In HOLD, the speed and direction control lever is centered. No signal is sent from the control
lever to the Machine ECM. The pump control valves are held in the center position by spring
force and no pressure oil is directed to the actuator pistons. With no signal pressure oil to
either end of the actuator pistons, the pistons stay in the centered or neutral position.
The centering springs for the actuator pistons hold the swashplates at zero angle. With the
swashplates in this position, the hydrostatic drive pump group does not produce oil flow.
Without output from the hydrostatic drive pump group to turn the drive motors, the machine
does not move.
The crossover relief and makeup valves protect the system by either maintaining a minimum
drive loop pressure or reducing pressure spikes in the drive loops. At machine start-up, the
makeup valves open to direct charge oil to both sides of the pump and motor. The oil in both
lines to the drive motors are approximately at charge pressure.
The main purpose of the travel motor flushing valve is to drain used hot oil from the low
pressure side of the drive loop to case drain, which purges heat and contamination from the
drive loop. The relief valve in the flushing valve also controls the minimum pressure in the
low pressure side drive loop when the machine moves.
SERV1833-01 - 55 - Text Reference
07/08