HYDROSTATIC POWER TRAIN
4203780 First Edition 5-7
5
Theory of Operation and
Sub-Circuit Schematics
Traction Circuit Neutral Schematic
and Theory of Operation
System Conditions:
• Engine running
• Park brake released
• Traction pedal in NEUTRAL position
Charge Pressure Circuit
The traction pump and gear pump are both driven by the
engine. Section 1 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.
The 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
port “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 Pump Circuit
With the traction pedal in the NEUTRAL position, the
swash plate is in a neutral position; therefore, oil simply
flows through the traction pump without an increase in
pressure. Charge pressure oil flows to the relief valves,
front axle motor, and ports “A” and “B” of the 4WD valve.
Since oil pressure is equal on both sides of the front and
rear drive motors, no movement occurs.
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