Defogger Inoperative - Rear Window

DEFOGGER INOPERATIVE - REAR WINDOW

DIAGNOSTIC AIDS
The vehicle is equipped with one of two different types of heating, ventilation and air conditioning (HVAC) systems which affect rear window defog operation. These are:
- Manual air temperature control (RPO C60)
- Automatic air temperature control (RPO C68)

Specific circuit and component failures may result in different symptoms depending upon which type of HVAC system the vehicle uses. Carefully follow the Test Descriptions, as well as the instructions contained in each table step in order to avoid misdiagnosis.

TEST DESCRIPTION

Step 1 - Step 10:

Step 11 - Step 23:

Step 24 - Step 26:

The numbers below refer to the step numbers on the diagnostic table.
5. The rear window defog feature is operated by activating the rear defog switch, which is an integral part of the HVAC control module. This step begins to determine if the ground circuit to the HVAC control module, which affects defog switch operation, has failed. If this ground has failed, both the AC and the rear window defog systems and indicators may still operate when the headlamps are turned OFF, but not when the headlamps are ON. Turning On the headlamps verifies that the test results are accurate.
10. Listen for an audible click to help determine if the rear defog relay operates.
11. Listen for an audible click to help determine if the rear defog relay operates.
14. On vehicles equipped with RPO C60, this step will determine if the failure is in the defog switch signal circuit. However, vehicles equipped with RPO C68 do not include this circuit. Therefore, on vehicles with C68, you must answer No to the question or omit this step and proceed directly to the next step.
15. On vehicles equipped with RPO C60, this step will determine if the failure is in the defog indicator supply voltage circuit. However, vehicles equipped with RPO C68 do not include this circuit. Therefore, on vehicles with C68, you must answer No to the question and proceed to replacement of the HVAC control module.