TM 5-3895-346-14
HYDROSTATIC SYSTEM
TROUBLESHOOTING
TROUBLESHOOTING PROCEDURE
HYDROSTATIC SYSTEM
NOTE
Before proceeding with troubleshooting, read the following information.
Sundstrand heavy-duty transmissions must maintain various pressures to function properly. Any disturbance of
the proper pressure levels will lead to an inoperable transmission.
Four pressures normally must be monitored to accurately diagnose a malfunction in the transmission:
1. CHARGE PUMP INLET SUCTION: The maximum vacuum at the charge pump inlet should not exceed 10-
inches of mercury at normal operating conditions. It is acceptable for the inlet vacuum to exceed 10-inches of
mercury during cold startup.
2. CHARGE PRESSURE: The minimum allowable charge pressure is 130 psi above case pressure. Normal
charge pressure is 160 psi above motor case pressure when the motor shaft is turning and 190 psi above pump
case pressure when the pump is in neutral.
3. SYSTEM OR HIGH PRESSURE: The maximum system pressure obtainable is controlled by the high-
pressure relief valves located in the motor manifold. The relief valves have a two-digit number stamped on the
exposed end stating valve setting (i.e. "50" = 5000 psi).
4. CASE PRESSURE:
The transmission case pressures should not exceed 40 psi gage under normal
operating conditions.
Proper troubleshooting procedures dictate that the pressure levels developed in the circuit must be known by
the installation and interpretation of the pressure gages.
The necessary gages and complimentary equipment required are depicted in Fig. 1. Their proper installation in
the circuit is depicted in Fig. 2.
NOTE
For accurate gage interpretation, it is recommended that the pump drive shaft be turning at or near maximum
rpm.
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