GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 1999 🢒 GMC C-Series Medium Duty Truck B7 🢒 Driveline/Axle 🢒 Rear Axle Controls 🢒 Two Speed Rear Axle Shift Motor Diagnosis
  1. Block the wheels.
  2. Start the engine. Charge the system to the air compressor governor cutout point.
  3. Stop the engine.
  4. Shift the two speed rear axle shift control valve to HIGH. Verify that air pressure is present in the entire system.
  5. Inspect for air leaks using a soap solution. Repair any air leaks. Refer to Air Brake System Leak Test . In Air Brake.
  6. Shift the two speed rear axle shift control valve to LOW.
  7. Verify that air pressure is present ONLY up to the two speed rear axle shift control valve. If air pressure is present in the entire system, replace the two speed rear axle shift control valve.
  8. Verify that the two speed rear axle shift motor is operating properly.
    9. • If the two speed rear axle shift motor is operating properly, you will hear a definite reaction by the sound of parts moving.
    • If you do not hear the sound of parts moving, Refer to Two Speed Rear Axle System Check .
  9. Remove the wheel blocks.