Two Speed Rear Axle Inoperative (Air-Operated)

Two Speed Rear Axle Inoperative (Air-Operated)

Circuit Description

This symptom is usually the result of actions that were expected to take place during the transition between rear axle gears that has not occurred. This circuit will only affect speedometer operation, which assumes the two speed rear axle system is operating normally. When the rear axle switches from LOW to HIGH the two speed rear axle air shift switch opens, removing the voltage at the speedo adapter on vehicles with L18 and early LG5 options. On late vehicles with the LG5 option voltage will no longer be sent to the powertrain control module (PCM). The speedo adapter/powertrain control module then changes the calibration so that the speedometer reading matches road speed. When the two speed rear axle shift control valve is placed back in the LOW position, the shift system's air supply is closed off. The loss of air pressure closes the two speed rear axle air shift switch, allowing current to flow to the speedo adapter, or the PCM. Applying voltage to the speedo adapter or the PCM changes the speedo calibration to match the low gear.

Test Description

The numbers below refer to the step numbers on the diagnostic table.
3. This step tests for proper ignition 3 voltage at the two speed rear axle air shift switch.
4. This step tests for proper operation of the two speed rear axle air shift switch.
5. This step tests for proper operation of the two speed rear axle air shift switch.
6. This step tests the ignition 3 voltage circuit for an open or high resistance.
7. This step tests the air shift switch signal circuit for an open or high resistance.