Rear Window Defogger

WARNING: This page is about a different car, the 2006 Buick LaCrosse and 2006 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.

The rear window defogger system consists of the following components:

Body Control Module (BCM)
Electronic compass
HVAC control module
Instrument Panel Cluster (IPC)
Rear window defogger grid
REAR WINDOW DEFOGGER relay

Battery positive voltage is supplied at all times from the BATT MAIN 3 fuse in the underhood fuse block to the coil and switch side of the REAR WINDOW DEFOGGER relay in the instrument panel (I/P) fuse block. When the rear defogger switch is pressed in the HVAC control module, a class 2 data signal is sent to the BCM. The BCM then applies a ground to the REAR WINDOW DEFOGGER relay. This energizes the relay and allows the battery positive voltage to be applied through the switched side of the relay to the rear window defogger grid. The rear window defogger grid is grounded at all times at G301.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle last for approximately 15 minutes. Further operations result in a 7.5 minute cycle. The defogger cycle is reset to 15 minutes when you cycle the ignition to the OFF position then back to the ON position.

If the battery is low, the BCM will disable the rear window defogger by removing the ground supplied to the REAR WINDOW DEFOGGER relay.