JLG 120HX Service Manual - Manual (Page 31)

SECTION 2 - PROCEDURES
3120819 – JLG Lift – 2-9
replacement, or cylinder rod is scored and cylinder
requires overhaul or replacement. If boom has
retracted and oil is leaking from counterbalance
valve, the valve is either improperly adjusted, or
defective and requires replacement.
9. Connect hydraulic lines to control valve.
Lift Cylinder
NOTE: Switches referenced in this procedure are located on
the Ground Control Panel.
1. Activate hydraulic system, properly set extendable
axles and position boom in stowed position; adhere
to all safety precautions.
NOTE: Tape measure or cord should be at least 2.1 meters
(7 feet) long for use in this test.
2. Attach tape measure or cord to bottom of platform.
BEFORE RAISING BOOM, ENSURE THAT AREAS ABOVE AND
BELOW BOOM AND PLATFORM ARE CLEAR OF ALL OBSTRUC-
TIONS AND PERSONNEL.
3. With boom fully retracted, place LIFT control switch
to UP and hold until platform is approximately 6 feet
(2 meters) above ground level. Shut down engine.
4. Tag and carefully disconnect hydraulic lines to lift
cylinder at control valve. Use a suitable container to
retain any residual hydraulic fluid.
5. Observe oil flow from cylinder lines. Oil leaking from
extend port hose indicates a leaking counterbalance
valve. Oil leaking from retract port hose indicates
leakage by cylinder piston.
6. Leave boom elevated in test position for approxi-
mately one (1) hour.
7. With tape measure or cord used for reference,
check to see whether boom has lowered (crept)
more than 7.6 cm (3 inches).
8. If boom has lowered and oil is leaking around rod-
end cap of cylinder, seals in cylinder are defective
and require replacement. If boom has lowered and
oil is leaking from the counterbalance valve, the
valve is either improperly adjusted or defective and
requires replacement.
E N S U R E T H AT H Y D R A U LI C L I N E S A R E C O N N E C TE D A S
MARKED PRIOR TO BEING DISCONNECTED.
9. Connect hydraulic lines to control valve.
2.9 CYLINDER CHECKING PROCEDURES
NOTE: Cylinder checks must be performed any time a cylin-
der component is replaced or when improper system
operation is suspected.
Cylinder w/o Counterbalance Valves - Steer
Cylinders, Master Level Cylinder, Frame
Jack Cylinders.
OPERATE FUNCTIONS FROM GROUND CONTROL STATION
ONLY.
1. Using all applicable safety precautions, activate
engine and fully extend cylinder to be checked. Shut
down engine.
2. Carefully disconnect hydraulic hose from retract port
of cylinder. There will be initial weeping of hydraulic
fluid which can be caught in a suitable container.
After the initial discharge, there should be no further
leakage from the retract port.
3. Activate engine and activate cylinder extend func-
tion. Check retract port for leakage.
4. If cylinder leakage is 6-8 drops per minute or more,
piston seals are defective and must be replaced. If
cylinder retract port leakage is less than 6-8 drops
per minute, carefully reconnect hose to retract port
and retract cylinder.
5. With cylinder fully retracted, shut down engine and
carefully disconnect hydraulic hose from cylinder
extend port.
6. Activate engine and activate cylinder retract func-
tion. Check extend port for leakage.
7. If cylinder leakage is 6-8 drops per minute or more,
piston seals are defective and must be replaced. If
extend port leakage is less than 6-8 drops per
minute, carefully reconnect hose to extend port,
then activate cylinder through one complete cycle
and check for leaks.