Manitowoc Published 10-01-2012, Control # 045-08 1-33
777 SERVICE MANUAL INTRODUCTION
1
Boom Lower
See Figures 1-20 and 1-21 for following procedures.
When the boom control handle is off or in neutral, the PC
monitors the boom cylinder pressure sender for pressure
level required to support the boom. This pressure level is the
pressure memory.
The boom control handle neutral switch closes when the
handle is moved forward for down operation. An analog input
of 5 volts or less is sent to the PC and processed. The PC
sends a regulated voltage to the boom pump EDC. The
boom pump swashplate tilts to stroke the pump in the down
direction. When the hydraulic pressure in the system equals
the pressure memory of the boom cylinders, the PC allows
boom control handle operation. The pump circuit operates
only if the boom park switch is in the OFF position, the boom
down limit switch is closed, and there are no shut down
operating faults present.
Boom pump pressure sender supplies the PC with pressure
information that the PC compares to the load holding
pressure, monitored by the boom cylinder pressure sender.
When the monitored pressure is enough to hold the load, the
PC closes the circuit to the boom load holding solenoid valve
HS18. The valve shifts to block the tank ports and supply
pilot pressure from the boom charge pump to the load
holding valve at the rod end of each boom cylinder. The pilot
pressure shifts the two position boom load holding valve and
allows oil to exit the rod end of the cylinders. The oil in the
rod end of the cylinders exhaust to the boom pump inlet and
also through hot oil shuttle valve to pressure relief valve.
The weight of the boom attempts to extend the boom
cylinders when lowering. The PC controls the boom lowering
speed by varying the voltage to the boom pump EDC in
proportion with control handle movement. The pump
swashplate tilt angle is increased as the control handle is
moved forward, pumping more oil in the piston end of the
boom cylinders and lowering the boom faster. As the boom
control handle is returned to the off or neutral position, the
PC decreases the boom pump swashplate tilt angle. This
change in swashplate tilt angle reduces pump output oil flow
and slows boom movement. Replenishing oil is supplied to
the circuit by the accessory pump. When the boom control
handle is off or in neutral, the PC monitors the boom cylinder
pressure sender for information required to support the
boom. The pressure level is the pressure memory.
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