GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 GMC_Truck 🢒 2004 🢒 TopKick C-Series (Conventional) C4/C5 🢒 Driveline/Axle 🢒 Drive Axle - Locking/Limited Slip 🢒 Differential Lock Relay Valve Diagnosis

Operational Test
  1. Apply the parking brake.
  2. Block the wheels.

    3. Remove the differential lock relay valve. Refer to Differential Lock Relay Valve Replacement .

  3. Install an air pressure gage in a common control and supply air line connected to the control and supply ports of the differential lock relay valve.
  4. Install an air pressure gage into the delivery port of the differential lock relay valve.
  5. Gradually apply air pressure to the common control and supply line. As the pressure increases, observe the two air pressure gages.
    6. 5.1. Note the common control and supply line air pressure gage when air pressure is delivered to the delivery port.
    5.2. The common control and supply line inlet pressure should be at 290 kPa (42 psi) when air pressure is provided to the delivery port of the differential lock relay valve.
    5.3. If the test does not result in the above conditions, repair or replace the differential lock relay valve.
  6. Slowly reduce the common control and supply line inlet air pressure. As the pressure decreases, observe the two air pressure gages.
    7. 6.1. Note the common control and supply line air pressure gage when air pressure from the delivery port is exhausted from the exhaust port.
    6.2. The common control and supply line inlet pressure should be at 193 kPa (28 psi) when air pressure is vented through the exhaust port.
    6.3. If the test does not result in the above conditions, repair or replace the differential lock relay valve.
  7. Remove the air pressure gage from the delivery port of the differential lock relay valve.
  8. Remove the air pressure gage in a common control and supply air line connected to the control and supply ports of the differential lock relay valve.
  9. Install the differential lock relay valve. Refer to Differential Lock Relay Valve Replacement .

Air Leakage Test
  1. Apply the parking brake.
  2. Block the wheels.
  3. Start the engine. Charge the air system to the air compressor governor cutout point.
  4. Stop the engine.
  5. Switch the differential lock control valve lever to the ON position.
  6. Inspect the supply, control and delivery ports for air leaks using a soap solution. Air leakage is not allowed. If any leaks are present, repair or replace the differential lock relay valve.
  7. Inspect the exhaust port for air leaks using a soap solution. A slight bubble leakage is allowed. If air leakage is excessive, repair or replace the differential lock relay valve.
  8. Drain all of the air reservoirs. Refer to Draining Reservoirs .
  9. Remove the supply and control air lines from the differential lock relay valve. Refer to Air Brakes.
  10. Install an auxiliary common air supply and control line into the differential lock relay valve.
  11. Remove the delivery port air line from the differential lock relay valve.
  12. Install a plug into the delivery port of the differential lock relay valve.
  13. Apply 69 kPa (10 psi) of air pressure to the auxiliary common air supply and control line.
  14. Inspect the exhaust port for air leakage. Air leakage less than a 25.4 mm (1.0 in) bubble in 5 seconds is acceptable.
  15. If the air leakage is too much, repair or replace the differential lock relay valve.
  16. Remove the air pressure from the auxiliary common air supply and control line.
  17. Remove the plug from the delivery port of the differential lock relay valve.
  18. Install the delivery port air line into the differential lock relay valve.
  19. Install the supply and control air lines into the differential lock relay valve.
  20. Start the engine. Charge the system to the air compressor governor cutout point.
  21. Stop the engine.
  22. Inspect for air leaks using a soap solution.
  23. Remove the wheel blocks.