HYDROSTATIC POWER TRAIN
4203780 First Edition 5-19
5
Traction Circuit—Forward 4WD
Schematic and Theory of Operation
System Conditions:
• Engine running
• Park brake released
• Traction pedal in FORWARD position
• 2WD/4WD switch in 4WD position
• Differential lock pedal in LOCKED position
(differential lock mode only)
Charge Pressure Circuit
The traction pump and gear pump are both driven by the
engine. One section of the gear pump provides operating
pressure oil for the steering and lift systems, and charge
pressure oil for the traction pump.
Oil is drawn from the hydraulic tank through a 100-mesh
screen, and enters the gear pump, supplying oil to all four
pump sections. Operating-pressure oil from section 1 of
the gear pump is routed to port “P” of the control valve,
flows through the 4.5-gpm priority valve, and out port
“ST” to the steering valve.
Oil flows through the steering valve to the charge filter.
The charge filter uses a 10-micron filter to remove debris
from the oil. In the event of the charge filter becoming
plugged, a 25-psi pressure switch will activate the
“Charge Filter” light on the control panel, and a bypass
valve will open to provide oil to the traction pump and
other systems. Charge-pressure oil is then routed to
ports “B” and “C” of the 4WD valve, port “F” of the control
valve, and the traction pump.
The control valve also incorporates a charge pressure
switch that will activate the “Charge Pressure” light on the
control panel if the charge oil pressure drops below 125
psi. The control valve also includes a charge relief valve
that will open to dump oil to the hydraulic tank when the
charge pressure reaches 250 psi.
Traction Drive Circuit
When the traction pedal is moved to FORWARD position,
the swash plate is moved, stroking the pump. This
change of the swash plate angle produces an increase of
oil flow on one side of the pump. Oil at operating pressure
is then routed against the check valve and relief valve
(3500 psi), and exits the traction pump through port “A.”
Oil is then routed to the front axle drive motor, driving the
motor. Operating-pressure oil is also routed to port “A” of
the 4WD valve. Oil exiting the front axle motor is returned
to the traction pump to replenish the intake side of the
pump.
When the 2WD/4WD switch is placed in the 4WD
position, the 4WD solenoid in the control valve shifts,
allowing oil to flow through the valve. Oil is then routed to
the pilot line of the secondary 4WD shuttle valve in the
4WD valve, causing the valve to shift. Operating-pressure
oil from port “A” flows through the 4WD shuttle valve
through ports “R1” (right motor) and “L1” (left motor) to
the right and left rear wheel motors, driving the rear
wheels.
Oil exiting the rear wheel motors enters the 4WD valve at
ports “R2” (right motor) and “L2” (left motor), and flows
through the 4WD reverse solenoid valve, exiting the 4WD
valve at port “C.” A check valve will also open, allowing oil
to exit the 4WD valve at port “B.” Oil exiting the rear wheel
motors is returned to the traction pump to replenish the
intake side of the pump.
Circuit Relief
The traction circuit is protected by a relief valve. The relief
valve will open if the pressure rises to 3500 psi, allowing
oil to dump to the charge side of the circuit.
Differential Lock
Charge-pressure oil is supplied to port “P” of the control
valve by section 1 of the gear pump, and is routed to the
pilot line of the charge relief valve and the differential lock
valve. When the differential lock pedal is depressed, the
solenoid engages, shifting the valve. This allows
charge-pressure oil to flow to the differential lock cylinder,
engaging the differential lock.
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