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Presentacion de Cargador 924g.

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0% found this document useful (0 votes)

69 views81 pages

Presentacion de Cargador 924g.

Uploaded by

Cristian Neira

We take content rights seriously. If you suspect this is your content, claim it here.

Available Formats

Download as PDF, TXT or read online on Scribd

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Service Training SERV1767

Meeting Guide January 2003

TECHNICAL PRESENTATION

924G AND 924Gz WHEEL LOADERS

STEERING, BRAKE, FAN, AND IMPLEMENT
HYDRAULIC SYSTEMS
924G AND 924Gz WHEEL LOADERS
STEERING, BRAKE, FAN, AND IMPLEMENT
HYDRAULIC SYSTEMS
MEETING GUIDE 767 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 operation of the updated steering, brake, fan, and implement
systems for the 924G and 924Gz Wheel Loaders.

OBJECTIVES
After learning the information in this presentation, the technician will be able to:
1. identify the major components in the steering, brake, fan, and implement systems;
2. explain the operation of each component in steering, brake, fan, and implement systems;
and
3. trace the flow of oil through the steering, brake, fan, and implement systems.

REFERENCES
924G/924Gz/928G and IT28G Braking and Hydraulic Fan System, SOTA RENR6428
924G and 92Gz Hydraulic System Testing and Adjusting RENR6431
STMG 717 "924G/924Gz Wheel Loaders--Introduction" SESV1717
CD ROM version of STMG 717 SERV1717

PREREQUISITES
Interactive Video Course "Fundamentals of Mobile Hydraulics” TEMV9001
STMG 546 "Graphic Fluid Power Symbols” SESV1546

Estimated Time: 2 Hours

Visuals: 60
Handouts: 2
Form: SESV1767
Date: 01/03
© 2003 Caterpillar Inc.
STMG 767 -3-
1/03

TABLE OF CONTENTS

924G AND 924Gz UPDATES......................................................................................................5

IMPLEMENT HYDRAULIC SYSTEMS ...................................................................................6

THE IMPLEMENT PUMP ..........................................................................................................9

Implement Pump Operation..................................................................................................10

PILOT VALVE OPERATION ....................................................................................................18

IMPLEMENT CONTROL VALVE OPERATION ....................................................................21

924G IMPLEMENT CONTROL VALVE OPERATION ..........................................................28

924G Tilt Valve.....................................................................................................................28
924G Lift Valve ....................................................................................................................35
Auxiliary Valves ...................................................................................................................42
Additional Component Operation.........................................................................................46

924Gz IMPLEMENT HYDRAULIC SYSTEM ........................................................................49

924Gz IMPLEMENT CONTROL VALVE OPERATION.........................................................50

924Gz Tilt Valve ...................................................................................................................50
924Gz Lift Valve...................................................................................................................57

IMPLEMENT HYDRAULIC SYSTEM TESTING AND ADJUSTING ................................65

STEERING, BRAKE, AND HYDRAULIC FAN SYSTEMS ..................................................66

Steering System ....................................................................................................................67
Brake System ........................................................................................................................70
Hydraulic Fan System...........................................................................................................74

CONCLUSION...........................................................................................................................77

SLIDE LISTS .............................................................................................................................78

HANDOUTS...............................................................................................................................79
STMG 767 -4-
1/03

NOTES
STMG 767 -5-
1/03

924G AND 924Gz WHEEL LOADERS

STEERING, BRAKE, FAN, AND IMPLEMENT
HYDRAULIC SYSTEMS

© 2002 Caterpillar Inc.

924G AND 924Gz UPDATES

In early 2002 a different main control valve group was put into production on the 924G and
924Gz Wheel Loaders. Operation of the valve group is similar to the valve group operation for
the M300 Wheel Excavators. The system is a variation of Caterpillar's Proportional Priority
Pressure Compensated (PPPC) Hydraulic System, which the suppliers describes as "Load
Pressure - Independent Flow Distribution." This type of system is currently not used on any
other Caterpillar wheel loader.

In late 2002, these models had additional changes made to their fan, steering, and brake
systems.

This presentation will provide update information on changes to these systems.

NOTE: The cutaway graphics are only representative illustrations of the actual valves
and may not show all of the component detail of the production valves.

For color codes used in this presentation refer to Handout No. 1.

STMG 767 -6-
1/03

FROM
STEERING
RIDE 924G IMPLEMENT
QUICK
COUPLER
CONTROL
COUPLER
CONTROL
GROUP HYDRAULIC SYSTEM
VALVE

TILT LIFT
RETURN CYLINDER CYLINDERS
COMBINATION THIRD FOURTH
RESTRICTOR COMBINATION
VALVE FUNCTION FUNCTION
TO VALVE
STEERING MANIFOLD

SIGNAL
LIMITER

LIFT AUXILIARY
TILT AUXILIARY
CONTROL CONTROL
CONTROL CONTROL
VALVE VALVE
VALVE VALVE
SIGNAL
DRAIN
VALVE

TANK

PRESSURE
REDUCING
VALVE

TILT
HYDRAULIC WIRED TO PILOT
LOCKOUT CONTROLLER VALVE
VALVE PILOT CONTROL VALVES

IMPLEMENT HYDRAULIC SYSTEMS

The 924G and 924Gz implement hydraulic systems are equipped with a Load Sensing (LS)
variable displacement pump to provide flow and to maintain system pressure at a fixed value
above the highest work port pressure. The 924G hydraulic system is shown.

A signal limiter is used to control the maximum work port pressure and works with the pump
control valve to limit the maximum system pressure. A signal limiter is required for a PPPC
system to ensure that additional implements can be operated when one circuit is stalled.

A separate Load Sensing (LS) variable displacement pump is used to provide pilot oil, supply
oil to operate the quick coupler, and supply oil for steering.

The four closed-center control valves are controlled by pilot valves. The control valves are
used to control the bucket tilt, loader lift arms, and two auxiliary circuits. All machines may
not be equipped with both auxiliary circuits.

STMG 767 -7-
1/03

Ride Control is an option. Ride Control improves machine roading by cushioning the load.

The coupler circuit is used to quickly attach work tools to the machine on the 924G.

A return restrictor creates a back pressure on the return oil to assist in preventing overrunning
conditions and cylinder cavitation.

The combination valve for the tilt circuit on the 924G provides a head end line relief function
for back dragging.

The hydraulic (pilot) lockout valve blocks the flow of pilot oil to the pilot valves.

The signal drain valve provides a means for the signal oil to flow to the tank when all circuits
are returned to HOLD.
STMG 767 -8-
1/03

FROM QUICK
STEERING COUPLER

RIDE
CONTROL
924G IMPLEMENT
COUPLER
CONTROL
GROUP
COMBINATION
HYDRAULIC SYSTEM
VALVE VALVE
HOLD
TILT
CYLINDER
RETURN LIFT
MANIFOLD RESTRICTOR CYLINDERS THIRD FOURTH
FUNCTION FUNCTION
TO
STEERING DEAD ENGINE
LOWER VALVE PRESSURE
SIGNAL COMPENSATOR
LIMITER

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN MAKEUP
TANK VALVE VALVE
PRESSURE
REDUCING
VALVE REGEN. BACK PRESSURE
LINE RELIEF
VALVE CHECK VALVE
AND MAKEUP
VALVES

SHOCK
REDUCING
VALVE

TILT LIFT AUX AUX

HYDRAULIC WIRED TO PILOT PILOT PILOT PILOT
LOCKOUT CONTROLLER VALVE VALVE VALVE VALVE
VALVE

A throttle pin in the signal line is used to control the stroking speed of the pump.

Some of the pilot valves are equipped with solenoid detents which will hold the pilot levers in a
detent position, such as BOOM FLOAT. The valves also have travel limiters to limit maximum
spool travel.

The implement control valves may be equipped with shock reducing valves. The shock
reducing valve returns the control spool to HOLD gradually to prevent high pressure surges in
the pump supply circuit.

The lift circuit features a regeneration valve. The regeneration valve is used to take return oil
from the rod end of the cylinder and direct it to the head end to prevent cylinder cavitation.

A back pressure check valve is also used to prevent cylinder cavitation for the lift circuit and
the auxiliary valves.

A pilot accumulator provides some assistance in lowering the implements with a dead engine.
A manual dead engine lowering valve can also be used to lower the loader arms.
STMG 767 -9-
1/03

IMPLEMENT PUMP
PUMP
CONSTANT FLOW CONTROL
VALVE SIGNAL
PRESSURE
ACTUATOR MARGIN
PISTON SPRING
BIAS
SPRING

ORIFICES

POPPET SIGNAL
LIMITER

MARGIN
PRESSURE SPOOL
TO CONTROL
SWASHPLATE
VALVE GROUP CUTOFF SPOOL

THE IMPLEMENT PUMP

The implement pump control valve contains two spools. The margin spool regulates output
flow of the pump to keep the pump supply pressure at a fixed value above the signal pressure.
The difference between the supply pressure and signal pressure is called "margin pressure."
The pressure cutoff spool limits the maximum system pressure and serves as a back-up relief
valve for PPPC hydraulic systems. The signal limiter and margin spring control the maximum
system pressure in most instances.

The pump is designed to maintain flow. Whenever the forces above and below the margin
spool are not balanced due to changes in the flow demand, the pump will upstroke or destroke
to meet the flow demand.

When the pump supply pressure equals the sum of the signal pressure plus the margin spring
value, the margin spool moves to a metering position to control oil to and from the actuator.
This action stabilizes the system. The swashplate is held at a relatively constant angle to
maintain the required flow. This is called "CONSTANT FLOW."

The pump control valve has a stability orifice in the passage to the actuator piston. The orifice
is used to regulate the response rate of the actuator piston by creating a constant leakage path to
drain.
STMG 767 - 10 -
1/03

IMPLEMENT PUMP
ENGINE OFF

ACTUATOR NO SIGNAL
PISTON PRESSURE
BIAS
SPRING

MARGIN
ORIFICES
SPRING

SHAFT

MARGIN
PISTON AND TO CONTROL PRESSURE SPOOL
SWASHPLATE BARREL ASSEMBLY VALVE GROUP CUTOFF SPOOL

Implement Pump Operation

When the engine is OFF, no signal pressure is sent to the pump control valve. The margin
spring pushes the margin spool down.

