JLG sky trak mmv Service Manual page 324

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.
9.26
If the extend/retract cylinder
does not retract, check the
pilot-operated check valve on
the extend/retract 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
extend/retract cylinder.
If the extend/retract cylinder
drifts or reacts slowly, check
counterbalance and pilot-
operated check valves on the
extend/retract cylinder.
If OK
Check extend/retract
cylinder packing.
Replace the pilot-operated
Not
check valve.
OK
Refer to Section 10.7.18,
Not
Boom Extend/Retract Circuit
OK
and Troubleshooting .
If OK
Not
Replace solenoid cartridge.
OK
Adjust/repair/replace the relief
Not
valve(s) on extend/retract
OK
section of 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
Not
OK extend/retract cylinder.
Replace counterbalance and/or
Not
pilot-operated check valves on
OK
the extend/retract cylinder.
Repair/replace
Not
extend/retract cylinder.
OK
Model MMV Rev. 12/04