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) |
On both systems, the body control module (BCM) supplies voltage to energize the rear window defog relay, which then suplies voltage to the rear window defog grid. However, on vehicles with C60, the BCM acts upon receiving a rear window defog switch signal input from the HVAC control module. On vehicles with C68, the BCM acts upon receiving a GMLAN message from the HVAC control module.
On either system, a short to voltage in the rear defog element supply voltage circuit may cause the symptom addressed in this table The rear window defog grid will be ON anytime the ignition is ON, however, the rear window defog indicator will operate normally.
Vehicles equipped with RPO C60 are equipped with two additional circuits associated with the rear window defog feature. However, of these two circuits, only the rear window defog switch signal circuit can have a failure which results in the symptom covered here. If this circuit shorts to ground the rear window defog grid will be ON and the rear window defog indicator LED will be illuminated anytime the ignition is ON . A short to voltage in this circuit can produce the same effect if the defog switch contacts overheat and fuse together.
Test Description
The number below refers to the step number on the diagnostic table.
Step | Action | Yes | No |
|---|---|---|---|
Schematic Reference: Defogger Schematics Connector End View Reference: Stationary Windows Connector End Views | |||
1 | Did you perform the Diagnostic System Check - Vehicle? | Go to Step 2 | Go to Diagnostic System Check - Vehicle in Vehicle DTC Information |
2 |
Important: Do not attempt to operate the rear window defog switch during this test. Verify the complaint is present by performing the following steps:
Does the test lamp illuminate? | Go to Step 3 | Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems |
3 | Is the vehicle equipped with RPO C60? | Go to Step 4 | Go to Step 6 |
4 |
Does the test lamp illuminate? | Go to Step 8 | Go to Step 5 |
5 | With the test lamp connected to battery positive voltage, again probe the rear defog switch signal circuit at the harness connector. Does the test lamp illuminate? | Go to Step 7 | Go to Step 6 |
6 |
Is the test lamp illuminated? | Go to Step 11 | Go to Step 9 |
7 | Test the rear defog switch signal circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 14 | Go to Step 10 |
|
Important: A short to voltage may damage the defog switch circuits within the HVAC control module and require control module replacement. Test the rear defog switch signal circuit for a short to voltage. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 14 | Go to Step 10 | |
9 | Inspect for a faulty connection at the rear defog relay. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 14 | Go to Step 12 |
10 | Inspect for a faulty connection at the harness connector of the HVAC control module. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems. Did you find and correct the condition? | Go to Step 14 | Go to Step 13 |
11 | Repair the short to voltage in the rear defog element supply voltage circuit. Refer to Wiring Repairs in Wiring Systems. Did you complete the repair? | Go to Step 14 | -- |
12 | Replace the rear defog relay. Did you complete the replacement? | Go to Step 14 | -- |
13 | Replace the HVAC control module. Refer to Control Module References in Computer/Integrating Systems for replacement, setup, and programming. Did you complete the replacement? | Go to Step 14 | -- |
14 | Operate the system in order to verify the repair. Did you correct the condition? | System OK | Go to Step 2 |
