Overview
This section provides general steps to follow if certain undesirable system conditions occur. Follow the
steps in a section until you solve the problem. Some of the items are system specific. For areas covered in
this manual, we reference the section. Always observe the safety precautions listed in the Introduction
section and those relating to your specific equipment.
Neutral difficult or impossible to find
Item Description Action
Input to pump control Input to control module is operating improperly. Check control input and repair or replace as necessary.
Pump displacement control Control linages are not secure, control orifices are
blocked.
Adjust, repair, or replace control module as necessary.
Multifunction valve Multifunction valve is leaking. Swap multifunction valves. If system changes direction,
replace defective valve.
System operating hot
Item Description Action
Oil level in reservoir Insufficient hydraulic fluid will not meet cooling
demands of system.
Fill reservoir to proper level.
Heat exchanger Heat exchanger is not sufficiently cooling the system. Check air flow and input air temperature for heat
exchanger. Clean, repair, or replace heat exchanger.
Charge pressure Low charge pressure will overwork system. Measure charge pressure. Inspect and adjust or replace
charge relief valve, or repair leaky charge pump.
Charge pump inlet vacuum High inlet vacuum will overwork system. A dirty filter
will increase the inlet vacuum. Inadequate line size
will restrict flow.
Check charge inlet vacuum. If high, inspect inlet filter and
replace as necessary. Check for adequate line size, length
or other restrictions.
System relief pressure settings If the system relief settings are too low, the relief
valves will be overworked.
Verify settings of pressure limiters and high pressure relief
valves and adjust or replace multi-function valves as
necessary.
For internal leakage in motor Leakage will reduce low side system pressure and
overwork the system.
Monitor motor case flow without loop flushing in the
circuit (use defeat spool). If flow is excessive, replace motor.
System pressure High system pressure will overheat system. Measure system pressure. If pressure is high, reduce loads.
Transmission operates normally in one direction only
Item Description Action
Input to pump control Input to control module is operating improperly. Check control input and repair or replace as necessary.
Pump displacement control Control linkages are not secure, control orifices are
blocked.
Repair or replace control module as necessary.
System pressure limiters, high
pressure relief valves, or system
check valves
Interchanging the multi-function valves will show if the
problem is related to the valve functions contained in
the multi-function valves.
Interchange multi-function valves. If the problem changes
direction, repair or replace the valve on the side that does
not operate.
Charge pressure If charge pressure decays in one direction, the loop
flushing valve may be sticking in one direction.
Measure charge pressure in forward and reverse. If pressure
decays in one direction, inspect and repair the motor loop
flushing valve.
Service Manual Series 90 Pumps
Troubleshooting
520L0818 • Rev 0101 • August 2015 25
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