Section 8. Theory of Operation
Model 4700 Maintenance Manual
Hydraulic System
Hydraulic System Lift/Lower NOTE: A priority valve is used to ensure adequate hydraulic pressure to the steering system at all times. If adequate flow is not available for steering, the priority valve diverts fluid from any other requested function.
Standard Lever Controls Lift The VM monitors the lift control lever switch (SW11) and the voltage from the wiper of the lift pot (P3). When lift is requested, SW11 closes, which is sensed by the VM. The VM checks battery voltage (for lift reduction) and the Lift Limit option that could prevent activation of the lift system. Output voltage from the lift pot is also sensed by the VM. It is compared to the previously Learned value of the pot and the VM converts the voltage value into a percentage of total request. The Pump Power Amplifier (PPA) receives the lift request via the CAN-Bus. The PPA generates the correct voltage to the pump motor to run the pump at the requested RPM. From the pump, fluid flows first to the priority valve. When the lift/lower valve is in the neutral position, fluid bypasses the lift/lower valve and is either available for other functions or returned to the reservoir. When lift is requested, fluid flows to the lift cylinders. Lift motor RPM is determined by the position of the lift lever, which determines the output voltage of P3. The center cylinders elevate first because less pressure is required. Once the inner cylinders reach the stops, the pressure increases enough to start elevating the outer cylinders. Lower When lower is requested, there is no electrical response from the truck. When the valve is stroked to the lower position, fluid returns to
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Lift/Lower the reservoir. The rate of lower is dependent on the position of the lever, but is restricted to a maximum rate of 115 ft./min. by a flow restrictor valve located in the distribution manifold at the base of the main frame, between the lift cylinders. Flow controls in each cylinder restrict lowering speed.
Optional Fingertip Controls Lift The Armrest Board monitors the lift switch (SW17) and the voltage from the wiper of lift/lower pot (P4). When lift is requested, SW17 closes. This is sensed by the Armrest Board. The Armrest Board encodes the lift request into a digital message and communicates it to the VM via the CAN-Bus. The VM checks the battery voltage (for lift reduction) and the Lift Limit option, which could prevent the activation of the lift system. Output voltage from P4 is interpreted by the Armrest Board. The Armrest Board converts the voltage value into a percentage of total request. The PPA receives the lift request via the CAN-Bus. The PPA generates the correct voltage to the pump motor to run the pump at the requested RPM. From the pump, fluid flows first to the priority valve. The lift request is also sent to the Hydraulic Control Board. The Hydraulic Control Board interprets the message and provides controlling voltage to the lifting solenoid EV2, causing EV2 to open, providing fluid to the lift cylinders. The Hydraulic Control Board also energizes the proportional solenoid valve, EVP. This valve opens proportionally to the lift request from P4 and provides a path back to the reservoir for the fluid not used by the lift cylinders. For maximum lift, EVP is fully energized. When the lift solenoid valve is de-energized, fluid bypasses the lift/lower valve and is either available for other functions or returned to the reservoir through the proportional lift solenoid valve (EVP). Motor RPM and position of the EVP are determined by the position of the fingertip control, which determines the output of P4. The center cylinders elevate first because less
Publication 1160093, Issued: 20 May 2013
