5-8 673831-Rev A
HYDROSTATIC POWER TRAIN
5
Theory of Operation and
Sub-Circuit Schematics
Traction Circuit—Neutral Schematic
and Theory of Operation
System Conditions:
• Engine running
• Parking brake switch ON
• Traction pedal in the NEUTRAL position
Charge Pressure Circuit
The traction pump and gear pump are both driven by the
engine. Section 2 of the gear pump provides operating
pressure oil for the rear reel valve, 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
three pump sections. Operating pressure oil from section
2 of the gear pump is routed to port “P” of the rear reel
valve and out port “T” to the charge filter and the brake
valve port “CHG”.
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 (1.7 bar) pressure indicator is installed
on the filter head, and a bypass valve will open to provide
oil to the traction pump. Charge pressure oil is then
routed to the traction pump.
The traction pump includes a charge relief valve that will
open to dump oil to the case drain when the charge
pressure reaches 200 psi (14 bar).
Brake Circuit
When the parking brake switch is in the ON position, the
brake valve solenoid coil is not energized. The charge
pressure flow is blocked at the solenoid valve. Oil from
left and right front motors Port X are allowed to flow
through the needle valve, solenoid valve and to the tank.
The brake spring in the wheel motors puts pressure on
the friction discs, preventing the wheel motors from
rotating.
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,
wheel motors, 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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