the case drain filter (1) for the right steering pump
1. Right steering mounted to the right side of the rear frame near the rear tire. The case
pump case drain drain filter (2) for the left steering pump is mounted on the left side of the
filter rear frame near the rear tire.
2. Left steering pump
The case drain filters are canister type filters with replaceable 177 micron
case drain filter
elements.

The case drain filters have bypass valves that are monitored by VIMS.
STMG 779 - 100 -
2/04

Right Turn Diverter Valve

Secondary
Steering Steering
Cylinders Pump
Unloader
Relief
STEERING
Ball Spool
Crossover
Relief
Resolver Valve SYSTEM
Valve Direction Control Spool HOLD
Valve

St eering Back-up
Relief
Cont rol Valve
Valve
Control
Spool
Check Left Pump
Valve Compensator Valve

Quad
Check valve

Right Pump
Compensator Check
Valve Valve
Left Right
Left Pump
Neutralizer Neutralizer
Valve Valve

Steering
Pilot
Valve Right Pump
Steering
Warning
Switch

Pressure Pressure
and and
Selector Selector Steering and
Valve 2 Valve 1 Brake Tank

73

• Identify components Steering Hydraulic System Schematics

and explain function

- Right steering pump When the engine is running and the steering system is in HOLD, pilot oil
from the right pump is blocked at the steering control valve spool. Oil
- Left steering pump
from the left and right steering pumps flows through the respective check
- Steering valve spool valves to the steering control valve. The control valve spool blocks oil
- Steering cylinders flow to the steering cylinders and no signal pressure is generated.

System pressure is sensed at the margin spool, the pressure compensator

- Margin spool
spool, and the small actuator piston (rod end symbol) of each pump . As
- Pressure system pressure increases, the margin spool moves against the spring
compensator spool force and opens a passage for pump oil to flow to the large actuator piston
- Actuator piston (head end symbol). The pressure in the large actuator piston overcomes
the combined force of the actuator spring and the pressure in the small
piston and moves the swashplate to the LOW PRESSURE STANDBY
position.
STMG 779 - 101 -
2/04

In LOW PRESSURE STANDBY, the pump produces enough flow to

compensate for system leakage and sufficient pressure to provide for
instantaneous response when the steering control valve is moved.

Machine pressures are found in the 994D Wheel Loader Steering Systems
Operation, Testing and Adjusting Module (Form RENR2509).

NOTE: In the above schematic, only one actuator piston is shown.

The small actuator piston is represented by the rod end of the
actuator piston and the large actuator piston is represented by the
head end of the actuator piston.
STMG 779 - 102 -
2/04

Right Turn Diverter Valve

Secondary
Steering Steering
Cylinders Pump
Unloader
Relief
STEERING
Ball Spool
Crossover
Relief
Resolver Valve SYSTEM
Valve Direction Control Spool
Valve GRADUAL RIGHT TURN

Back-up
St eering Relief
Cont rol Valve Valve
Control
Spool
Check Left Pump
Valve Compensator Valve

Quad
Check valve

Right Pump
Compensator Check
Valve Valve
Left Right
Neutralizer Left Pump
Neutralizer
Valve Valve

Steering
Pilot
Valve Right Pump
Steering
Warning
Switch

Pressure Pressure
and and
Selector Selector Steering and
Valve 2 Valve 1 Brake Tank

74

Explain schematic When the operator moves the STIC to the right, pilot oil flows through the
pilot control valve and the right neutralizer valve to the right side of the
steering control spool. Pilot oil pressure moves the steering control spool
to the right.

System oil from the steering pumps flows through the check valves and
the control spool orifice to the steering cylinders. As pressure increases in
the steering cylinders, the pressure (signal pressure) is sensed in the
margin valve spring chamber at each pump.

The signal pressure combines with the force of the margin spool spring
and moves the margin spool down. The margin spool restricts the flow of
oil to the large actuator piston (head end). The spring and pressure in the
small actuator piston overcome the pressure in the large piston to move
the swashplate toward maximum angle.
STMG 779 - 103 -
2/04

The increase in swashplate angle increases pump oil flow. The increase
in oil flow through the control spool orifice increases the system pressure.
The system pressure is sensed at the margin spool.