Any pressure behind the actuator piston goes to case drain across the margin spool. With no
pressure behind the actuator piston, the bias spring in the pump holds the swashplate at
maximum angle.
STMG 767 - 11 -
1/03

IMPLEMENT PUMP
LOW PRESSURE STANDBY

ACTUATOR NO SIGNAL
PISTON PRESSURE
PASSAGE
BIAS
SPRING

ORIFICES
MARGIN
SPRING

MARGIN
SWASHPLATE TO CONTROL PRESSURE SPOOL
VALVE GROUP CUTOFF SPOOL

When the engine is started, the pump drive shaft begins rotating. Tank oil is drawn into the
piston bore from the pump inlet. As the pistons and barrel assembly rotate, the oil is forced
from the pump outlet into the hydraulic system.

The hydraulic system pressure begins to increase because the flow is blocked at the implement
control valve group. The increased pressure is felt below the margin spool. The margin spool
moves up against the margin spring and permits some system output oil (red and white stripes)
to fill the chamber behind the actuator piston.

The pressure behind the actuator piston increases, overcomes the force of the bias spring, and
moves the swashplate to a minimum angle. When the passage in the actuator opens to the
pump case, the actuator piston travel stops.

STMG 767 - 12 -
1/03

At this minimum angle, the pump produces enough flow to compensate for system leakage and
sufficient pressure to provide instantaneous response when a control lever is activated. With no
flow demand from a circuit, no signal pressure is generated. The pump output pressure has to
overcome only the margin spring value. This condition is called "LOW PRESSURE
STANDBY."

In this system, LOW PRESSURE STANDBY is higher than margin pressure. This
characteristic is due to a higher back pressure created by the oil which is blocked in the
hydraulic system. During LOW PRESSURE STANDBY, the pump output oil pushes the
margin spool farther to the right and compresses the margin spring. More supply oil now goes
to the actuator piston and slightly destrokes the pump.

NOTE: Depending on the adjustments made to the margin spool and the amount of
pump leakage, low pressure standby and margin pressure can be equal. Margin pressure
can never be higher than low pressure standby. The specification for low pressure
standby and margin pressure are almost the same on the 924G/924Gz.
STMG 767 - 13 -
1/03

IMPLEMENT PUMP
UPSTROKE

ACTUATOR
PISTON SIGNAL
PRESSURE
BIAS
SPRING

ORIFICES
MARGIN
SPRING

MARGIN
TO CONTROL PRESSURE SPOOL
SWASHPLATE VALVE GROUP CUTOFF SPOOL

The following conditions can result in UPSTROKING the pump:

- a circuit is activated when the system is at LOW PRESSURE STANDBY;

- an additional circuit is activated;

- a control lever is moved for additional flow; or

- engine rpm decreases.

When a circuit is activated from LOW PRESSURE STANDBY, the signal pressure plus the
margin spring force above the margin spool becomes greater than the pump output pressure
below the spool.

STMG 767 - 14 -
1/03

The greater force (margin spring plus signal pressure) moves the spool down, blocking the flow
of oil to the actuator piston. The oil (green) behind the actuator piston is vented to case drain
across the margin spool.
The pressure behind the actuator piston is reduced or eliminated which allows the bias spring to
move the swashplate to an increased angle. The pump will now produce more flow. This
condition is called "UPSTROKING."
The signal does not have to increase for the pump to upstroke. An increase in flow demand is
what causes the pump to upstroke. If the pump output pressure below the spool becomes less
than the signal pressure and margin spring force above the spool due to activation of another
circuit or reduced engine rpm, the pump will also UPSTROKE.
STMG 767 - 15 -
1/03

IMPLEMENT PUMP
CONSTANT FLOW

ACTUATOR
SIGNAL
PISTON
PRESSURE
BIAS
SPRING

ORIFICES MARGIN
SPRING

MARGIN
TO CONTROL PRESSURE SPOOL
SWASHPLATE
VALVE GROUP CUTOFF SPOOL

As the pump output flow increases (upstroke condition) or decreases (destroke condition) to
meet the system demand, the forces acting above and below the margin spool will equalize and
the margin spool will move to a metering position. The system stabilizes. The swashplate is
held at a relatively constant angle to maintain the required flow.

The difference between the signal pressure and the pump supply pressure is "margin pressure."
Margin pressure is the value of the margin spring.

The margin pressure and standby pressure is adjusted by turning the margin spool adjustment
screw.
STMG 767 - 16 -
1/03

IMPLEMENT PUMP
DESTROKE

ACTUATOR SIGNAL
PISTON PRESSURE
BIAS
SPRING

ORIFICES
MARGIN
SPRING

MARGIN
SWASHPLATE TO CONTROL PRESSURE SPOOL
VALVE GROUP CUTOFF SPOOL

The following conditions can result in destroking the pump:

- all the control levers are moved to the HOLD position and the pump returns to low
pressure standby;

- a control lever is moved to reduce flow;

- an additional circuit is deactivated; or

- engine rpm increases.

When less flow is needed, the pump destrokes. The pump destrokes when the force below the
margin spool becomes greater than the force above the spool. The margin spool moves up and
allows more output pressure (red) behind the actuator piston.

Pressure behind the actuator piston is now increased. The increased pressure overcomes the
force of the bias spring and moves the swashplate to a reduced angle. When the new pump
output pressure matches the force below the margin spool, the spool returns to a metering
position. The pump again maintains a constant flow. The signal pressure does not have to
decrease for the pump to destroke.
STMG 767 - 17 -
1/03

IMPLEMENT PUMP
MAXIMUM SYSTEM PRESSURE
SIGNAL
PRESSURE
MARGIN
SPRING
ACTUATOR
PISTON
BIAS
SPRING

ORIFICES

SIGNAL
POPPET
LIMITER

MARGIN
SPOOL
SWASHPLATE TO CONTROL PRESSURE
VALVE GROUP CUTOFF SPOOL

The signal limiter valve limits the maximum load sensing signal pressure. The signal limiter
valve works with the margin spring to control the maximum system pressure.

The pressure cutoff serves as a backup to the signal limiter and margin spring. The pressure
cutoff is set above the combined spring settings of the signal limiter and the margin spring.

When a circuit is stalled, the signal limiter opens to limit the maximum system pressure.

Initially, the combined forces of the signal pressure and margin spring are less than pump
output pressure. The supply pressure moves the margin spool up to destroke the pump. Once
the pump destrokes to provide very little flow at a high pressure, the margin spool moves to a
metering position to maintain the desired minimum flow rate. In the metering position the
margin spring and the signal limiter equal the pump output pressure.

If a second circuit is activated while another circuit is stalled, the pump will upstroke to meet
the new flow requirements.

NOTE: Without a signal limiter in a PPPC system, if a single circuit is stalled, no other
circuit will work due to the implement valve pressure compensators sensing the same high
pressure signal.
STMG 767 - 18 -
1/03

LEVER

PILOT VALVE DETENT PLUNGER

HOLD
PLATE
ADJUSTMENT PIN

RETAINER PLUNGER

DETENT COIL

SPOOL

ORIFICE
PILOT SUPPLY

FROM TO TO
CONTROL TANK CONTROL
VALVE VALVE

PILOT VALVE OPERATION

In HOLD, springs hold the plungers and spools up. Pilot supply oil is blocked by the spools.
The pilot lines from the control valve are open to the tank around each spool.

Some of the circuits may use detent coils to hold the lever for certain functions such as FLOAT
or for the bucket kickouts.

The detent plungers provide feedback to the operator, as to starting to enter a detent coil
position.

The adjustment pin is used to adjust the point at which the plunger contact begins when the
lever is shifted.

The orifice in the lower end of the spool dampens the downward spool travel when the pilot
valve is shifted. Oil trapped in the spring chamber is forced up through the orifice as the spool
moves down.
STMG 767 - 19 -
1/03

PILOT VALVE LEVER

DETENT PLUNGER
SHIFT
ADJUSTMENT PIN PLATE

RETAINER
PLUNGER

DETENT COIL

SPOOL

ORIFICE
PILOT SUPPLY

FROM TO TO
CONTROL TANK CONTROL
VALVE VALVE

When a lever is shifted, the adjustment pin contacts the plunger and pushes it down against its
spring. The plunger will contact the spool and move it down against its spring. Depending on
how far the lever is shifted determines how far the spool moves.

As the spool moves down, it will close off the drain passage for the oil to the control valve and
meter pilot oil to the control valve to cause the control spool (not shown) to shift. The greater
the pilot oil flow to the control spool the greater the control spool travel.

As pressure increases in the pilot line to the control valve, the pressure works on the spool to
move the spool up to a balance position against the spool and plunger springs to maintain the
pilot pressure in the pilot line. This will maintain the position of the control spool in the
control valve.

In summary, once the pilot control lever is shifted the pilot valve becomes a pressure reducing
valve which maintains a downstream pressure equal to the spring forces above the spool.
STMG 767 - 20 -
1/03

PILOT VALVE LEVER

IN DETENT DETENT PLUNGER

ADJUSTMENT PIN
PLATE
RETAINER
PLUNGER

DETENT COIL

SPOOL

ORIFICE
PILOT SUPPLY

FROM TO TO
CONTROL TANK CONTROL
VALVE VALVE

When the key start switch is in the ON position, the detent coils are energized.

As the operator shifts the lever further, the detent plunger begin to provide feedback resistance.

The retainer contacts the detent coil. The retainer and lever is then held by the detent coil until
the operator moves the lever out of detent or power to the detent is stopped.

Power may be stopped by "kickout" switches mounted on the cylinders or loader linkage.
STMG 767 - 21 -
1/03

IMPLEMENT HYDRAULIC SYSTEM

HOLD
LIFT
CYLINDERS
TILT
SIGNAL PRESSURE CYLINDER
LIMITER COMPENSATOR

LOAD CHECK
VALVE
SIGNAL
DRAIN
TANK LINE RELIEF
VALVE
AND MAKEUP BACK PRESSURE
VALVES CHECK VALVE

IMPLEMENT CONTROL VALVE OPERATION

The following visuals will cover the basic valve operation for this system. This illustration
shows two control valves with corresponding compensators. Many of the components shown in
an earlier illustration have been removed to simplify the explanation.

In HOLD there is no signal created. The pump is at LOW PRESSURE STANDBY.

STMG 767 - 22 -
1/03

CONTROL VALVE
HOLD
SIGNAL
TO / FROM TO / FROM
PRESSURE
ATTACHMENT ATTACHMENT
COMPENSATOR
VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

TRAVEL LIMITER
SHOCK REDUCING LOAD CHECK FEEDER PUMP SCREW
PASSAGE SPOOL
CHECK VALVE VALVE SUPPLY

In HOLD, the control valve centering springs, at each end, center the spool in the valve body.

