Group 30, Hydraulic Control Valve/Lift Circuit
SM 765, Sep ’06 Hydraulic System Troubleshooting • 30-3-1
The following is a list of problems and solutions relating
to the main hydraulic control valve and associated compo-
nents. For other hydraulic system troubleshooting, refer to
Groups 29 and 34.
No lift, tilt, or auxiliary function
• Hydraulic fluid very low; check and fill to correct
level.
• Hose or fittings broken; replace component.
• Defective main lift valve; check other Trouble-
shooting items for possible cause, then consider
rebuilding or replacing main lift valve.
• Hydraulic pump defective: check other Trouble-
shooting items for possible cause, then consider
rebuilding or replacing pump.
No motion, slow or jerky action of
hydraulic system
• Spool not moved to full stroke; check travel and
linkage adjustment.
• Relief valve not properly set, stuck in place, and/or
worn; check and clean valve, replace if necessary.
• Dirt or foreign particles lodged between relief
valve control poppet and seat: check valve and
clean.
• Valve body cracked inside; check and replace
entire valve.
Foaming hydraulic fluid
• Low oil level; check and fill to correct level.
• Wrong fluid; drain and refill with correct oil.
• Oil too heavy; change to correct viscosity.
• Pump inlet line restriction or line kinked; clean line
and suction screen or repair kinked hose.
• Hydraulic pump cavitating (pumping air with
fluid); check hydraulic plumbing for airtight hoses
and connections.
Overheated hydraulic fluid
• Thin fluid; drain and fill with correct fluid.
• Fluid contaminated; drain sump, clean suction
screen, replace filter, and refill.
• Cavitating pump; check hydraulic plumbing for
airtight hoses and connections.
• Pump driveshaft misaligned; check mounting and
alignment.
• Axial loading on drive shaft; check shaft end clear-
ance and shaft alignment; check for worn key/
spline.
• Relief valve in bypass; check relief setting.
Load cannot be lifted to maximum height
• Hydraulic fluid low: check and fill to correct level.
• Hydraulic pump defective; check other Trouble-
shooting items for possible cause, then consider
rebuilding or replacing pump.
Oil leaks at top of lift (secondary) cylinder(s)
• Plugged vent line; check and clear line.
• Worn or damaged piston seal: rebuild cylinder.
• Scored cylinder wall; replace cylinder.
See Group 34, “Cylinder Removal, Overhaul, and
Replacement.”
Oil leak at tilt or auxiliary function cylinder
• Worn or damaged seal; rebuild cylinder.
• Scored piston rod; repair or replace rod.
See Group 34, “Cylinder Removal, Overhaul, and
Replacement.”
Load will not hold
• .Oil bypassing between lift spool and valve body; over-
haul valve and spool.
• Spool not centered; see spool remedies for correct-
ing problems when spools do not return to neutral.
• Oil bypassing piston in cylinder; repair or replace
cylinder.
Oil leaks at either end of main hydraulic
valve spool
• Defective O-ring seals; rebuild valve.
Section 3
Hydraulic System Troubleshooting
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