The increased pressure moves the margin spool against the combined
forces of the spring and signal pressure and sends oil to the large actuator
piston. The actuator piston moves the swashplate to a reduced angle that
produces flow relative to the position of the control spool.
STMG 779 - 104 -
2/04

Right Turn Diverter Valve

Secondary
Steering Steering
Cylinders Pump

Unloader
Relief
STEERING
Ball Spool
Crossover
Relief
Resolver Valve SYSTEM
Valve Direction Control Spool FULL RIGHT TURN
Valve

Back-up
St eering Relief
Cont rol Valve Valve
Control
Spool
Check Left Pump
Valve Compensator Valve

Quad
Check valve

Right Pump
Compensator Check
Valve Valve
Left Right
Neutralizer Left Pump
Neutralizer
Valve Valve

Steering
Pilot
Valve
Right Pump
Steering
Warning
Switch

Pressure Pressure
and and
Selector Selector Steering and
Valve 2 Valve 1 Brake Tank

75

Explain FULL RIGHT When making a FULL RIGHT TURN, the right striker (not shown)
TURN function contacts the right neutralizer valve. Oil flow from the pilot control valve
to the steering control valve is blocked by the movement of the neutralizer
valve.

The steering control spool returns to the center position. Flow to the
steering cylinders is blocked and the machine stops turning. The steering
pumps return to the LOW PRESSURE STANDBY position.

The neutralizer valves prevent the machine front frame from contacting
the machine rear frame when turning FULL RIGHT or FULL LEFT.
Refer to the Service Manual for the correct adjustments.
STMG 779 - 105 -
2/04

Diverter Valve
Secondary
Steering
Steering
Cylinders
Pump
Unloader
Relief
STEERING
Ball
Crossover
Relief
Resolver
Spool
Valve SYSTEM
Valve Direction Control Spool SECONDARY STEERING
Valve

Back-up
St eering Relief
Cont rol Valve Valve
Control
Spool
Check Left Pump
Valve Compensator Valve

Quad
Check valve

Right Pump
Compensator Check
Valve Valve
Left Right
Neutralizer Left Pump
Neutralizer
Valve Valve

Steering
Pilot
Valve Right Pump
Steering
Warning
Switch

Pressure Pressure
and and
Selector Selector Steering and
Valve 2 Valve 1 Brake Tank

76

• Explain schematic This schematic shows the 994D steering system when the Secondary
Steering System is active. The bi-directional secondary steering pump is
splined to the output transfer gears and turns whenever the machine is
rolling.

- Diverter valve The diverter valve directs oil from the tank to the input side of the pump
directs oil and the oil from the output side of the pump to the main steering system
depending on if the machine is rolling FORWARD or REVERSE. The
diverter valve also sends oil from the secondary steering pump to tank if
the main steering system is pressurized.

- Secondary steering The secondary steering relief valve limits the maximum pressure in the
relief valve limits secondary steering system. The unloader spool senses the pressure in the
maximum pressure primary steering system from the pressure and selector valve 1.

The machine is rolling and the main steering pumps are not providing
flow with the engine off. The main steering pump output is shown
blocked by thecheck valves but bleeds down to essentially zero pressure.
STMG 779 - 106 -
2/04

- Unloader spool Spring force moves the unloader spool up and blocks the secondary
moves up steering pump flow. The pressure unseats the check valve and directs oil
to the main steering system. The steering system uses flow from the
secondary system to function as normal.

- Main pump output The check valves on the output of the main steering pumps seat and block
check valves seat the oil from entering the pumps.

The steering warning switch senses the main steering system pressure
after the selector and pressure control valve. The steering warning switch
is monitored by VIMS. When the main system pressure drops, the switch
opens. The VIMS alerts the operator with a Level 3 warning that the
main steering system pressure is low.
STMG 779 - 107 -
2/04

Oil Cooler

Cooler Bypass
Valve

Filters Filter
Bypass
Switch
Filter Bypass 994D
Valve STEERING AND BRAKE
OIL COOLING SYSTEM
Fluid Sampling
Valve
Breather
Steering and Brake
Oil cooler Pump

Steering and
Filter Brake Tank

77

• Identify components STEERING AND BRAKE OIL COOLING SYSTEM

and explain function

- Gear pump Shown is a block diagram of the steering and brake hydraulic oil cooling
system.
- Hydraulic tank

- Fluid sampling valve The gear pump draws oil from the steering and brake hydraulic tank.
Pump oil flows past the fluid sampling valve, through the filter, through
- Filter
the three oil cooler cores, and back to the steering and brake hydraulic
- Oil cooler core tank.
- Cooler bypass relief
The cooler bypass valve allows pump oil to bypass the coolers at machine
valve
start-up or when the oil is cold. The cooler bypass valve is set to open at
approximately 345 kPa (50 psi).
STMG 779 - 108 -
2/04