The control valve may have travel limiter screws to limit the maximum travel of the spool;
thereby, limiting the flow through the valve.

The valve shown has two combination line relief and makeup valves. The pressure setting of
each line relief valve is adjustable.

Located in the ends of the control valve are the two shock reducing valves which allow the
main spool to return to the HOLD or center position slowly.

The pressure compensator valve maintains a controlled pressure differential across the spool to
control flow to the circuit. If more than one circuit is used at a time, the circuit with the highest
workport pressure is used to regulate the flow through each control valve.

STMG 767 - 23 -
1/03

The 924G implement hydraulic system is similar to the Proportional Priority Pressure
Compensated (PPPC) hydraulic system used on other Caterpillar products. The system shown
incorporates the flow control, signal duplication, and resolver into one component called a
pressure compensator valve.

The pressure compensator valve acts as a resolver to compare the different circuit workport
pressures and to send a signal pressure equal to the highest circuit pressure back to the pump.

As shown in HOLD, another circuit has been activated and the load sensing signal working
with the spring above the pressure compensator moves the pressure compensator valve down.

Due to the signal duplication feature, some of the supply oil acting below the pressure
compensator valve having the highest circuit pressure flows through the load sensing port into
the signal network to become signal oil. The signal network consists of all of the pressure
compensators, internal passages, and signal lines. On other systems, the actual highest circuit
workport pressure is directed through the signal network on other load sensing systems. This
signal is sent to the pump control and to all of the other pressure compensator valves.

For the pressure compensator with the highest circuit pressure requirement, the spring above
the pressure compensator controls the maximum pressure differential across the pressure
compensator, which results in the signal to the pump being the same value as the highest circuit
pressure requirement.

For example, if the pressure in the feeder passage is 14150 kPa (2050 psi) and the spring above
the pressure compensator is 350 kPa (50 psi), both the circuit workport pressure and the load
sensing signal pressure working above pressure compensator with the spring to balance the
pressure compensator, would be the same value of 13800 kPa (2000 psi).

In summary, the pressure compensator duplicates the highest circuit workport pressure to create
a signal.
STMG 767 - 24 -
1/03

PRESSURE COMPENSATOR OPERATION

TO PUMP,
SIGNAL LIMITER, SIGNAL
AND
SIGNAL DRAIN VALVE A VALVE B VALVE C
VALVE
PRESSURE
COMPENSATOR

SLOTS

LOAD CHECK
VALVE

FEEDER
PASSAGE SPOOL SPOOL
PUMP PUMP PUMP
SUPPLY SUPPLY SUPPLY

HOLD LOW PRESSURE HIGH PRESSURE

The pressure compensator valve for valve "A" is in HOLD. The duplicated signal pressure
from the circuit with the highest workport pressure keeps the pressure compensator valve
closed.

The signal pressure from the circuit with the highest workport pressure regulates the flow
through all activated control valves.

When a single hydraulic circuit is activated from the HOLD position to the position as shown
for valve "C" from a pump STANDBY condition, the control spool is shifted and pump supply
oil enters the feeder passage below the pressure compensator valve. Pressure increases and the
pressure compensator valve moves up. When the valve moves up, the center passage in the
pressure compensator opens the cross-drilled passage to the load sensing port.

Supply oil enters the load sensing line and becomes signal oil to the pump controls and also
flows to the spring chamber above the pressure compensator valve.

STMG 767 - 25 -
1/03

The signal oil flows to the margin spool in the main pump control valve. The pump control
valve reacts to the change in flow demand and the pump UPSTROKES to increase flow. The
increased flow increases the pressure in the feeder passage below the pressure compensator.

When the supply pressure in the feeder passage increases to more than the circuit pressure, the
load check valve (previously shown) opens and pump flow goes past the load check valve and
to the circuit.

Signal oil also flows to the spring chamber above the compensator through slots machined into
the compensator. The spring on the top of the pressure compensator valve works with the
signal oil to balance the forces working below the pressure compensator. When the forces are
in balance, the supply oil is metered through the cross-drilled hole in the pressure compensator
to provide signal oil. The signal oil pressure, when the pressure compensator is balanced, is
equal to the oil pressure in the feeder passage less the spring value.

For the circuit with the highest circuit pressure, the pressure compensator spring controls the
maximum pressure differential between the feeder passage and the passage to the cylinder, so
the signal pressure and the circuit pressure will be the same. The pressure compensator valve
duplicates the actual circuit workport pressure using supply oil from the feeder passage.

With more than one circuit activated at a time, the highest circuit pressure is directed through
the slots in the compensators to the spring chamber at the top of all pressure compensators
valves.

With the same circuit pressure working on all pressure compensators, the pressure differential
across all shifted control spools is the same, as shown in the illustration for the pressure
compensator for valve "C" and for valve "B." The pressure differential across the control
spools will be the same value whether the pump can satisfy the flow demand for all activated
circuits or not.

For example, if the margin pressure is 2100 kPA (300 psi) and the pressure compensator spring
is 350 kpa (50 psi), the pressure differential between the pump supply passage and the feeder
passage will be approximately 1750 (250 psi) for a given circuit pressure.

When the pump cannot meet the flow needs of all activated circuits, the pressure compensators
will move down to proportion the pump flow in relation to the amount of control spool travel
for each circuit. The pressure differential will be less than shown in the example, but the
pressure differential will be the same for all spools.

Valve "B" pressure compensator shows what occurs when an additional circuit is activated with
a lower circuit pressure than the first activated valve.

STMG 767 - 26 -
1/03

The load sensing signal from valve "C" pressure compensator is directed to the top of the valve
"B" pressure compensator valve with the low circuit pressure. When the control spool is
moved, pressure oil in the feeder passage moves the pressure compensator valve up. The
pressure compensator valve does not move up enough to open the load sensing signal line to
feeder passage pressure oil due to the higher forces working above the pressure compensator
spool.

The pressure compensator valve will respond to changes in the circuit pressure by opening and
closing off the passage between the feeder passage to the cylinder to maintain a constant flow
rate for a given control spool displacement. As the compensator opens and closes, the pressure
differential across the compensator will vary in order to maintain the constant flow rate.
STMG 767 - 27 -
1/03

IMPLEMENT HYDRAULIC SYSTEM

RAISE / DUMP

TILT LIFT
CYLINDER CYLINDERS
SIGNAL PRESSURE
LIMITER COMPENSATOR

LOAD CHECK
VALVE
SIGNAL
DRAIN
TANK LINE RELIEF
VALVE
AND MAKEUP BACK PRESSURE
VALVES CHECK VALVE

As previously discussed, when two circuits are operated at the same time, the circuit with the
highest workport or load pressure is sent by that circuit's compensator through the signal
network to all other compensators and to the pump.

In this illustration, the load pressure from the lift circuit is greater than the load pressure from
the tilt circuit. The lift circuit load pressure becomes the signal pressure that is sent through the
signal line to the pump and causes the tilt valve compensator to shift down.

For the lift compensator shifts to the upper envelope, while the tilt compensator shift to the
middle envelope.
STMG 767 - 28 -
1/03

924G TILT VALVE

HOLD
SIGNAL
TO / FROM TO / FROM PRESSURE
HEAD END ROD END COMPENSATOR
VALVE

LINE RELIEF /
MAKEUP
VALVE

SPOOL
LOAD CHECK FEEDER PUMP PILOT OPERATED
VALVE PASSAGE SUPPLY LOCK VALVE

924G IMPLEMENT CONTROL VALVE OPERATION

924G Tilt Valve

The following illustrations show the tilt, lift, and auxiliary control valves in various possible
states for the 924G. Corresponding systems schematics will also be shown.

A pilot operated lock valve is used to prevent cylinder drift. In HOLD, the tilt valve centering
springs center the spool in the valve body.

Shock reducing valves are not used for the 924G tilt valve.

NOTE: The ISO symbol of the pilot operated lock valve shown is a simplified
representation of a more complex compound valve.
STMG 767 - 29 -
1/03

924G IMPLEMENT HYDRAULIC SYSTEM

TILT BACK
FROM QUICK
STEERING COUPLER

RIDE
CONTROL
COUPLER GROUP
CONTROL COMBINATION
VALVE VALVE

TILT LIFT
RETURN CYLINDERS
MANIFOLD CYLINDER
RESTRICTOR
TO
STEERING DEAD ENGINE
SIGNAL PRESSURE LOWER VALVE
LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE SIGNAL
PIN DRAIN MAKEUP
TANK VALVE VALVE

PRESSURE
REDUCING LINE RELIEF BACK PRESSURE
REGEN
VALVE AND MAKEUP CHECK VALVE
VALVE
VALVES

SHOCK
REDUCING
PRESSURE
VALVE
SWITCH

TILT LIFT
HYDRAULIC WIRED TO PILOT
LOCKOUT PILOT
CONTROLLER VALVE
VALVE VALVE

When the tilt valve is shifted for TILT BACK, pilot oil from the tilt pilot valve is directed to the
bottom of the tilt valve and shifts it up.

Supply oil flows past the spool and flows to the pressure compensator. The pressure
compensator shifts to direct some of the oil into the signal network. Oil in the signal network
flows to all other pressure compensators, the signal limiter, the signal drain valve, through the
throttle pin, and to the pump to cause the pump to UPSTROKE.

When the signal is sensed at the signal drain valve, the valve restricts the signal oil from going
to the tank.

The signal limiter is closed unless the signal oil pressure increases to its spring setting. If the
valve opens, the signal pressure is limited. The circuit that has the highest signal pressure will
be in a stall condition. Other circuits will continue to function.

The orifices in the throttle pin control the destroking and upstroking speed of the pump by
regulating the signal flow to and from the pump. The orifices are sized differently.

STMG 767 - 30 -
1/03

The rest of the oil through the pressure compensator, unseats a load check valve and is directed
to the pilot operated lock valve. This valve unseats and oil flows to the tilt cylinder to tilt the
bucket back. Return oil from the tilt cylinder is directed back to the tank through the spool.

The return restrictor provides a slight back pressure on the return oil to prevent cylinder
cavitation.
STMG 767 - 31 -
1/03

924G TILT VALVE

TILT BACK
SIGNAL
FROM TO ROD PRESSURE
HEAD END END COMPENSATOR
VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

SPOOL

LOAD CHECK PUMP PILOT OPERATED

VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool to the left, pump supply oil flows into the feeder
passage.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the spool
unseating the pilot operated lock valve and to the cylinder rod end. Return oil from the circuit
flows around the spool to the tank.

