Defogger Always On - Rear Window

DEFOGGER ALWAYS ON - 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)

On both systems, the body control module (BCM) supplies voltage to energize the rear window defog relay, which then supplies 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

Steps 1 - 12:

Steps 13 - 14:

The number below refers to the step number on the diagnostic table.
8. This step will determine if the rear defog switch signal circuit is shorted to voltage. This condition may damage the rear window defog switch circuits internal to the HVAC control module, requiring control module replacement.