JLG sky trak mmv Service Manual page 328

Hydraulic System
Before troubleshooting any
hydraulic circuit, ALWAYS
begin by checking the
following:
1. Hydraulic oil level in the
reservoir
2. All hoses, tubes, fittings
for leaks, kinks,
interference, etc.
3. Air in the lines
NOTE:
When checking pressures in
circuits, refer to Section 9.7,
Hydraulic System Testing for
correct pressure readings.
9.30
If the lift/lower cylinder
does not raise, check the
pilot-operated check valve
on the lift/lower cylinder.
If OK
Check pilot pressure at the
main control valve.
Check solenoid valve cartridge
in pilot select manifold.
If OK
Check port relief at
main control valve.
If OK
Check pilot pressure at
"PPG" port of secondary
function manifold.
If OK
Check main relief pressure at
"VG" port of unloader valve.
If OK
Check differential pressure
sense valve on unloader valve.
If OK
Check pump flow.
If OK
Check packing on
lift/lower cylinder.
If the lift/lower cylinder
drifts or reacts slowly, check
counterbalance valve and
pilot-operated check valve
on lift/lower cylinder.
If OK
Check lift/lower cylinder
packing.
Replace pilot-operated
Not
OK check valve.
Refer to Section 10.7.19,
Not
Boom Lift/Lower Circuit and
OK
Troubleshooting .
If OK
Not
Replace solenoid cartridge.
OK
Adjust/repair/replace relief
Not
valve on lift/lower section of
OK
main control valve.
Adjust/repair/replace
Not
pressure reducing valve on
OK
secondary function manifold.
Adjust/repair/replace relief
Not
OK valve on unloader valve.
Adjust/replace differential
Not
pressure sense valve on
OK
unloader valve.
Not
Repair/replace pump.
OK
Repair/replace lift/lower
Not
OK cylinder.
Replace counterbalance
valve and/or
Not
OK pilot-operated check
valve on lift/lower cylinder.
Repair/replace lift/lower
Not
OK cylinder
Model MMV Rev. 12/04