When the pressure compensator valve moves up, the center passage in the pressure
compensator opens the cross-drilled passage to the load sensing port.

Supply oil enters the load sensing line and becomes signal oil to the pump controls and also
flows to the spring chamber above the pressure compensator valve.

Signal oil flows to the margin spool in the pump control valve. The pump control valve reacts
to the change in flow demand and the pump UPSTROKES to increase flow. The increased
flow increases the pressure in the feeder passage below the pressure compensator.

STMG 767 - 32 -
1/03

When the supply pressure in the feeder passage increases to more than the circuit pressure, the
load check valve opens and pump flow goes past the load check valve and to the circuit.

The spring on the top of the pressure compensator valve works with the signal oil to balance
the forces working below the pressure compensator. When the forces are in balance, the supply
oil is metered through the cross-drilled hole in the pressure compensator to provide signal oil.
The signal oil pressure, when the pressure compensator is balanced, is equal to the oil pressure
in the feeder passage less the spring value.

For the circuit with the highest circuit pressure, the pressure compensator spring controls the
maximum pressure differential between the feeder passage and the passage to the cylinder.

The pressure compensator valves in each circuit will respond to changes in the circuit pressure
by opening and closing off the passage between the feeder passage to the cylinders to maintain
a constant flow rate for the given control spool displacement.

NOTE: Due to all pressure compensators sensing the same load sensing pressure, a signal
limiter is required in a PPPC system. Without the signal limiter, if a circuit was stalled
the system would react as if all activated circuits were in a stall condition.
STMG 767 - 33 -
1/03

924G IMPLEMENT HYDRAULIC SYSTEM

DUMP / REGENERATION
FROM QUICK
STEERING COUPLER

RIDE
CONTROL
COUPLER GROUP
CONTROL COMBINATION
VALVE VALVE

RETURN LIFT
MANIFOLD TILT CYLINDERS
RESTRICTOR CYLINDER
TO
STEERING DEAD ENGINE
PRESSURE LOWER VALVE
SIGNAL
LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL MAKEUP
DRAIN VALVE
TANK VALVE

PRESSURE
REDUCING BACK PRESSURE
LINE RELIEF REGEN.
VALVE CHECK VALVE
AND MAKEUP VALVE
VALVES

SHOCK
REDUCING
VALVE

TILT LIFT
HYDRAULIC WIRED TO PILOT
PILOT
LOCKOUT CONTROLLER VALVE
VALVE
VALVE

When the tilt valve is shifted for a full DUMP, pilot oil from the tilt pilot valve is directed
above the tilt valve and moves the spool down. At the same time the pilot oil is used to unseat
the pilot operated lock valve so return oil from the cylinders can flow to the tank.

Supply oil flows past the spool to the pressure compensator. The pressure compensator shifts to
direct some of the oil into the signal network. Oil in the signal network flows to all other
pressure compensators, the signal limiter, the signal drain valve, through the throttle pin, and to
the pump to cause the pump to UPSTROKE to increase flow.

The rest of the oil through the pressure compensator, unseats a load check valve and is directed
to the pilot operated lock valve. This valve unseats and oil flows to the cylinders to dump the
bucket. Return oil from the tilt cylinder rod end flows past the pilot operated lock valve to the
spool. The spool blocks the return oil from going to tank and directs the oil to the head end of
the tilt cylinder to reduce cavitation and to provide a powered dump, versus the force of gravity
causing the bucket to dump. A powered dump, results in faster cycle times.

The return restrictor provides a slight back pressure on the return oil to prevent cylinder
cavitation.
STMG 767 - 34 -
1/03

924G TILT VALVE

DUMP / REGENERATION
SIGNAL
PRESSURE
TO HEAD FROM ROD COMPENSATOR
END END VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

SPOOL
LOAD CHECK PUMP PILOT OPERATED
VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool fully to the right, the pump supply oil flows into
the feeder passage.

Operation of the pressure compensator valve is as described earlier.

Return oil from the tilt cylinder rod end flows past the pilot operated lock valve to the spool.
The spool blocks the return oil from going to tank and directs the oil to the head end of the tilt
cylinder to reduce cavitation and to provide a powered dump.
STMG 767 - 35 -
1/03

924G LIFT VALVE

HOLD
SIGNAL PRESSURE
TO / FROM TO / FROM COMPENSATOR
BACK PRESSURE ROD ENDS HEAD ENDS VALVE
CHECK VALVE

LOAD CHECK MAKEUP

VALVE VALVE

PILOT OPERATED PISTON

LOCK VALVE
SHOCK REDUCING REGENERATION PUMP SUPPLY SPOOL
CHECK VALVE VALVE PASSAGE

924G Lift Valve

In HOLD, the lift valve centering springs center the spool in the valve body. A shock reducing
check valve is used to dampen the pilot oil as it returns to the tank when the pilot lever is
returned to HOLD from the LOWER or FLOAT positions.

The lift valve is equipped with a regeneration valve which is used to reduce cylinder cavitation
when the lift valve is moved to the LOWER position.

A pilot operated lock valve is used to prevent cylinder drift. The makeup valve is used to
reduce cylinder cavitation when the lift circuit is lowered rapidly.

The pressure compensator valve maintains a controlled pressure differential across the main
spool to control flow to the circuit. If more than one circuit is used at a time, the circuit with
the highest workport pressure is used to regulate the flow through each control valve.
Operation of the pressure compensator is the same as described for the tilt valve.

The back pressure check valve restricts the return oil from the lift cylinders to prevent cylinder
cavitation.
STMG 767 - 36 -
1/03

924G IMPLEMENT HYDRAULIC SYSTEM

RAISE
FROM QUICK
STEERING COUPLER

RIDE
CONTROL
COUPLER GROUP
CONTROL COMBINATION
VALVE VALVE

TILT LIFT
RETURN
MANIFOLD CYLINDER CYLINDERS
RESTRICTOR
TO
STEERING DEAD ENGINE
SIGNAL PRESSURE LOWER VALVE
LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN MAKEUP
VALVE VALVE
TANK

PRESSURE BACK PRESSURE

REDUCING LINE RELIEF REGEN.
VALVE CHECK VALVE
VALVE AND MAKEUP
VALVES

SHOCK
REDUCING
PRESSURE VALVE
SWITCH

TILT LIFT
HYDRAULIC WIRED TO PILOT
LOCKOUT PILOT
CONTROLLER VALVE VALVE
VALVE

When the lift valve is shifted to RAISE, pilot oil from the lift pilot valve is directed to the
bottom of the lift valve and shifts the spool up.

The return restrictor provides a slight back pressure on the return oil to prevent cylinder
cavitation.
STMG 767 - 37 -
1/03

924G LIFT VALVE

RAISE PRESSURE
BACK PRESSURE SIGNAL COMPENSATOR
FROM TO HEAD
CHECK VALVE VALVE
ROD ENDS ENDS

LOAD CHECK MAKEUP

VALVE VALVE

PILOT OPERATED PISTON

PUMP SPOOL LOCK VALVE
SHOCK REDUCING REGENERATION
CHECK VALVE VALVE SUPPLY

When pilot pressure moves the control spool to the left, the pump supply oil flows into the
feeder passage.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the control
spool unseating the pilot operated lock valve and flows to the cylinders. Operation of the
pressure compensator valve is as described earlier.

Return oil from the lift cylinder rod ends flows around the spool to the tank.
STMG 767 - 38 -
1/03

924G IMPLEMENT HYDRAULIC SYSTEM

LOWER
FROM QUICK
STEERING COUPLER

RIDE
CONTROL
COUPLER GROUP
CONTROL COMBINATION
VALVE VALVE

TILT
CYLINDER
RETURN LIFT
MANIFOLD RESTRICTOR CYLINDERS
TO
STEERING DEAD ENGINE
SIGNAL PRESSURE LOWER VALVE
LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN MAKEUP
TANK VALVE VALVE

PRESSURE BACK PRESSURE

REDUCING LINE RELIEF REGEN.
VALVE CHECK VALVE
VALVE & MAKEUP
VALVES

SHOCK
REDUCING
PRESSURE VALVE
SWITCH

HYDRAULIC TILT LIFT

LOCKOUT WIRED TO PILOT PILOT
VALVE CONTROLLER VALVE VALVE

When the lift valve is shifted to LOWER the implements, pilot oil from the lift pilot valve is
directed above the lift valve and shifts it down. At the same time the pilot oil is used to unseat
the pilot operated lock valve so return oil from the cylinders can flow to the tank.

The rest of the oil through the pressure compensator and oil flows to the cylinders to lower the
implements. Return oil from the lift cylinders flows past the pilot lock valve and is directed to
the back pressure check valve. The check valve creates a back pressure on the return oil, which
helps to open the regeneration valve.

The oil unseats the regeneration valve and is used as makeup oil for oil being directed to the
rod end of the cylinders to reduce cavitation and to provide a controlled lower.
STMG 767 - 39 -
1/03

924G LIFT VALVE

LOWER
SIGNAL
TO ROD FROM PRESSURE
BACK PRESSURE ENDS HEAD ENDS COMPENSATOR
CHECK VALVE VALVE

LOAD CHECK
VALVE MAKEUP
VALVE

PISTON
SHOCK REDUCING REGENERATION PUMP SPOOL PILOT OPERATED
CHECK VALVE VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool to the right, the pump supply oil flows into the
feeder passage. The pilot pressure also unseats the pilot operated lock valve.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the control
spool, past the pilot operated lock valve, and to the cylinders.

Pressure compensator operation is as described before.

Return oil from the head ends is blocked by the spool and is restricted by the back pressure
check valve from going to the tank. The regeneration valve opens and the return oil flows to
the rod ends of the lift cylinders to prevent cavitation and to provide a faster cylinder fill than
just using the pump supply oil.
STMG 767 - 40 -
1/03

924G IMPLEMENT HYDRAULIC SYSTEM

FLOAT
FROM QUICK
STEERING COUPLER

RIDE
CONTROL
COUPLER GROUP COMBINATION
CONTROL VALVE
VALVE

TILT
CYLINDER
LIFT
RETURN CYLINDERS
MANIFOLD RESTRICTOR
TO
STEERING DEAD ENGINE
SIGNAL PRESSURE LOWER VALVE
LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL
MAKEUP
DRAIN
VALVE
TANK VALVE

PRESSURE REGEN. BACK PRESSURE

REDUCING LINE RELIEF CHECK VALVE
AND MAKEUP VALVE
VALVE
VALVES

SHOCK
REDUCING
VALVE

TILT LIFT
HYDRAULIC WIRED TO PILOT
LOCKOUT CONTROLLER PILOT
VALVE VALVE
VALVE

When the lift is put in FLOAT, full pilot oil flows to the pilot check valve opening a return
passage for oil from the cylinders. The pilot lever is held in the float detent by a detent coil.

