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Troubleshooting - hydraulic Motors
condition
Possible cause
correction
A. Oil motor stalls at bottom (no
unusual heat problems).
Fluid pump seat unthreaded.
Valve sticking or oil motor trip rod shifter
assembly separated.
If connecting rod is okay, remove cylinder
head plug and pop valve down. Replace
plug and start machine. If machine
cycles up and stops at bottom again, then
problem is piston seat on fluid pump.
Check piston seat. Repair or replace
as necessary. If piston seat is okay and
problem does not change, check oil motor.
Remove valve and check for scratches
and rough movement when sliding it up
and down. Replace valve and spool in
this condition. Check trip rod for possible
separation.
B. Oil motor stops at top (no unusual
heat problems).
Valve sticking
Broken spring retainer (valve rod
assembly)
Broken spring or valve rod
Air in hydraulic motor
Air in fluid pump
Remove valve and check for scratches
and rough movement when sliding it up
and down. Replace valve and spool in this
condition.
Replace valve rod assembly.
Replace valve rod assembly.
Reset valve. Purge Air, generally
accomplished by low pressure cycling of
motor/pump assembly for 5–10 minutes.
Check for causes of air introduction:
• Loose fittings in tank.
• Loose fittings on hydraulic pump.
• Loose hose connections.
• Low oil in reservoir.
Stall at top can occur randomly when fluid
pump picks up air. Reset valve. Avoid air
in the fluid pump.
C. Low pressure (okay on down stroke,
sluggish on up stroke - high heat)
NOTE: Engine labors on upstroke,
idles back at stall on the down stroke.
Blown piston seal
Cracked piston
Before dismantling oil motor, start
machine. With pump cycling under
pressure, touch the hydraulic cylinder and
the head to see if cylinder or head gets
hotter. This will help determine if piston
seal is blown or piston nut is broken. If
heat is on the head, check the o-rings on
spool valve.
Dismantle oil motor and check piston seals
cylinder bore and piston nut. Pay special
attention to piston nut. It can be cracked
and not show externally.
D. Low pressure (both strokes - high
heat)
NOTE: Engine labors at stall on both
strokes.
Blown center o-rings on spool valve
Bad hydraulic pump
Before dismantling oil motor, start
machine. With pump cycling under
pressure, touch the head to see if the head
becomes hotter. This will help determine
if center o-ring is blown on spool valve. If
hot, remove and replace o-ring.
Replace hydraulic pump.
