JLG 450A Service And Maintenance Manual page 87

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TROUBLE
Valve spool sticking.
Valve leaking.
No response to lift control switch.
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Table 3-2.Boom Assembly - Troubleshooting
PROBABLE CAUSE
CONTROL VALVES
Dirt in oil causing excessive temperature build-
up.
Moisture in oil.
Incorrect valve mounting causing warping of the
unit.
Valve spool scored.
Tie-bolts in valve over torqued.
Return spring weak or broken.
Relief valve malfunctioning causing excessive
pressure within valve.
Dirt or other foreign material under seal.
Valve spool scored.
Excessive back pressure caused by restricted
return line to reservoir.
Damaged valve seals.
BOOM ELEVATION SYSTEM.
Lift function not activated within 7 seconds after
footswitch was depressed.
Lift control switch inoperative.
Lift cylinder holding valve inoperative.
Dump valve (bypass) not operating.
Electrical malfunction.
Hydraulic system oil low.
Restricted or broken supply line on valve bank or
hydraulic pump.
– JLG Lift –
SECTION 3 - TROUBLESHOOTING
REMEDY
Flush system and change oil using recom-
mended viscosity
Flush system and change oil using recom-
mended viscosity
Loosen valve and check mounting. Repair as
necessary.
Remove valve and repair or replace as neces-
sary.
Correctly torque bolts.
Remove valve and repair or replace as neces-
sary.
Check pressure delivery to and from valve and
repair or replace as necessary.
Remove and repair valve as necessary.
Remove valve and repair or replace as neces-
sary.
Remove line and clear obstruction or replace line
as necessary.
Remove valve and repair or replace as neces-
sary.
Recycle footswitch.
Repair or replace control switch.
Repair or replace holding valve.
Determine cause and repair or replace valve.
See wiring diagram.
Replenish oil as necessary.
Clean or replace line.
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