994D
BRAKE SYSTEM Left Rear Left Front
ENGINE OFF Service Brake Service Brake

Brake Pressure
Switch

Parking
Brake Brake Brake
Accumulators Pedals

Service Right Rear Right Front

Brake Service Brake Service Brake
Valve

Pressure
Compensator
Valve
Parking Brake Oil
Brake Cooler Core
Valve Group
Variable Brake
Displacement Cooling
Piston Pump Breather
Pump
Pump
Actuator

Breather
Brake Cooling
Oil Tank

Steering and
Brake Oil Tank

78

• Identify components BRAKE SYSTEM

• Service brake system Brake System Schematic

- Steering and brake
oil tank Shown is a schematic of the service brake system, the parking brake
system, and the brake cooling system.
- Pump

- Brake accumulators
Components of the service brake system are the steering and brake oil
tank, the variable displacement pump, the brake accumulators, the right
- Right and left brake
and left brake pedals, the service brake valve, and the four service brakes.
pedals

- Service brake valve The screens and check valves for the brake oil cooling circuit are new for
the 994D.
- Service brakes
STMG 779 - 109 -
2/04

• Parking brake system Components of the parking brake system are the steering and brake oil
- Tank tank, the variable displacement pump, the brake accumulators, the parking
- Pump
brake valve, and the parking brake.
- Brake accumulators
- Parking brake valve Components of the brake cooling system are the brake cooling oil tank,
- Parking brake
the brake cooling pump, the brake oil cooler core group, and the four
• Brake cooling system
service brakes.
- Tank
- Pump
- Cooler core group
- Service brakes
STMG 779 - 110 -
2/04

79

• Identify components Brake Component Locations

and explain function

1. Service brake valve The service brake valve (1) is bolted to the inside of the left frame below
the access panels. When either the left or right service brake pedal is
2. Parking brake
valve depressed, the service brake valve sends oil to engage the wheel brakes.

The parking brake valve (2) is bolted to the lower right corner of the
brake panel. When the parking brake control lever is moved to the
BRAKE OFF position, the parking brake valve directs oil to release the
parking brake.
STMG 779 - 111 -
2/04

2
2

80

1. Front service brake The screens (1) for the front service brakes are mounted to the front axle
cooling screens housing. Each screen has a check valve (2) to prevent oil from flowing in
2. Check valves the reverse direction.

The brake cooling screens are canister type screens with replaceable 500
micron elements.
STMG 779 - 112 -
2/04

81

• Right rear screen for This view shows the screen (arrow) for the right rear wheel brake cooling
brake cooling circuit circuit. It is mounted to the axle housing between the axle housing and
(arrow)
the trunion.

The rear brake cooling screens are also equipped with check valves (not
shown) to prevent the oil from flowing in the wrong direction.

The rear brake cooling screens are also canister type screens with
replaceable 500 micron elements.
STMG 779 - 113 -
2/04

82

• Left rear screen for This view shows the screen (arrow) for the rear left wheel brake cooling
brake cooling circuit circuit. It is mounted to the axle housing between the axle housing and
(arrow)
the trunion.
STMG 779 - 114 -
2/04

83

CONCLUSION

This concludes this presentation on the 994D wheel loader.

This presentation described the location of the basic components on the

engine, the operation of the power train, the implement, the steering, and
the brake systems for the 994D Wheel Loader.
STMG 779 - 115 -
2/04

