Rear Window Defogger Operation

WARNING: This page is about a different car, the 2003 Pontiac Aztek and 2003 Buick Rendezvous. However, it is still accessible from the selected car via links, so may be relevant.

Battery positive voltage is supplied through the BATT 3 fuse in the underhood fuse block, to the Rear Defog relay switched input. With the engine started, the BCM supplies battery positive voltage to Rear Defog relay coil. When you depress the rear window defogger switch the HVAC control assembly illuminates the rear window defogger indicator and energizes the Rear 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 G403.

When you start the engine and press the rear window defogger switch for the first time, the defogger cycle lasts for 10 minutes. Further operation results in 5 minute defogger cycles. The defogger cycle resets to 10 minutes when you cycle the ignition to the OFF position and then start the engine.

If the battery voltage is very low, the rear defogger feature is disabled.