Hyundai HL955 Manual page 514

Problem
Function drifts down
Boom drifts up
Boom down does not
work (engine off)
Oil overheats
Function drops before
raising when valve is
activated
Cause
Leaking cylinders.
Leaking seals in circuit relief valve (port
relief valve) or valve stuck open.
Leaking loader control valve (MCV).
Leakage in boom down spool.
Safety valve not operated.
Stuck pilot control valve.
Faulty line filter.
Accumulation not operated.
MCV spool stuck.
Low oil viscosity in hot weather.
Excessive load.
Holding hydraulic system over relief.
Leakage in work circuit.
Plugged fins in oil cooler.
Internally plugged oil cooler.
Incorrect system or circuit relief valve
setting.
Restriction in oil lines or loader control
valve (MCV).
Malfunctioning steering valve.
Leaking system main relief valve.
Worn hydraulic pump (internal leakage).
Stuck open lift check valve.
6-49
Remedy
Do cylinder leakage checks in group 3.
Inspect seals. Replace relief valve.
Repair or replace valve section.
Remove and inspect boom down spool.
Operate valve.
Inspect.
Remove and inspect filter.
Inspect.
Inspect and repair valve.
Use recommended oil.
Reduce load.
Reduce load.
Do boom and bucket cylinder leakage
test in group 3.
Inspect and clean oil cooler.
Do hydraulic oil cooler restriction test.
Do loader system and circuit relief valve
pressure test in group 3.
Inspect for dented or kinked lines.
Do hydraulic system restriction test in
group 3.
Do hydraulic system restriction test in
group 3.
Remove and inspect valve and seals.
Do hydraulic pump performance check
in group 2.
Do control valve lift check in group 2.