Supply oil through the lift valve, in FLOAT, helps create a slight down pressure on the bucket
to improve back dragging operation.
STMG 767 - 41 -
1/03

924G LIFT VALVE

FLOAT
SIGNAL
TO / FROM TO / FROM PRESSURE
BACK PRESSURE HEAD ENDS
ROD ENDS COMPENSATOR
CHECK VALVE
VALVE

LOAD CHECK MAKEUP

VALVE VALVE

PISTON
SHOCK REDUCING REGENERATION PUMP METERING PILOT OPERATED
CHECK VALVE VALVE SUPPLY SLOT LOCK VALVE

Full pilot oil shifts the spool fully to the right. The pilot oil also unseats the pilot operated lock
valve to allow the head end of the cylinders to be connected to the tank.

Supply oil flows through the metering slot into the the valve passages. The supply oil and the
return oil from the cylinders flows through the back pressure check valve to the tank. The back
pressure check valve creates a back pressure in the lift cylinders in FLOAT.

This back pressure creates a down pressure on the buckets which helps in back dragging
operation.
STMG 767 - 42 -
1/03

AUXILIARY VALVE
HOLD
SIGNAL
BACK PRESSURE TO / FROM TO / FROM PRESSURE
CHECK VALVE ATTACHMENT ATTACHMENT COMPENSATOR
VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

SHOCK REDUCING LOAD CHECK PUMP SPOOL

CHECK VALVE VALVE SUPPLY

Auxiliary Valves

The third and fourth function auxiliary valves are identical for either the 924G or the 924Gz.

The valves are equipped with components already discussed such as shock reducing check
valves, line relief and makeup valves, a load check valve, and a pressure compensator.

Operation of the pressure compensator is the same as previously discussed.

The auxiliary valves are not equipped with regeneration valves.

For a shift to the right, the auxiliary valves are affected by a back pressure check valve as
previously discussed.

The return restrictor (not shown) will assist the line relief and makeup valves by opening the
makeup valves to prevent cylinder voiding.
STMG 767 - 43 -
1/03

FROM QUICK
STEERING COUPLER

RIDE
CONTROL
924G IMPLEMENT
COUPLER
CONTROL
GROUP
COMBINATION
HYDRAULIC SYSTEM
VALVE VALVE
THIRD FUNCTION ACTIVATED

TILT LIFT
RETURN
MANIFOLD CYLINDER CYLINDERS
RESTRICTOR THIRD FOURTH
FUNCTION FUNCTION
TO
STEERING DEAD ENGINE
PRESSURE
LOWER VALVE
SIGNAL COMPENSATOR
LIMITER

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN MAKEUP
TANK VALVE VALVE

PRESSURE
REDUCING BACK PRESSURE
LINE RELIEF REGEN.
VALVE CHECK VALVE
AND MAKEUP VALVE
VALVES

SHOCK
REDUCING
PRESSURE VALVE
SWITCH

TILT LIFT AUX

HYDRAULIC AUX
WIRED TO PILOT PILOT PILOT
LOCKOUT PILOT
CONTROLLER VALVE VALVE VALVE VALVE
VALVE

When the auxiliary pilot lever is shifted, pilot oil is directed to one side of the control spool.

The rest of the oil through the pressure compensator, unseats a load check valve and oil flows
to the attachment. Return oil from the lift cylinders is directed back to the tank through the
spool.

The return restrictor provides a slight back pressure on the return oil to assist the makeup
valves in preventing cylinder cavitation.
STMG 767 - 44 -
1/03

AUXILIARY VALVE
SHIFT LEFT
BACK PRESSURE SIGNAL PRESSURE
CHECK VALVE FROM TO COMPENSATOR
ATTACHMENT ATTACHMENT VALVE
LINE RELIEF /
MAKEUP
LINE RELIEF /
VALVE
MAKEUP
VALVE

SHOCK REDUCING LOAD CHECK PUMP

SPOOL
CHECK VALVE VALVE SUPPLY

When pilot pressure moves the control spool to the left, the pump supply oil flows into the
feeder passage.

Pressure compensator operation is as described before.

Return oil from the attachment flows past the spool to the tank.
STMG 767 - 45 -
1/03

AUXILIARY VALVE
SHIFT RIGHT
SIGNAL
BACK PRESSURE TO FROM PRESSURE
CHECK VALVE ATTACHMENT ATTACHMENT COMPENSATOR
VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

SHOCK REDUCING LOAD CHECK PUMP

SPOOL
CHECK VALVE VALVE SUPPLY

When pilot pressure moves the control spool to the right, the pump supply oil flows into the
feeder passage.

Pressure compensator operation is as described before.

Return oil from the attachment flows past the spool to the back pressure valve. The back
pressure valve creates a slight back pressure on the return oil before allowing the return oil to
flow to the tank.
STMG 767 - 46 -
1/03

924G QUICK COUPLER AND CONTROL VALVE

ENGAGED RELEASED
CHECK
VALVE

RELIEF
VALVE

SHUTTLE
VALVE

FROM FROM
STEERING STEERING
SOLENOID
CONTROL
VALVE

STEERING
AND PILOT
PUMP

Additional Component Operation

The quick coupler cylinder is controlled by a solenoid control valve. When one of the
solenoids in the valve is energized as shown for RELEASE, the spool shifts and supply oil from
the steering and pilot pump is directed to the coupler.

Since the valve is Load Sensing (LS), a signal is sent to the pump to cause the pump to increase
flow.

One of the two shuttle valves senses the highest work port pressure between the two lines to the
coupler cylinder. The other shuttle valve is used to send either the signal from the coupler or
from the steering circuit to the pump.

Some of the oil to the coupler cylinder is used as pilot oil to unseat the check valve so that
return oil from the coupler cylinder can return to the tank through the solenoid control valve.

The relief valve is used to protect the quick coupler cylinder from high pressures.
STMG 767 - 47 -
1/03

924G COMBINATION VALVE

BACK DRAG OPERATION

RELIEF
VALVE
TILT
CYLINDER

DIVERTER
VALVE

TO TILT
CONTROL VALVE

Plumbed into the tilt cylinder on the 924G is a combination valve with a diverter valve and a
relief valve. The combination valve performs the relief function on the head end of the cylinder
during back dragging.

This illustration shows the combination valve during a back drag operation. The bucket is
forcing the cylinder piston in, thus causing a void on the rod end and pressure on the head end
of the cylinder.

The diverter valve spool is connected to each end of the tilt cylinder. The function of the
diverter is to sense the pressure differential between the head end and the rod end of the
cylinder. If the pressure differential exceeds 3000 KPa (435 psi), the diverter opens and oil
from the head end of the cylinder flows to the relief valve. If the pressure on the relief valve
exceeds 12,300 KPa (1780 psi), the relief valve opens and the oil is returned to tank.

The reason for the diverter valve is to keep the relief valve setting low for back dragging
operation and allow for a higher circuit operating pressure during normal operations. The
higher pressure is controlled by the rod end line relief valve in the main control valve group.
STMG 767 - 48 -
1/03

924G RIDE CONTROL SYSTEM

SELECTOR
SELECTOR
VALVE
VALVE

RIDE RIDE
SOLENOID CONTROL SOLENOID CONTROL
GROUP GROUP

PUMP
SUPPLY

SHUTTLE SHUTTLE
CHECK VALVE CHECK VALVE
VALVE VALVE
LIFT LIFT
CYLINDERS CYLINDERS

DEAD ENGINE
PUMP LOWER VALVE
PUMP
PILOT PILOT SUPPLY
SUPPLY
OIL OIL

SIGNAL SIGNAL

PILOT PILOT
LOCK LOCK
VALVE VALVE
IMPLEMENT
SHUTOFF
VALVE

MAKEUP MAKEUP
VALVE VALVE
TO TANK TO TANK
REGEN REGEN
VALVE VALVE

BACK PRESSURE
SHOCK CHECK VALVE
REDUCING
VALVE

ENABLED DISABLED
LIFT LIFT
PILOT PILOT
VALVE VALVE

The Ride Control System allows the lift cylinders to act as shock absorbers when roading the
machine for smoother travel.

When the Ride Control solenoid is ENERGIZED for ENABLE, the valve shifts and directs
pilot oil to the shuttle valve and the check valve. At the check valve, the pilot oil unseats the
valve to open the rod end of the cylinders to the tank. The pilot oil also causes the shuttle valve
to shift to direct pilot oil to the selector valve causing the selector valve to shift up, connecting
the Ride Control accumulator to the head end of the lift cylinders. The accumulator will now
cushion the load.

The Ride Control solenoid is DE-ENERGIZED when the operator DISABLES Ride Control,
operates the tilt for TILT BACK, or if the transmission is in the Automatic Mode in a low
ground speed. In this illustration, the pilot valve has been moved to the RAISE position to
charge the Ride Control Accumulator.

When the transmission is in the Automatic Mode, the Ride Control System will turn on and off
based on a preset ground speed, which can be changed through Electronic Technician (ET).
STMG 767 - 49 -
1/03

RIDE
CONTROL
924Gz IMPLEMENT
GROUP
HYDRAULIC SYSTEM
HOLD
TILT
CYLINDER
MANIFOLD RETURN LIFT
RESTRICTOR CYLINDERS THIRD FOURTH
TO FUNCTION FUNCTION
STEERING DEAD ENGINE
SIGNAL LOWER VALVE PRESSURE
LS LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE
PRESSURE
REDUCING
VALVE FLOAT
LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE POSITION
VALVES CHECK VALVE VALVES

SHOCK
REDUCING
VALVE

TILT LIFT AUX AUX

HYDRAULIC WIRED TO PILOT PILOT PILOT PILOT
LOCKOUT CONTROLLER VALVE VALVE VALVE VALVE
VALVE

924Gz IMPLEMENT HYDRAULIC SYSTEM

The 924Gz implement hydraulic system is similar to the 924G. There are some differences.
These differences are:

- a regeneration valve for the tilt circuit;

- no combination valve for the tilt circuit;

- a second float position valve for the lift circuit;

- the lift control spool does not shift in FLOAT. Instead, two float position valves are
used; and

- no quick coupler and coupler control valve.

