GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Brakes 🢒 Air Compressor 🢒 Air Compressor Governor Valve Diagnosis

Operation Test

The air compressor should be capable of building the air system from 586-690 kPa (85-100 psi) in 40 seconds with the engine at the full governed RPM.

    • Inspect for air leaks in the system.
        Refer to Air Brake System Testing . In order to check the air lines and for overall system leakage.
    • Install a test gauge in the air compressor discharge line.
    •  Start the engine and build up the air pressure.
    • Observe the reading on the test gauge when the air compressor cuts out. Compare the reading with the governor cut-out specification. Refer to Air Compressor Governor Valve Specifications .
    • Slowly reduce the air pressure in the system by applying and releasing the brakes while the engine is running.
    • Observe the reading on the test gauge when the air compressor cuts in. Compare the reading with the governor cut-in specification. Refer to Air Compressor Governor Valve Specifications .
    • Adjust the governor if the cut-in or cut-out pressures are not within the specifications. Refer to Air Compressor Governor Valve Adjustment .

Leak Test

Leakage tests on the Bendix® D-2 governor should be made in both the cut-in and the cut-out positions.

Cut-In Position

Apply a soap solution around the cover and to the exhaust port. A slight bubble leakage is permitted. Too much leakage indicates a faulty exhaust valve seat, exhaust stem O-ring or O-ring at the top of the piston.

Cut-Out Position

Apply a soap solution around the cover and to the exhaust port. A slight bubble leakage is permitted. Too much leakage indicates a faulty exhaust valve seat, exhaust stem O-ring or O-ring at the top of the piston.

If the governor does not function as described or if the leakage is too much, overhaul or replace the governor. Refer to Air Compressor Governor Valve Replacement .