ILLUSTRATION LIST
1. Introduction 42. Lockup clutch enable switch
2. Similarities and differences 43. Reduced rimpull selection switch
3. Additional filtration summary 44. Impeller clutch solenoid valve
4. Component location 45. Lockup clutch enable switch
5. Front pump drive components (illustration) 46. Transmission solenoid valves
6. Rear pump drive components (illustration) 47. Park brake knob
7. Front pump drive filter 48 Implement hydraulic system components
8. Engine left side 49. Hydraulic system color code
9. Engine right side 50. Implement pilot system basic schematic
10. Primary fuel filter 51. Implement pilot pump
11. Engine electrical block diagram 52. Pilot relief valve
12. Throttle lock set and resume switches 53. Pilot control valve - LOWER (cutaway)
13. Ground level shutdown switch 54. Pilot control valve - FLOAT (cutaway)
14. Engine derates (illustration) 55. Implement hydraulic system basic schematic
15. Cooling system block diagram 56. Main implement hydraulic pumps
16. Engine and SCAC radiators 57. Main implement hydraulic relief valves
17. Hydraulic, steering, and brake oil coolers 58. Main implement pump case drain filters
18. Radiator expansion tanks 59. Implement makeup and vent valves
19. Power train components (illustration) 60. Implement control valve - FLOAT
20. Torque converter (cutaway)
21. Impeller clutch torque converter (cutaway) 61. Implement hydraulic system schematic -
22. Transmission BUCKET LOWER
23. Transmission hydraulic schematic 62. Implement hydraulic system schematic -
24. Transmission pump BUCKET FLOAT
25. Power train filters 63. Implement oil cooling system schematic
26. Priority valve 64. Auto lube system pump
27. Impeller clutch solenoid valve (cutaway) 65. Auto lube system reservoir
28. Lockup clutch solenoid valve (cutaway) 66. Steering hydraulic system components
29. Torque converter outlet relief valve 67. Steering pilot control valve
30. Transmission hydraulic control valve 68. Steering neutralizer valves
31. Transmission control valve (illustration) 69. Steering control valve
32. Transmission schem. - FIRST FORWARD 70. Steering pumps
33. Transmission schem. - SPEED SHIFT 71. Steering pumps high pressure screens
34. Transmission schem. - DIRECT DRIVE 72. Steering pumps case drain filters
35. Transmission schem. - DIRECTION SHIFT 73. Steering hydraulic system schematic -
36. Power train electrical system block diagram HOLD
37. Power Train ECM 74. Steering hydraulic system schematic -
38. STIC GRADUAL RIGHT TURN
39. Gear indicator lamps 75. Steering hydraulic system schematic -
40. Transmission lock lever FULL RIGHT TURN
41. Key start switch
STMG 779 - 116 -
2/04

SLIDE LIST
76. Steering hydraulic system schematic -
SECONDARY STEERING
77. Steering and brake oil cooling system
schematic
78. Brake system schematic - ENGINE OFF
79. Steering control valve
80. Front service brake cooling circuit high
pressure screens
81. Right rear service brake cooling circuit high
pressure screen
82. Left rear service brake cooling circuit high
pressure screen
83. Conclusion
 2/04
STMG 779

ADDITIONAL FILTRATION SUMMARY

SYSTEM LOCATION QTY MICRON DESCRIPTION
Implement System
Near Control Valve Canister with
High Pressure Screens 3 200
on NEEF Element
Near Main Implement Canister with
Case Drain filters 3 177
Pumps Element
Steering System
- 117 -

Under Left Side of Canister with

High Pressure Screens 2 200
Platform Element
Left and Right Side of Canister with
Case Drain filters 2 177
EEF Element
Brake Cooling system
Canister with
Front Brake Oil Screens Back Side of Front Axle 2 500
Element
Canister with
Rear Brake Oil Screens Front Side of Rear Axle 2 500
Element
Other
Front Pump Drive Lube Filter Top Side of NEEF 1 6 Spin on Filter
Handout No. 1
 2/04
STMG 779

994D WHEEL LOADER

COMPONENT LOCATION
Auxiliary
Rear Input Drive Input
Pump Torque Drive Transfer Transmission
Shaft
Drive Converter Shaft Gear
Front
Pump
Drive Output
Transfer
Gear
- 118 -

3516B
Engine

Spring Final Transmission Secondary Parking Drive Final

Coupling Drive Pump Steer Pump Brake Shaft Drive
Engine Power Train Hydraulics
Handout No. 2
 2/04
STMG 779

Front
of
Machine 1 1 1

994D
Front Pump Drive
- 119 -

2 4
3

1 Main Implement Hydraulic Pumps

2 Implement Oil Cooling Pump
3 Implement Pilot Oil Pump
4 Front Pump Drive Lubrication Pump
Handout No. 3
 2/04
STMG 779

Front
of
1
Machine

2 2

994D
- 120 -

Rear Pump Drive

3 4

1 Service Brake Cooling Pump

2 Steering Hydraulic Oil Pumps
3 Steering And Brake Oil Cooling Pump
4 Brake Application Oil Pump
Handout No. 4
 2/04
STMG 779

994D BASIC ENGINE BLOCK DIAGRAM

J2
16 Electronic Engine
J1 ECM
Unit Injectors
Ground Bolt
Jacket Water
Temperature Sensor
Speed / Timing
Sensor Aftercooler Coolant
Temperature Sensor
Machine Interface
Connector Permanent Timing
Calibration Sensor
Machine Interface
Connector Crankcase
Pressure Sensor
- 121 -

