Normal Operation

When the engine is running and the defrost switch (integral to the HVAC module or Front Controls Interface Module (FCIM) if equipped with navigation system) is pressed, the HVAC module activates the rear window defrost relay. For vehicles equipped with navigation system, the FCIM sends the defrost request message to the HVAC module over the Controller Area Network (CAN). When the rear window defrost relay is active, voltage is supplied to the rear window defrost grid. For vehicles equipped with a power sliding rear window, the rear window defrost grid does not activate unless the rear sliding window glass is in the fully CLOSED position due to a glass position switch in series with the grid circuit. The glass position switch is integral to the power sliding rear window assembly. There is a feedback circuit incorporated into the glass position switch which illuminates the rear window defrost switch LED when the defrost grid is active and the sliding glass is fully closed. If the sliding glass is opened while the defrost system is active, the heated mirrors continue to operate regardless of the rear window defrost switch LED status.

This pinpoint test is intended to diagnose the following: 

• Wiring, terminals or connectors
• Rear window defrost relay
• HVAC module