JLG G12-55A Service Manual - Manual (Page 124)

Hydraulic System
8-4 G10-55A, G12-55A
8.3.1 Pressure Checks and Adjustments
When diagnosing trouble in the hydraulic system, use
the hydraulic testing information in Section 8.4.1,
“Hydraulic Pressures.”
In general, follow the steps below whenever conducting
pressure checks and performing adjustments:
1. Park machine on a firm, level surface. Engage park
brake, place transmission control lever in (N)
NEUTRAL, level boom and turn engine OFF.
2. Locate proper test port. Install a pressure gauge
capable of measuring at least 10% more pressure
than the circuit being checked operates under.
3. Start engine. Operate machine functions several
times to allow hydraulic oil to reach operating
temperature. The hydraulic oil temperature should
be a minimum of 130°F (54° C). If a temperature
gauge or thermometer is unavailable, hydraulic oil
reservoir should be warm to touch.
4. Refer to Section 8.4.1, “Hydraulic Pressures.” for
testing procedures.
5. Fully depress accelerator pedal if required. Place
and hold joystick in position needed to operate
particular machine function being checked. Continue
holding joystick in position until pressure readings
are taken.
6. Check pressure gauge reading. It should read as
specified in the Pressure Readings column of the
charts found in Section 8.4.3, “Pressure
Specifications,” If the reading is not as specified,
turn the engine OFF and check other components in
the system. Verify that all related hydraulic
components and electrical switches, sensors,
solenoids, etc. are operating correctly.
7. Adjust relief valve by turning adjustment screw
Turning clockwise will increase pressure; turning
screw counterclockwise will decrease pressure.
8. Start engine and check pressure again. Turn engine
OFF. If there is pressure reading in gauge, bleed it
off then disconnect or remove the pressure gauge
from the machine.
8.4 HYDRAULIC CIRCUITS
This section covers the hydraulic circuits and includes
listings for all hydraulic function pressures, where and
how to check those pressures and a hydraulic schematic.
Electrical and hydraulic functions are often related. Verify
that electrical components of the circuit are functioning
properly whenever troubleshooting hydraulic circuit.
Always check following before beginning to troubleshoot
a circuit that is not functioning correctly.
1. Check hydraulic oil reservoir level. Oil level should be
to middle of sight glass with all cylinders retracted.
2. Check hoses, tubes, fittings and other hydraulic
components for leaks, bends, kinks, interference, etc.
3. Check for air in hydraulic system. Erratic machine
performance and/or spongy cylinder operation are
signs of air in hydraulic system.
If air in hydraulic system is suspected, you will hear
air leakage when hydraulic fittings are loosened and
see air bubbles in hydraulic fluid.
Loose fittings, faulty o-rings or seals, trapped oil,
leaks, system opened for service, etc., can cause air
in system. Determine what is causing air to enter
system and correct it. Bleed air from system.
8.4.1 Hydraulic Pressures
a. Checking Pressure
1. Start the machine and warm the hydraulic system to
operating temperature.
2. Shut off machine and install a digital or a 5000 psi
(345 bar) gauge to appropriate test port on the
hydraulic manifold.
3. Start machine, run engine at high idle and bottom
appropriate hydraulic function. Refer to Section
8.4.3, “Pressure Specifications,” for the correct
pressure rating.
b. Adjusting Hydraulic Pressure
1. Shut the machine off. Remove the cap on the relief
(if necessary).
2. Start machine and loosen the jam nut on the relief.
Turn the relief clockwise to increase pressure or
counter-clockwise to decrease pressure. Set to the
correct pressure.