STMG 767 - 50 -
1/03

924Gz TILT VALVE

HOLD
SIGNAL
TO / FROM TO / FROM PRESSURE
ROD END HEAD END COMPENSATOR
VALVE
BACK PRESSURE
CHECK VALVE
LINE RELIEF /
MAKEUP
VALVE

SPOOL
SHOCK REDUCING LOAD CHECK REGENERATION PUMP PILOT OPERATED
CHECK VALVE VALVE VALVE SUPPLY LOCK VALVE

924Gz IMPLEMENT CONTROL VALVE OPERATION

924Gz Tilt Valve

The following illustrations will show the tilt and lift control valves in various possible states for
the 924Gz. Corresponding systems schematics will also be shown.

The auxiliary valves on the 924Gz work the same as they do on the 924G.

A pilot operated lock valve is used to prevent cylinder drift.

In HOLD, the tilt valve centering springs center the spool in the valve body. Shock reducing
valves are used for the 924Gz tilt valve along with a regeneration valve.
STMG 767 - 51 -
1/03

924Gz IMPLEMENT HYDRAULIC SYSTEM

RIDE TILT BACK
CONTROL
GROUP

LIFT
MANIFOLD RETURN TILT CYLINDERS
RESTRICTOR CYLINDER
TO DEAD ENGINE
STEERING SIGNAL PRESSURE LOWER VALVE
LS LIMITER COMPENSATOR

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE
PRESSURE
REDUCING
VALVE
LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE
VALVES CHECK VALVE

SHOCK FLOAT
REDUCING POSITION
VALVE VALVES

PRESSURE
SWITCH

HYDRAULIC TILT LIFT

LOCKOUT WIRED TO PILOT PILOT
VALVE CONTROLLER VALVE VALVE

When the tilt valve is shifted for TILT BACK, pilot oil from the tilt pilot valve is directed to the
bottom of the tilt valve and shifts it up.

When the signal is sensed at the signal drain valve, the valve restricts the signal oil from going
to the tank.

The orifices in the throttle pin control the destroking and upstroking speed of the pump by
regulating the signal flow to and from the pump. The orifices are sized differently.

STMG 767 - 52 -
1/03

The return restrictor provides a slight back pressure on the return oil to prevent cylinder
cavitation.
STMG 767 - 53 -
1/03

924Gz TILT VALVE

TILT BACK
SIGNAL PRESSURE
BACK PRESSURE FROM TO
ROD END COMPENSATOR
CHECK VALVE HEAD END
VALVE
LINE RELIEF /
LINE RELIEF /
MAKEUP
MAKEUP
VALVE
VALVE

SPOOL
SHOCK REDUCING LOAD CHECK REGENERATION PUMP PILOT OPERATED
CHECK VALVE VALVE VALVE SUPPLY LOCK VALVE

When pilot pressure flows through the shock reducing check valve and moves the control spool
to the left, the pump supply oil flows into the feeder passage.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the control
spool unseating the pilot operated lock valve and to the cylinder rod end. Return oil from the
circuit flows around the spool to the tank.

When the pressure compensator valve moves up, the center passage in the pressure
compensator opens the cross-drilled passage to the load sensing port.

Supply oil enters the load sensing line and becomes signal oil to the pump controls and also
flows to the spring chamber above the pressure compensator valve.

Signal oil flows to the margin spool in the pump control valve. The pump control reacts to the
change in flow demand and the pump UPSTROKES to increase flow. The increased flow
increases the pressure in the feeder passage below the pressure compensator.

STMG 767 - 54 -
1/03

924Gz IMPLEMENT HYDRAULIC SYSTEM

DUMP / REGENERATION
RIDE
CONTROL
GROUP

LIFT
RETURN TILT CYLINDERS
MANIFOLD RESTRICTOR CYLINDER

DEAD ENGINE
TO LOWER VALVE
STEERING SIGNAL PRESSURE
LIMITER COMPENSATOR
LS

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE
PRESSURE
REDUCING
VALVE LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE
VALVES CHECK VALVE

SHOCK FLOAT
REDUCING POSITION
VALVE VALVES

PRESSURE
SWITCH

HYDRAULIC TILT LIFT

LOCKOUT WIRED TO PILOT PILOT
VALVE CONTROLLER VALVE VALVE

When the tilt valve is shifted for a full DUMP, pilot oil from the tilt pilot valve is directed
above the tilt valve and shifts it down. At the same time the pilot oil is used to unseat the pilot
operated lock valve so return oil from the cylinders can flow to the tank.

The rest of the oil through the pressure compensator, unseats a load check valve and is directed
to the pilot operated lock valve. This valve unseats and oil flows to the cylinders to dump the
bucket. Return oil from the tilt cylinder rod end flows past the pilot operated lock valve to the
spool. The spool blocks the return oil from going to tank and directs the oil to the head end of
the tilt cylinder through the regeneration valve to reduce cavitation and to provide a powered
dump, versus the force of gravity causing the bucket to dump. A powered dump, results in
faster cycle times.
STMG 767 - 56 -
1/03

924Gz TILT VALVE

DUMP / REGENERATION
SIGNAL
TO ROD PRESSURE
BACK PRESSURE FROM
END COMPENSATOR
CHECK VALVE HEAD END
VALVE
LINE RELIEF /
MAKEUP LINE RELIEF /
VALVE MAKEUP
VALVE

SPOOL
SHOCK REDUCING LOAD CHECK REGENERATION PUMP PILOT OPERATED
CHECK VALVE VALVE VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool to the right, the pump supply oil flows into the
feeder passage.

Operation of the pressure compensator valve is as described earlier.

Return oil from the tilt cylinder rod ends flows past the pilot operated lock valve to the spool.
The spool blocks the return oil from going to tank and directs the oil to the cylinder head end
through the regeneration check valve to reduce cavitation and to provide a powered dump.
STMG 767 - 57 -
1/03

924Gz LIFT VALVE

HOLD
BACK PRESSURE SIGNAL
CHECK VALVE
TO / FROM TO / FROM PRESSURE
ROD ENDS HEAD ENDS COMPENSATOR
LOAD CHECK VALVE
VALVE
FLOAT POSITION
FLOAT POSITION
SOLENOID
SOLENOID
VALVE
VALVE

SHOCK REDUCING REGENERATION PUMP SUPPLY SPOOL PILOT OPERATED

VALVE PASSAGE LOCK VALVE
CHECK VALVE

924Gz Lift Valve

In HOLD, the lift valve centering springs center the spool in the valve body. Shock reducing
valves are used to dampen the pilot oil as it returns to the tank when the pilot lever is returned
to HOLD.

The lift valve is equipped with two regeneration valves which are used to reduce cylinder
cavitation when the lift valve is moved to the LOWER position.

Two float solenoid valves are used to open a direct path to the tank when the lift valve is shifted
to the FLOAT position instead of having to flow around the spool.

The piston and additional spring on the right end are used to assist in returning the spool to the
HOLD position from the FLOAT position.

A pilot operated lock valve is used to prevent cylinder drift.

STMG 767 - 58 -
1/03

Operation of the pressure compensator is the same as described for the tilt valve.

The back pressure check valve restricts the return oil from the lift cylinders to prevent cylinder
cavitation.
STMG 767 - 59 -
1/03

924Gz IMPLEMENT HYDRAULIC SYSTEM

RAISE
RIDE
CONTROL
GROUP

LIFT
RETURN TILT CYLINDERS
MANIFOLD RESTRICTOR CYLINDER
TO DEAD ENGINE
STEERING SIGNAL LOWER VALVE
PRESSURE
LIMITER COMPENSATOR
LS

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE
PRESSURE
REDUCING
VALVE LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE
VALVES CHECK VALVE

SHOCK FLOAT
REDUCING POSITION
VALVE VALVES

TILT LIFT
HYDRAULIC PILOT
WIRED TO PILOT
LOCKOUT VALVE
CONTROLLER VALVE
VALVE

When the lift valve is shifted to RAISE, pilot oil from the lift pilot valve is directed to the
bottom of the lift valve and shifts it up.

The return restrictor provides a slight back pressure on the return oil to prevent cylinder
cavitation.
STMG 767 - 60 -
1/03

924Gz LIFT VALVE

RAISE
BACK PRESSURE SIGNAL
CHECK VALVE TO PRESSURE
FROM COMPENSATOR
ROD ENDS HEAD ENDS
LOAD CHECK VALVE
VALVE
FLOAT POSITION
FLOAT POSITION SOLENOID
SOLENOID VALVE
VALVE

SHOCK REDUCING REGENERATION PUMP SPOOL PILOT OPERATED

CHECK VALVE VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool to the left, the pump supply oil flows into the
feeder passage.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the control
spool unseating the pilot operated lock valve and to the cylinders. Operation of the pressure
compensator valve is as described earlier.

Return oil from the lift cylinder rod ends flows around the spool to the tank.
STMG 767 - 61 -
1/03

924Gz IMPLEMENT HYDRAULIC SYSTEM

RIDE
LOWER
CONTROL
GROUP

RETURN LIFT
RESTRICTOR TILT CYLINDERS
MANIFOLD CYLINDER

TO DEAD ENGINE
STEERING PRESSURE LOWER VALVE
SIGNAL
LIMITER COMPENSATOR
LS

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE

PRESSURE
REDUCING
VALVE LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE
VALVES CHECK VALVE

SHOCK FLOAT
REDUCING POSITION
VALVE VALVES

HYDRAULIC TILT LIFT

LOCKOUT WIRED TO PILOT PILOT
VALVE CONTROLLER VALVE VALVE

The rest of the oil through the pressure compensator flows to the cylinders to lower the
implements. Return oil from the lift cylinders flows past the pilot lock valve and is directed to
the back pressure check valve. The check valve creates a back pressure on the return oil, which
helps to open the regeneration valve.

