Circuit/System Description

WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.

With the ignition in the RUN position, and the rear window defogger switch depressed, a class 2 serial data message is sent from the HVAC control module to the body control module (BCM) requesting rear defog operation. The BCM responds by providing ground to the rear defog control signal circuit energizing the R/DEFOG relay. The R/DEFOG relay then provides voltage to the rear defog element supply voltage circuit operating the rear defog system. The HVAC control module illuminates the rear window defogger indicator when a class 2 serial data message is received indicating that the BCM is requesting rear defog operation. The R/DEFOG relay receives battery positive voltage to both the coil and switch input sides from the BATT MAIN 3 fuse in the underhood fuse block. Ground for the rear window defogger is provided at G301.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle lasts for 15 minutes. Further operation results in 7.5 minute defogger cycles. The defogger cycle resets to 15 minutes when you cycle the ignition to the OFF position and then back to the ON position.

If battery voltage is low, the BCM will disable the rear window defogger