Model 4700 Maintenance Manual

Section 8.Theory of Operation

Emergency Lower Hydraulic System pressure is required. Once the inner cylinders reach the stops, the pressure increases enough to start elevating the outer cylinders.

Lower

The Armrest Board monitors the lower switch (SW18) and the voltage from the wiper of lift/lower pot (P4). When lower is requested, SW18 closes and P4 voltage changes, which is sensed by the Armrest Board. The lower request is encoded into a digital message that is communicated to the Hydraulic Control Board via the CAN-Bus. The Hydraulic Control Board interprets the message and provides controlling voltage to the lowering solenoid EVP1. This causes EVP1 to open proportionally to the request and allows fluid from the cylinders to return to the reservoir. The rate of lower is dependent on the position of the fingertip control, which determines the output of P4, 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.

Emergency Lower

An emergency lower valve, located at the base of the solenoid valve assembly, allows the carriage to be lowered in the event of a loss of power to the truck. See Figure8-1.

When the emergency lower valve is opened, hydraulic fluid is allowed to pass through the emergency lower valve at a constant rate.

Publication 1160093, Issued: 20 May 2013