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Service Manual
Troubleshooting
System will not operate in either direction
Item
Oil level in reservoir
Input to pump control
Pump displacement control
Bypass valve(s) open
Charge pressure with pump in
neutral
Charge pressure with pump in
stroke
Pump charge relief valve
Charge pump inlet filter
Charge pumps
Pump displacement control
System pressure
System multi-function valves
Low motor output torque
Item
System pressure at motor
Variable motor stuck at minimum
displacement
Internal leakage
Improper motor output speed
Item
Oil level in reservoir
Pump output flow
Variable motor displacement
control
Internal leakage
26
Series 90 Pumps
Description
Insufficient hydraulic fluid to supply system loop
Input to control module is operating improperly
Control linkage is not secure, control orifices are
blocked, etc.
If bypass valve(s) are open, the system loop will be
depressurized.
Low charge pressure is insufficient to recharge system
loop.
Low charge pressure with the pump in stroke indicates
a motor charge relief valve or system pressure relief
valve may be improperly set.
A pump charge relief valve that is leaky or set too low
will depressurize the system.
A clogged filter will under supply system loop.
A malfunctioning charge pump will provide insufficient
charge flow.
Control linkages are not secure, control orifices are
blocked.
Low system pressure will not provide power necessary
to move load.
Defective multi-function valves will cause system to
pressure to be low.
Description
Low system pressure at the motor will reduce torque
Minimum motor displacement yields low output
torque.
Internal leakage will reduce system pressure.
Description
Insufficient hydraulic fluid will reduce motor speed.
Incorrect outflow will affect output speed. Incorrect
output flow indicates the swashplate is out of position.
If variable motor displacement control is not
functioning correctly, variable motor swashplate may
be in wrong position.
Internal leakage will reduce system pressure.
520L0818 • Rev 0101 • August 2015
Action
Fill reservoir to proper level.
Check control input and repair replace as necessary.
Repair or replace control module as necessary.
Close bypass valves. Replace multi-function valve if
defective.
Measure charge pressure with the pump in neutral. If
pressure is low, go to Pump charge relief valve; otherwise
continue with Charge pressure with pump in stroke.
Measure charge pressure with pump in stroke. If pressure is
low, adjust or replace motor charge relief valve, otherwise
go to Charge pumps.
Adjust or replace pump charge relief valve as necessary.
Inspect filter and replace if necessary.
Repair or replace the charge pump.
Repair or replace control module as necessary.
Measure system pressure. Continue to next step.
Repair or replace multi-function valve(s).
Action
Measure system pressure at motor. If pressure limiter
setting is low, increase setting.
Check control supply pressure or repair displacement
control. Check motor control orifices.
Check for leaking O-rings, gaskets and other fittings. Repair
unit as required, or replace leaking unit.
Action
Fill oil to proper level.
Measure pump output and check for proper pump speed.
Ensure the pump is in full stroke.
See if variable motor displacement control is responding. If
not, repair or replace control.
Check for leaking O-rings, gaskets, and other fittings. Repair
unit as required, or replace leaky unit.
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