The oil unseats the regeneration valve and is used as makeup oil for oil being directed to the
rod end of the cylinders to reduce cavitation and to provide a controlled lower.
STMG 767 - 62 -
1/03

924Gz LIFT VALVE

BACK PRESSURE LOWER
CHECK VALVE
SIGNAL
TO FROM PRESSURE
LOAD CHECK ROD ENDS HEAD ENDS COMPENSATOR
VALVE VALVE

FLOAT POSITION FLOAT POSITION

SOLENOID SOLENOID
VALVE VALVE

SHOCK REDUCING REGENERATION PUMP SPOOL PILOT OPERATED

CHECK VALVE VALVE SUPPLY LOCK VALVE

When pilot pressure moves the control spool to the right, the pump supply oil flows into the
feeder passage. The pilot pressure also unseats the pilot operated lock valve.

The pressure compensator valve moves up to provide signal oil and to allow oil flow to the load
check valve. The load check valve unseats. Oil flows around additional lands on the control
spool, past the pilot operated lock valve, and to the cylinders.

Pressure compensator operation is as described before.

Return oil from the head ends is blocked by the spool and is restricted by the back flow check
valve from going to the tank. The regeneration valve opens and the return oil flows to the rod
ends of the lift cylinders to prevent cavitation and to provide a faster cylinder fill than just using
the pump supply oil.
STMG 767 - 63 -
1/03

924Gz IMPLEMENT HYDRAULIC SYSTEM

FLOAT
RIDE
CONTROL
GROUP

LIFT
RETURN TILT CYLINDERS
MANIFOLD RESTRICTOR CYLINDER
TO DEAD ENGINE
STEERING SIGNAL PRESSURE LOWER VALVE
LIMITER COMPENSATOR
LS

LOCK
VALVE

THROTTLE
PIN
SIGNAL
DRAIN
TANK VALVE

PRESSURE
REDUCING
VALVE LINE RELIEF REGEN.
AND MAKEUP VALVE BACK PRESSURE
VALVES CHECK VALVE

SHOCK FLOAT
REDUCING POSITION
VALVE VALVES

PRESSURE
SWITCH

HYDRAULIC TILT LIFT

LOCKOUT WIRED TO PILOT PILOT
VALVE CONTROLLER VALVE VALVE

When the lift is put in FLOAT, full pilot oil flows to the pilot check valve opening a return
passage for oil from the cylinders. The pilot lever is held in the float detent by a detent coil.

As pilot pressure increases, a pressure switch closes. Current is sent to energize both float
position solenoids and shifts the valves to allow oil from the lift cylinders to be connected to
the tank passage. Due to the size of the control spool and the pilot operated lock valve, the
float position valve is required to handle the return flow from the lift cylinders in FLOAT.

Supply oil through the valve, in FLOAT, helps create a slight down pressure on the bucket to
improve back dragging operation.
STMG 767 - 64 -
1/03

924Gz LIFT VALVE

FLOAT
BACK PRESSURE SIGNAL
CHECK VALVE TO / FROM TO / FROM
CYLINDER CYLINDER PRESSURE
LOAD CHECK COMPENSATOR
VALVE VALVE
FLOAT POSITION FLOAT POSITION
SOLENOID SOLENOID
VALVE VALVE

SHOCK REDUCING REGENERATION PUMP SUPPLY SPOOL PILOT OPERATED

CHECK VALVE VALVE PASSAGE LOCK VALVE

Full pilot oil shifts the spool fully to the right. The pilot oil also unseats the pilot operated lock
valve to allow the head ends of the cylinders to be connected to the tank.

In FLOAT, both float position solenoids are energized allowing the lift cylinder head ends to be
connected directly to the tank
STMG 767 - 65 -
1/03

TESTING AND ADJUSTING

FORMER SYSTEM VS. CURRENT SYSTEM

SIMILAR DIFFERENT
• Cycle Times • Signal Limiter
• Standby Pressure • Quick Coupler
• Pilot System Pressure • No Resolver Checks
• Maximum System Pressure • Pressure Cutoff Adjustments
• Margin Pressure
• 924G Combination Valve

IMPLEMENT HYDRAULIC SYSTEM TESTING AND ADJUSTING

Testing and adjusting procedures are very similar between the former and the current implement
hydraulic systems.

Maximum system pressure adjustments are slightly different on the current system due to the
addition of the signal limiter.

Also, if adjustments to the line relief valves with specifications above the maximum system
pressure are required, both the pressure cutoff and the signal limiter settings will have to be
increased. Make sure both the pressure cutoff and signal limiter are returned to their former
settings before returning the machine to operation.

NOTE: Since the specification for margin pressure and low pressure standby are almost
the same, only adjusting standby pressure is suggested if margin spool adjustments are
required.
STMG 767 - 66 -
1/03

QUICK
COUPLER

STEERING, BRAKE, AND STEERING

HYDRAULIC FAN SYSTEMS METERING
UNIT QUICK COUPLER
CONTROL

MANIFOLD

REAR PUMP
AUX. GROUP
BRAKES BRAKE STEERING
VALVE GROUP
M
FRONT
BRAKES

PUMP
AXLE OIL GROUP
COOLER
FAN REVERSING
MOTOR FAN CONTROL FAN CONTROL

COOLER

FILTER

STEERING, BRAKE, AND HYDRAULIC FAN SYSTEMS

The steering system consists of a Load Sensing/Pressure Compensated (LS/PC), variable

displacement pump, that is the same as the implement pump except for the pump control valve
pressure settings. The system uses a closed-center steering metering unit (SMU) to control the
steering cylinders.

An auxiliary steering pump and motor is available. The electric motor, when activated, causes
the auxiliary steering pump to provide additional supply oil for the steering system.

A separate gear-type pump provides oil to the brake, hydraulic fan, and optional axle oil cooler
circuits. The pump provides priority oil flow to the brake system. An external screen is used in
the pump inlet to reduce system contamination.

The hydraulic fan circuit has a fan control valve and may also be equipped with a reversing fan
control.

An optional heavy duty brake system with an axle oil cooler is available.
STMG 767 - 67 -
1/03

STEERING CIRCUIT
HOLD

QUICK
COUPLER

SHUTTLE
QUICK
COUPLER
CONTROL

STEERING
METERING
UNIT

AUX.
STEERING
GROUP
RELIEF
VALVE PUMP
GROUP
M

Steering System

In HOLD, the pump is at LOW PRESSURE STANDBY. The supply oil from the pump is
blocked along with oil in the steering cylinders by the SMU.
STMG 767 - 68 -
1/03

STEERING CIRCUIT
RIGHT TURN

QUICK
COUPLER

SHUTTLE QUICK
COUPLER
CONTROL

STEERING
METERING
UNIT

AUX.
STEERING
GROUP
RELIEF
VALVE PUMP
GROUP
M

When the steering wheel is turned, a passage to the cylinders is opened. A signal is sent to the
pump through the resolver. The pump increases flow to meet the steering flow needs.

The SMU meters oil to the steering cylinders to turn the machine.
STMG 767 - 69 -
1/03

STEERING AND PILOT PUMP

CONSTANT FLOW
PUMP
CONTROL
ACTUATOR VALVE
SIGNAL
PISTON
PRESSURE
BIAS
SPRING

ORIFICES
MARGIN
SPRING

MARGIN
SWASHPLATE TO CONTROL PRESSURE SPOOL
VALVE GROUP CUTOFF SPOOL

The steering and pilot pump control valve contains two spools. The margin spool regulates
output flow of the pump to keep the pump supply pressure at a fixed value above the signal
pressure. The difference between the supply pressure and signal pressure is called "margin
pressure." The pressure cutoff spool limits the maximum system pressure.

The pump is designed to maintain flow. Whenever the forces above and below the margin
spool are not balanced due to changes in the flow demand the pump will upstroke or destroke to
meet the flow demand.

The specification for low pressure standby is much higher for the steering pump than the
implement pump. This is due to the steering pump needing to provide a higher pressure for the
pilot system.

NOTE: Refer to the previous discussion on the implement pump operation for more
detailed information on the steering pump operation.
STMG 767 - 70 -
1/03

BRAKE BRAKE SYSTEM

REAR PRESSURE VALVE ACCUMULATORS CHARGING
SENSOR
BRAKES BRAKES RELEASED

BRAKE
ACCUMULATORS

FRONT INVERSE
BRAKES SHUTTLE PRESSURE
SENSOR

FLOW
CONTROL
MACHINE ECM CHECK
VALVE
TO FAN
CIRCUIT

SOLENOID
VALVE PUMP
GROUP

TRANSMISSION
SOLENOID
VALVES NEUTRALIZER SWITCH

Brake System

When the engine is first started or when the pressure sensor in the pump group senses low
brake pressure, the current from the Machine Electronic Control Module (ECM) is insufficient
to open the solenoid valve in the pump group. Pressure below the flow control valve shifts the
valve up preventing oil from flowing to the fan circuit. Flow from the pump is directed through
the check valve to the inverse shuttle valve.

The inverse shuttle valve directs the oil to the lower charged accumulator first. As the pressure
increases in this accumulator, the shuttle valve shifts and allows both accumulators to be
charged at equal pressures. As the accumulators charge, the pressure increases at the pressure
sensor. The pressure sensor communicates this information to the Machine ECM. At a
predetermined point the Machine ECM will increase the current to the solenoid valve, causing
it to open.

STMG 767 - 71 -
1/03

When the solenoid valve opens, oil below the flow control valve is drained to the tank.
Pressure above the spool moves the flow control valve down directing supply oil to the demand
fan circuit. The pump supply pressure drops in the passage to the check valve and the check
valve seats to maintain pressure in the accumulators. During this condition, the accumulator
charging portion of the system is in the "standby" mode.

After the accumulators are charged to the CUT-OUT pressure, as normal leakage in the system
occurs or as the brakes are applied, the inverse shuttle valve will shift left or right to direct the
lower accumulator pressure to the pressure sensor. The flow control valve directs pump oil to
the fan circuit.

A pressure sensor senses when the brakes are applied and sends a signal to the Machine ECM.
The Machine ECM neutralizes the transmission when it receives this input. If the transmission
neutralizer override switch is activated, the Machine ECM will not neutralize the transmission
when the brakes are engaged.
STMG 767 - 72 -
1/03

BRAKE BRAKE SYSTEM

REAR
PRESSURE VALVE ACCUMULATORS CHARGED
SENSOR
BRAKES BRAKES ENGAGED

BRAKE
ACCUMULATORS

FRONT INVERSE
SHUTTLE PRESSURE
BRAKES
SENSOR

FLOW
CONTROL
CHECK
MACHINE ECM
VALVE
TO FAN
CIRCUIT

SOLENOID
VALVE PUMP
GROUP

TRANSMISSION
SOLENOID
VALVES NEUTRALIZER SWITCH

When the operator depresses either brake pedal, the tandem brake valve meters oil from the
accumulators to ENGAGE the brakes.

