Section 8. Theory of Operation Model 8210/8250 Maintenance Manual
Lift/Lower System
8-8 Publication: 1146945/001, Revised: 25 Sep 2018
Lift/Lower System
The lift/lower system consists of an electrically
operated hydraulic pump assembly and related
components.
The hydraulic pump assembly consists of a
positive displacement rotary gear pump with
reservoir mounted to an adapter. A DC electric
motor is mounted to the opposite side of the
pump adapter. An adjustable relief valve, check
valve, and a solenoid operated load holding
valve are installed within the adapter.
With the forks elevated, the normally closed
solenoid valve and the check valve prevent
hydraulic fluid from returning to the reservoir.
Lift
When the battery is connected, the truck is ON,
and the lift button is pressed:
• A lift request is detected by the VM via hall
effect sensors. The VM checks the battery
state-of-charge to verify lift is allowed. The
request for lift is then converted into a
CAN message and sent to the Traction
Amplifier.
• The TA checks the state of the Main
Contactor (MPC) and closes it if required.
• The TA supplies B– to the lift pump
contactor (LPC) and it closes, applying B+
to the lift pump motor (MP).
• Hydraulic fluid is drawn into the lift pump.
• As the pump rotates, oil is forced out the
pressure port through the lift hose to the
lift cylinder. Oil cannot return to the
reservoir because of the closed check valve
and closed load holding solenoid valve.
• Hydraulic pressure in the lift cylinder lifts
the forks.
• The relief valve opens if the hydraulic
pressure exceeds the preset limit while
lifting.
• Once maximum lift is reached and the lift
button remains pressed, the motor pump
will continue to pump hydraulic fluid
through the relief valve and back into the
reservoir until the lift timer expires. The
lift timer starts when lift is requested and
lasts until the lift request is removed or six
seconds passes, whichever occurs first.
When lower is requested, the lift timer
counts down for 1.8 seconds. If lower is
requested for 1.8 seconds or less the lift
timer counts back up to six seconds
starting at the remaining time (6 seconds
minus the time lowered). If lower is
requested for more than 1.8 seconds, the
lift timer resets at zero and allows a lift
request for the full 6 seconds.
When the lift button is released:
• The pump contactor coil (LPC) is
de-energized. This stops the lift motor and
pump. The forks are held in position by
hydraulic fluid trapped in the cylinder by
the check valve and the closed load
holding solenoid valve (SOL1).
Lower
When the battery is connected, the truck is ON,
and the lower button is pressed:
• A lower request is detected by the VM via
hall effect sensors. The request for lower is
converted into a CAN message and sent to
the TA.
• Load holding solenoid valve (SOL1) opens.
• Hydraulic fluid in the lift cylinder returns
to the hydraulic reservoir through the load
holding valve and the flow control valve.
The lowering speed is regulated by the flow
control valve.
• The forks lower.
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