Hydraulic Reservoir Replacement; Hydraulic System Pump; Pump Failure Analysis - JLG sky trak mmv Service Manual

Occasionally, fluid may seep, leak or be more forcefully
expelled from the filter head (6) when system pressure
exceeds the rating of the filter head or breather. If the re-
turn filter becomes plugged, return hydraulic oil will by-
pass the filter when pressure reaches 25 psi
(1,7 bar) and return to the reservoir unfiltered.
Carefully examine fluid seepage or leaks from the hy-
draulic reservoir to determine the exact cause. Clean the
reservoir and note where any seepage occurs.
DANGER:
or around the hydraulic reservoir. Hydraulic
fluid and fumes can cause the reservoir to
explode. This can result in death or serious
personal injury.
Leaks from a cracked or damaged reservoir require that
the reservoir be removed from the vehicle, flushed com-
pletely with water and repaired by a certified welder using
approved techniques. If these conditions cannot be met,
the reservoir must be replaced in its entirety. Contact
JLG or the nearest JLG Authorized Service Center
(ASC) should reservoir welding or replacement be re-
quired.
9.10.1

Hydraulic Reservoir Replacement

To remove the hydraulic reservoir, first remove the en-
gine and transmission assemblies, along with all brack-
ets, hoses, wiring, etc. Refer to Section 8.10, Engine
Replacement and Section 7.7.1, Transmission Removal .
After the engine and transmission assemblies and all re-
lated brackets, hoses, wiring, etc. are removed, the hy-
draulic reservoir must be supported with a suitable device
and unbolted from the vehicle frame.
Install the replacement reservoir (1) and secure it to the
vehicle with four 1-8 X 3" Grade 8 capscrews (3),
1" hardened washers (4) and 1-8 locknuts (5). Torque the
nuts to 680 lb/ft (922 Nm).
9.11

HYDRAULIC SYSTEM PUMP

The gear pump (7) is attached at the rear of the transmis-
sion and driven by the pump main shaft via meshing with
an internal transmission gear.
The pump draws fluid through a 2" inside diameter hose
via negative displacement.
Model MMV Rev. 12/04
NEVER weld in, on, near
7
Fluid is forced from the front section of the pump through
a 3/4" inside diameter hose to the unloader valve. The
rear section of the pump forces fluid through a 3/4" inside
diameter hose to the accumulator charge valve. Hydrau-
lic 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 trans-
mission input shaft (and dependent on engine rpm), the
internal pump gears and passageways and the differen-
tial between pump inlet (2" I.D.) and outlets (3/4" I.D.)
openings, all contribute to pressure supplied to the un-
loader valve and accumulator charge valve which sup-
plies pressure to the system. The unloader valve
ultimately regulates maximum system-operating pressure
of 3500 psi (241 bar) for various vehicle functions. The
accumulator charge valve regulates the steering pres-
sure at 2500 ±100 psi (172,3 ±7 bar) and maintains ser-
vice brake accumulator pressure. The accumulator
charge valve also supplies pressure to the secondary
function manifold.
9.11.1

Pump Failure Analysis

The pump is the "heart" of the hydraulic system, and
whenever there is a problem in the system, the pump of-
ten is blamed. Yet, pump failure is seldom due to failure
of pump components. Pump failure usually indicates an-
other problem in the hydraulic system.
According to pump manufacturer statistics, 90-95 percent
of pump failures are due to one or more of the following
causes:
• Aeration
• Cavitation
• Contamination
• Excessive Heat
• Over-Pressurization
• Improper Fluid
In the event of pump failure, investigate further to deter-
mine the cause of the problem.
Hydraulic System
MM3191
9.87