Oil Level Add Atmospheric

Switch Pressure Sensor

Coolant Flow Turbocharger Outlet

Switch Pressure Sensor

Filtered Oil
Left Exhaust Pressure Sensor
Temperature Sensor
Unfiltered Oil
Right Exhaust Pressure Sensor
Temperature Sensor
Left Turbocharger
Inlet Pressure Sensor
Cooling Fan
Speed Sensor
Right Turbocharger
Inlet Pressure Sensor
Cooling Fan
Proportional Valve Air Conditioning
ON Switch
Handout No. 5
 2/04
STMG 779

994D ENGINE COOLING SYSTEM

ADVANCED MODULAR COOLING SYSTEM (AMOCS)

Separate Circuit Engine Coolant

Aftercooler (SCAC) Radiator
Radiator

Regulator
Housing
Radiator
Aux. Coolant Bypass
Direction of Pump Direction of
Air Flow Main Air Flow
Coolant
Pump Brake
- 122 -

Oil Cooler
Aftercoolers

Hottest
Engine
Oil
Increasing Cooler
Coolant
Temperature Power Train
Oil Cooler
Coldest

Hot SCAC Coolant

Handout No. 6
 2/04
STMG 779

994D POWER TRAIN

SCHEMATIC Priority
Valve

Transmission
Control Valve
Lube Input
Transfer
T/C Trans.
LUC Solenoid IC Solenoid Filter Filter
- 123 -

Lockup Impeller Transmission

Clutch Clutch
PUMP
Torque
Converter

Output Transfer Gear

T/C Outlet
Relief Valve
Lube Output
Lube Rear Lube Output
Transfer
Pump Drive Transfer Gear
Bearings
Cooler
Handout No. 7
 2/04
STMG 779

POWER TRAIN Cat Data Link Control and Monitor

Power
ELECTRICAL SYSTEM Systems
Train
ECM
INPUT COMPONENTS OUTPUT COMPONENTS

STIC
Air Start
Upshift, Downshift,
Solenoid
Forward, Neutral,
Reverse
Reduced Rimpull
Ignition Keyswitch Indicator Lamp
Reduced Rimpull
Clutch 1
Enable Switch
Reverse Solenoid
Reduced Rimpull
Selection Switch Clutch 2
Forward Solenoid
Torque Converter Oil
Outlet Temperature Clutch 3
- 124 -

3rd Gear Solenoid

Parking Brake
Pressure Switch
Clutch 4
Lockup Clutch 2nd Gear Solenoid
Enable Lamp
Lockup Clutch Clutch 5
Enable Switch 1st Gear Solenoid
Steering/Transmission
Impeller Clutch
Lock Switch
Solenoid
Torque Converter Lockup Clutch
Pedal Position Sensor Solenoid
Torque Converter
Output Speed Sensor Gear Indicator
Transmission Output
Speed Sensor
Impeller Clutch Back-up Alarm
Pressure Sensor Relay
Handout No. 8
 2/04
STMG 779

HYDRAULIC SCHEMATIC COLOR CODE

Black - Mechanical connection or Seal Red - High pressure oil

Dark Gray - Cutaway section Red/White Stripes - 1st pressure reduction

Light Gray - Surface color Red Crosshatch - 2nd reduction in pressure

White - Atmosphere or Pink - 3rd reduction in pressure

Air (no pressure)
- 125 -

Purple - Pneumatic pressure Red/Pink Stripes - Secondary source oil pressure

Yellow - Moving or activated components Orange - Pilot, signal, or Torque Converter oil

Cat Yellow - (restricted usage) Orange/White Stripes -

Identification of components Reduced pilot, signal, or TC oil pressure
within a moving group
Orange Crosshatch - 2nd reduction in
Brown - Lubricating oil pilot, signal, or TC oil pressure.

Green - Tank, sump, or return oil Blue - Trapped oil

Green/White Stripes -
Scavenge Oil or Hydraulic Void
Handout No. 9
 2/04
STMG 779

From Pump MAKEUP AND VENT VALVE

CUTAWAY
To Pilot Control To Pilot Control
Valve Valve

From Pump

Rod End Rack Dump Head End

From To
Rod End Rod End
Cylinder Cylinder
Rod End
Head End

Lower Lift To Tank From Tank

- 126 -

FLOAT MAKEUP
994D WHEEL LOADER
From Rod End To Head End
MAIN CONTROL VALVE Cylinder To Tank Cylinder
FLOAT
To Line Relief
To Makeup And Load Check and
Vent Valve Valve Makeup Valve
Lift Cylinder Lift cylinder
Rod End Head End

LIFT CONTROL VALVE

CUTAWAY

To Tank
through
From Pilot
Pilot
Control Valve
Control Valve
Handout No. 10
STMG 779 - 127 -
2/04

INSTRUCTOR NOTES