Lull 944E-42 Service Manual - Manual (Page 417)

8.165 Model 644E-42/944E-42 Rev. 6/04
Hydraulic System
Occasionally, fluid may seep, leak or be more forcefully
expelled from the filter head when system pressure
exceeds the rating of the filter head or breather. If the
return filter becomes plugged the indicator light will come
ON, indicating that return hydraulic oil will bypass the
filter when pressure reaches 25 psi (1,7 bar) and return to
the reservoir unfiltered.
Carefully examine fluid seepage or leaks from the
hydraulic reservoir to determine the exact cause. Clean
the reservoir and note where any seepage occurs.
Leaks from a cracked or damaged reservoir require that
the reservoir be removed from the vehicle and must be
replaced in its entirety. Contact your local Lull distributor
or the JLG Service Department, should reservoir
replacement be required.
8.10.1 Hydraulic Reservoir Replacement
1. To remove the hydraulic reservoir Refer To Section
7.10.1, “Engine Removal,” for proper procedure on
hydraulic reservoir removal.
8.11 HYDRAULIC SYSTEM PUMP
The gear pump (4) is attached at the rear of the
transmission (5) and driven by the pump main shaft via
meshing with an internal transmission gear.
The pump draws fluid through a 2" inside diameter tube
via negative displacement.
Fluid is forced from the secondary section of the pump
through a 3/4" inside diameter hose to the Main Control
Valve. The primary section of the pump forces fluid
through a 3/4" inside diameter hose to the Accumulator
Charge/Secondary Function Valve. Hydraulic system
pressure begins at the pump. Various factors are
involved in creating the relatively high pressure used in
the hydraulic system. Pump rpm, controlled via a
transmission input shaft (and dependent on engine rpm),
the internal pump gears and passageways and the
differential between pump inlet (2" I.D.) and outlets (3/4"
I.D.) openings, all contribute to pressure supplied to the
main control valve and Accumulator Charge/Secondary
Function Valve which supplies pressure to the system.
The main control valve ultimately regulates maximum
system-operating pressure of 3400 psi (234 bar) for
various vehicle functions. The Accumulator Charge/
Secondary Function Valve regulates the steering
pressure at 2500 ±100 psi (172,3 ±7 bar) and maintains
service brake accumulator pressure. The Accumulator
Charge/Secondary Function Valve also supplies pilot
pressure to the joysticks.