Circuit Description
WARNING: This page is about a different car, the 2004 GMC Sierra, 2004 GMC Cab & Chassis Sierra, 2004 Chevrolet Silverado, and 2004 Chevrolet Cab & Chassis Silverado. However, it is still accessible from the selected car via links, so may be relevant.
The body control module (BCM) monitors the voltage level on the control circuit of the DEFOG relay. The voltage level should be near system voltage while the DEFOG relay is de-energized. The voltage will be pulled low when the BCM energizes the DEFOG relay. The BCM tests the control circuit of the DEFOG relay every 125 milliseconds.