INTRODUCTION 999 SERVICE/MAINTENANCE MANUAL
1-24
Published 05-16-17, Control # 233-03
The PC sends a positive output signal to stroke the boom
hoist pump momentarily in the up direction to satisfy
pressure memory. The PC compares boom hoist-holding
pressure to value stored in pressure memory. When system
pressure is high enough, the PC sends a positive output
signal to enable brake release solenoid HS-13. Brake
release solenoid HS-13 shifts, to block tank port and open
port from low pressure side of closed-loop to boom hoist
brake cylinder spring, releasing brake (2).
The PC sends a negative output signal to enable boom hoist
pump EDC that tilts pump swashplate to stroke pump in the
down direction. Hydraulic fluid flows from hoist pump port B
to port A of hoist motor (13). Fluid from motor port B returns
to boom hoist pump port A.
The PC controls lowering speed by varying the voltage to
pump EDC and motor PCP in relation to handle movement.
As boom hoist handle is pushed forward, the PC sends a
signal to increase the boom hoist pump swashplate angle.
The PC also sends a signal to reduce the displacement of
the motor. The PC is continuously balancing the system
pressure and the motor displacement angle so the motor
displacement goes to minimum when handle is all the way
back, if the torque on the motor is not too high. Knowing the
displacement of the motor, the PC may then control the
motor speed by regulating the flow through the pump.
The weight of boom attempts to drive motor faster than
return fluid can return to low-pressure side of pump. System
charge pump maintains fluid supply at a positive pressure to
motor. Pump swashplate position restricts the returning fluid
flow. Pressure builds on fluid return side of closed-loop,
acting as a hydraulic brake to control lowering speed.
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