Rear Window Defogger Operation

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 Chevrolet Tahoe, 2003 Chevrolet Suburban, and 2003 Chevrolet Avalanche. However, it is still accessible from the selected car via links, so may be relevant.

When you turn the ignition to the ON position, ignition voltage is supplied to the DEFOG relay coil. Battery positive voltage is supplied at all times to the DEFOG relay switched input. When the rear window defogger switch is depressed, the HVAC control module illuminates the rear window defogger indicator and sends a class 2 message to the BCM. The BCM energizes the DEFOG relay by grounding the control circuit of the relay. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger. Ground for the rear window defogger is provided by G306. If the vehicle is equipped with cargo doors, ground for the rear window defoggers will be provided by G410.

When you turn ON the ignition and press the rear window defogger switch for the first time, the defogger cycle lasts 10 minutes. Further operation results in 5 minute defogger cycles.