Rear Window Defogger Operation
WARNING: This page is about a different car, the 2009 GMC Sierra, 2009 GMC Cab & Chassis Sierra, 2009 Chevrolet Silverado, and 2009 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
When the rear defogger switch is pressed, the HVAC control module will illuminate the rear window defogger indicator and will supply voltage to the defog relay coil. Battery positive voltage is supplied at all times to the defog relay switched input and the defog relay coil is always grounded. This allows battery positive voltage from the relay switch contacts to the rear window defogger grid. The HVAC module also sends a request to the driver door module and passenger door module to activate the heated mirrors, if equipped.
When the rear window defogger is initially activated, the defogger cycle will last 10 minutes, further requests will result in 5 minute defogger cycles.