Chevrolet Sonic Repair Manual: Rear Window Defogger Description and Operation

Rear Window Defogger System Components

The rear window defogger system consist of the following components:

• The HVAC control module
• The body control module (BCM)
• The rear defogger relay
• The rear window grid
• Driver outside rearview mirror
• Passenger outside rearview mirror
• 30A fuse

Rear Window Defogger Operation

The BCM supplies 12?€‰V signal to the rear window defogger switch signal circuit to the HVAC control module. When you depress the rear window defogger switch, the rear window defogger switch pulls the signal circuit low. The BCM interprets as a request for the rear window defogger system. The BCM enables the rear window defogger system by supplying voltage to the rear defogger relay coil. The relay coil supply voltage is also spliced off internally in the BCM to the supply voltage circuit of the rear window defogger indicator. The rear defogger relay is energized and the rear window defogger indicator is illuminated. With the relay energized, battery positive voltage is allowed from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger grid.

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 rear defogger feature will not time out if vehicle speed is above 80?€‰km/h (50?€‰mph). The defogger cycle resets to 15?€‰minutes when you cycle the ignition to the OFF position and then to the ON position.

Heated Mirrors

The heated mirrors are also controlled through the rear defogger relay. Whenever the rear window defogger is turned on battery voltage is supplied to the mirror heater elements through the left and right mirror heater element control circuits.