Feedback oil pressure is sensed by each spool in the tandem brake valve to allow the operator
to feel the application of the brakes. The tandem brake valve acts as a variable pressure
reducing valve. The brake pressures will vary depending on how far the brake pedal is
depressed.

Due to the spring below the tandem brake valve, the maximum rear brake pressure is slightly
higher than the maximum front brake pressure.

When the accumulator pressure decreases below the CUT-IN pressure, the pressure sensor will
communicate this to the Machine ECM. The Machine ECM decreases the current to the
solenoid valve. The spring causes the valve to shift blocking flow to the tank. Pressure builds
below the flow control valve. The flow control valve moves up blocking oil to the fan and
cooler circuits. The charging cycle begins again.

The pressure sensor senses when the brakes are applied and sends a signal to the Machine
ECM. The Machine ECM neutralizes the transmission when it receives this input.
STMG 767 - 73 -
1/03

BRAKE CONTROL VALVE

BRAKES RELEASED BRAKES ENGAGED

SPRINGS

REAR BRAKE
SPOOL

FROM REAR TO REAR

BRAKES BRAKES

FRONT BRAKE FROM

SPOOL ACCUMULATOR

FROM FRONT TO FRONT

BRAKES BRAKES

FROM
ACCUMULATOR

SPRING

When the operator depresses either brake pedal (right view), the upper spring moves the two
brake spools down. The brake spools close the passages to the tank and open the passages from
the two accumulators. The oil from the rear accumulator flows through the rear brake spool to
ENGAGE the rear service brakes and to fill the chamber between the rear brake spool and the
front brake spool. The oil from the front brake accumulator flows through the front brake spool
to ENGAGE the front service brakes and to fill the chamber at the bottom of the front spool.

The pressure at the bottom of the front brake spool and the force of the spring move the front
brake spool up against the pressure in the chamber between the rear brake spool and the front
brake spool. The pressure in the chamber between the rear brake spool and the front brake
spool moves the rear brake spool up against the force on the brake pedal.

The force at the bottom of each brake spool balances that brake spool against the force at the
top of the brake spool. Each brake spool acts as a pressure reducing valve to limit the pressure
in the brakes proportionally to the force at the top of the respective brake spool.

When the brake pedal is RELEASED (left view), the oil passages from the accumulators to the
brakes are closed, and the passages from the brakes to the tank are opened. Oil in the rear and
front service brakes flows through the respective brake spools to the tank. Spring force at the
bottom of the lower brake spool moves the brake spools up.
STMG 767 - 74 -
1/03

HYDRAULIC FAN SYSTEM

COOLER PUMP
GROUP

BYPASS
MAKEUP FAN CONTROL FLOW
VALVE SOLENOID VALVE CONTROL

FILTER

ENGINE COOLANT TRANSMISSION OIL

TEMPERATURE SENSOR TEMPERATURE SENSOR

HYDRAULIC OIL AIR INLET

TEMPERATURE SENSOR MACHINE ECM TEMPERATURE SENSOR

Hydraulic Fan System

The Machine ECM will send the required signal to the hydraulic fan solenoid valve in order to
provide the proper fan speed for the cooling system. The Machine ECM will monitor the
following parameters in order to provide the proper fan speed:

- coolant temperature;

- hydraulic oil temperature;

- transmission oil temperature; and

- air inlet temperature.

The fan control solenoid valve controls the hydraulic fan speed by controlling the flow control
valve. The flow control valve bypasses some of the supply oil to the fan to the cooler. As more
oil is bypassed, the fan turns slower. The maximum fan speed is controlled by the fan control
solenoid valve.

STMG 767 - 75 -
1/03

At the rated engine rpm, the fan speed is maintained at a minimum of 600 rpm. The maximum
fan speed of approximately 1625 rpm is limited by the fan control solenoid valve. The return
oil from the fan motor is directed through the oil cooler and filter to the tank. Both the cooler
and the filter are equipped with bypass valves.

The makeup valve prevents cavitation in the fan motor. During a quick deceleration, the flow
of oil to fan motor can stop. The makeup valve will open. This allows oil to flow from the
outlet side of the fan motor to the inlet side of the fan motor.
STMG 767 - 76 -
1/03

REVERSING HYDRAULIC FAN SYSTEM

CROSSOVER REVERSING FLOW
COOLER RELIEF AND MAKEUP SOLENOID VALVE CONTROL

PUMP
GROUP

NORMAL

BYPASS
MAKEUP FAN CONTROL
VALVE SOLENOID VALVE

FILTER

FAN REVERSING FLOW

MOTOR SOLENOID VALVE CONTROL

PUMP
GROUP

REVERSING

MAKEUP FAN CONTROL

VALVE SOLENOID VALVE

The machine may be equipped with an optional reversing fan.

The fan reversing solenoid is controlled by the Machine ECM. When the top of the reversing
fan switch (not shown) is depressed, the fan reversing solenoid is energized in order to reverse
the fan for 10 seconds. Supply oil from the pump is directed to the other side of the fan motor,
while the other side of the fan motor is open to the cooler circuit.

When the bottom of the reversing fan switch is depressed, the fan reversing solenoid is
energized in order to reverse the fan for 10 seconds. This will repeat every 30 minutes until the
reversing fan switch is returned to the center position.

The crossover relief valve prevents cavitation in the fan motor. During a quick deceleration, the
flow of oil to fan motor can stop. The crossover relief valve will open. This allows oil to flow
from the outlet side of the fan motor to the inlet side of the fan motor.
STMG 767 - 77 -
1/03

CONCLUSION

This concludes the presentation on the updated steering, brake, fan, and implement hydraulic
systems for the 924G and 924Gz Caterpillar Wheel Loaders. The material presented covers the
systems operation portion of the different hydraulic systems on the machine.

These rubber tired machines provide Caterpillar customers with the versatility, performance,
and productivity they have come to expect from the Caterpillar wheel loaders.

A thorough understanding of the information provided in this Service Training Meeting Guide
can make troubleshooting, diagnosis, and testing easier and more accurate. However, before
servicing the different systems on these machines, always check the latest service information
to ensure that the most current specifications and test procedures are used.
STMG 767 - 78 -
1/03

SLIDE LIST
1. Model view 43. 924Gz lift valve - HOLD
2. 924G hydraulic system block diagram 44. 924Gz implement hydraulic system - RAISE
3. 924G hydraulic system 45. 924Gz lift valve - RAISE
4. Implement pump with signal limiter 46. 924Gz implement hydraulic system - LOWER
5. Implement pump - ENGINE OFF 47. 924Gz lift valve - LOWER
6. Implement pump - LOW PRESSURE 48. 924Gz implement hydraulic system - FLOAT
STANDBY 49. 924Gz lift valve - FLOAT
7. Implement pump - UPSTROKE 50. Test and adjust comparison
8. Implement pump - CONSTANT FLOW 51. Steering, brake and fan block diagram
9. Implement pump - DESTROKE 52. Steering system - HOLD
10. Implement pump - MAXIMUM PRESSURE 53. Steering system - RIGHT TURN
11. Pilot valve - HOLD 54. Steering and pilot pump - CONSTANT FLOW
12. Pilot valve - SHIFT 55. Brake System - ACCUMULATORS
13. Pilot valve - IN DETENT CHARGING
14. Implement hydraulic system - HOLD 56. Brake System - ACCUMULATORS
15. Control valve - HOLD CHARGED
16. Pressure compensator operation 57. Brake control valve
17. Implement hydraulic system - RAISE/DUMP 58. Hydraulic fan system
18. 924G tilt valve - HOLD 59. Reversing fan system
19. 924G implement hydraulic system - TILT 60. Model view
BACK
20. 924G tilt valve - TILT BACK
21. 924G implement hydraulic system - DUMP/
REGENERATION
22. 924G tilt valve - DUMP/ REGENERATION
23. 924G lift valve - HOLD
24. 924G implement hydraulic system - RAISE
25. 924G lift valve - RAISE
26. 924G implement hydraulic system - LOWER
27. 924G lift valve - LOWER
28. 924G implement hydraulic system - FLOAT
29. 924G lift valve - FLOAT
30. Auxiliary valve - HOLD
31. 924G implement hydraulic system - THIRD
FUNCTION ACTIVATED
32. Auxiliary valve - SHIFT LEFT
33. Auxiliary valve - SHIFT RIGHT
34. 924G quick coupler and control valve
35. Combination valve
36. Ride Control system
37. 924Gz Implement hydraulic system - HOLD
38. 924Gz tilt valve - HOLD
39. 924Gz implement hydraulic system - TILT
BACK
40. 924Gz tilt valve - TILT BACK
41. 924Gz implement hydraulic system - DUMP/
REGENERATION
42. 924Gz tilt valve - DUMP/ REGENERATION
1/03
STMG 767

Hydraulic Color Codes

Black - Mechanical connection. 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

- 79 -

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
Green - Tank, sump, or return oil pilot, signal or TC oil pressure.

Green/White Stripes - Scavenge oil Blue - Trapped oil

or hydraulic void

Brown - Lubricating oil

Handout No. 1
STMG 767 - 80 - Handout No. 2
1/03

924G/924Gz HYDRAULIC SYSTEM TESTS

DATE _ DEALER _

MACHINE MODEL ___________________ CUSTOMER ______________________________
NAME/CODE ________________________ JOB ______________________________________
MACH S/N (PIN) _____________________ APPLICATION ____________________________
S.M.U. _______________________________ SERVICEMAN ___________________________

Record Hydraulic System Pressures Below

HYDRAULIC SYSTEM TESTS BEFORE ADJUSTING AFTER ADJUSTING

SPEC INITIAL ADJUSTED
Pilot Pressure

Margin Pressure

Low Pressure Standby

Signal Limiter

Max. Pressure ( Signal Limiter)

Pressure Cutoff

Pump Case

Record Steering System Pressures Below

STEERING SYSTEM TESTS BEFORE ADJUSTING AFTER ADJUSTING

SPEC INITIAL ADJUSTED

Margin Pressure

Low Pressure Standby

Pressure Cutoff

Pump Case

Record Brake System Pressures Below

BRAKE SYSTEM TESTS BEFORE ADJUSTING AFTER ADJUSTING

SPEC INITIAL ADJUSTED

Accumulator Charge
Accumulator Pre-Charge

Rear Brakes

Front Brakes
SERV1767 Printed in U.S.A.
01/03

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