GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2001 🢒 GMC C-Series Medium Duty Truck B7 🢒 Driveline/Axle 🢒 Rear Axle Controls 🢒 Two Speed Rear Axle Pressure Protection Valve Diagnosis

Operational Test
  1. Block the wheels.
  2. Apply the parking brake.
  3. Drain all of the air reservoirs. Refer to Draining Reservoirs .
  4. Remove the air lines from the two-speed rear axle pressure protection valve supply and the delivery ports.
  5. Install the test gauges and the drain valves on the supply and the delivery ports.
  6. Start the engine. Charge the system to the air compressor governor cutout point.
  7. Stop the engine.
  8. Slowly exhaust pressure from the delivery side. Observe the test gauges.
    9. 8.1. Ensure that both gauges show a pressure loss until the closing pressure of the two speed rear axle pressure protection valve is reached.
    8.2. Ensure that the two-speed rear axle pressure protection valve closes within 34 kPa (5 psi) of the rated pressure.
    8.3. If the two-speed rear axle pressure protection valve does not close within the acceptable pressure range, replace the two-speed rear axle pressure protection valve.
  9. Drain all of the air reservoirs.
  10. Remove the drain valves and the test gauges.
  11. Install the air lines.
  12. Start the engine. Charge the system to the air compressor governor cutout point.
  13. Stop the engine.
  14. Inspect for air leakage.
  15. Verify proper two-speed rear axle operation.
  16. Remove the wheel blocks.

Air Leakage Test
  1. Block the wheels.
  2. Start the engine. Charge the system to the air compressor governor cutout point.
  3. Stop the engine.
  4. Inspect the two-speed rear axle pressure protection valve for air leakage. Air leakage less than a 25.4 mm (1.0 in) bubble in 3 seconds or longer is acceptable.
  5. If the air leakage is too much, replace the two-speed rear axle pressure protection valve.
  6. Drain the air pressure from the delivery port. Remove the air line connected to the delivery port.
  7. Inspect the delivery port for air leakage. Air leakage less than a 25.4 mm (1.0 in) bubble in 5 seconds or more is acceptable.
  8. If the air leakage is too much, replace the two-speed rear axle pressure protection valve.
  9. Install the air line to the delivery port.
  10. Remove the wheel blocks.