Manitowoc Published 10-01-2012, Control # 045-08 1-31
777 SERVICE MANUAL INTRODUCTION
1
Boom Raise
See Figures 1-18 and 1-19 for following procedures.
When the boom control handle is off or in neutral, the PC
monitors the boom cylinder pressure sender for the pressure
required to support the boom. This supporting pressure is
the pressure memory.
The boom control handle neutral switch closes when the
handle is moved back for up operation. An analog input of 5
volts or greater 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 boom pump in the
up direction. When the hydraulic pressure in the system
equals the pressure memory of the boom cylinders, the PC
allows boom control handle operation. The boom pump
circuit operates only if the boom park switch is in the off
position, the boom up 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.
High pressure oil flows through each check valve of each
counterbalance valve and enters the rod end of each
cylinder. The oil in the piston end of the cylinders exhaust to
the boom pump inlet and also through hot oil shuttle valve to
pressure relief valve.
The PC controls the boom raising 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 back, pumping
more oil in the rod end of the boom cylinders and raising 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.
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