SUNDSTRAND HYDROSTATIC
SERVICE BULLETIN
TROUBLE SHOOTING PROCEDURE
IMPORTANT PUMP AND MOTOR SERVICE NOTE: Always include pump or motor model and serial numbers along
with the mixer serial number when ordering pumps, motors or service parts. (Do not open any part of the system without
first cleaning the surrounding area.)
I.
System* Will Not Operate in Either Direction.
* The word system denotes both pump and motor plus all lines, valves, filters, controls, etc., leading to and in
between them.
Cause
Remedy
A. System Low on Oil.
1. Check oil level in reservoir and replenish, if necessary.
2. Locate and fix leak or leaks causing the loss of oil.
B. Faulty Control Linkage to Pump.
1. Check the entire linkage to make sure it is connected and
free to operate as it should.
C. Slump and/or Pump Dump Valve Stuck Open.
1. If used, make sure the Slump and/or Pump Dump Valve is
closing properly. Check valve for leakage.
D. Disconnected Coupling.
1. Check to see that the coupling from the prime mover to the
pump and the coupling from the motor shaft to the driven
mechanism is not slipping or broken.
E. Low or Zero Charge Pressure (Steady).
1. Install pressure gauge (capable of 600 PSI) in either the 1,
N.P.T. in the charge pump or in the side of the main pump.
NOTE: Charge pressure may also be taken by attaching a
pressure gauge to the port on the rear-of the motor manifold.
This port, however, is blocked by the shuttle valve when the
hydrostatic system is in neutral; therefore, the system must
be operating either in the forward or reverse direction to
obtain a pressure reading at this port.
2. Set pump speed to at least 500 RPM. Charge pressure
should read at least 120 PSI or more when main pump
control lever is in pumping position and fluid motor is
operating.
3. Low charge pressure may be caused by:
a. Charge pressure relief valve in charge pump stuck open
or relief valve seat damaged.
b. Charge pump drive shaft sheared.
c. Suction line or filter blocked or clogged.
d. Internal damage to pump or motor.
e. "0" ring damaged or missing between displacement
control valve and pump case
page66.
F. Low and Fluctuating Charge Pressure.
1. Air in system. Air will also cause system to be noisy. Check
all fittings, especially around filter, in the suction line and
locate the point or points where air is being drawn into the
system. Tighten fittings and joints where air leaks exist.
45
