The body control module (BCM) controls the rear window Defog feature through the rear window defog relay. The rear window defog switch, which is hard wired to the BCM, provides the request signal to the BCM. When the ignition switch is in Run, the BCM supplies battery voltage to one side of the rear defog switch contacts, via the rear defog switch signal circuit. When the defog switch button is pressed, the switch contacts close the circuit to ground and the BCM interprets the flow of voltage as a request to turn the rear defog feature ON. The BCM then provides battery voltage to one side of the rear window defog relay coil, via the rear defog relay control circuit, while the other side of the relay coil is grounded to G302. The rear defog indicator LED circuit, which is spliced into the rear defog relay control circuit, is simultaneously provided battery voltage to illuminate the indicator LED.

The BCM controls the rear window defog feature through the rear window defog relay. However, the rear window defog switch is not hard wired to the BCM. When the ignition switch is in Run and the defog switch button is pressed, the HVAC control module interprets the switch input and sends a GMLAN message to the BCM to turn the rear window defog feature ON. In response to the GMLAN message, the BCM provides battery voltage to one side of the rear window defog relay coil via the rear defog relay control circuit, while the other side of the rear window defog relay coil is grounded to G302. The HVAC control module internally supplies the voltage to turn on the rear defog indicator LED.

This diagnostic procedure supports the following DTC:

DTC B0283 Electric Rear Defrost Circuit

This vehicle has DTCs which include DTC Symptoms. For more information on DTC Symptoms, refer to DTC Symptom Description .

The system voltage must be between 9-16 volts.

The specific condition causing the DTC to set will vary depending upon which HVAC air control system the vehicle is equipped with.

If the vehicle is equipped with RPO C60, the DTC may be set by the following conditions:

If the vehicle is equipped with RPO C68, the DTC may be set by the following conditions:

On vehicles equipped with either C60 or C68, a short to voltage, or a short to ground in the rear defog relay control circuit will set DTC B0283. However, an open in this circuit will cause DTC B0283 to set on vehicles with C68, but not on vehicles with C60.

The numbers below refer to the step numbers on the diagnostic table.

2. For vehicles equipped with RPO C68, this step verifies that the rear window defog relay ground circuit is not the cause of the DTC. On vehicles equipped with RPO C60, an open in this circuit will not cause the DTC to set.

3. This step will determine if the rear defog relay control circuit is shorted to voltage. However, on vehicles equipped with RPO C60, the short to voltage may be in either the rear defog relay control circuit, or in the rear defog indicator supply voltage circuit, since these 2 circuits are spliced together within the BCM.

8. This step will determine if the rear defog relay control circuit is shorted to ground. However, on vehicles equipped with C60, the short to ground could be in either the rear defog relay control circuit or in the rear defog indicator supply voltage circuit, since these 2 circuits are spliced together within the BCM.

14. After replacement of the BCM you must reprogram the new module for proper operation.