Normal Operation

When the engine is running and the defrost switch is pressed, the HVAC module activates the rear window defrost relay. The defrost system is activated at one of the three following locations: HVAC  module, FCIM  or FDIM  (8-inch [203mm] touchscreen). The FCIM  and/or FDIM  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. The glass position switch sends a feedback signal to the HVAC module. When this signal is received, the HVAC module sends a message to the FCIM  to illuminate 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 and the rear window defrost switch LED is no longer illuminated.

This pinpoint test is intended to diagnose the following: 

• Fuse(s)
• Rear window defrost relay
• Wiring, terminals or connectors
• Rear sliding glass position switch (if equipped) (part of power sliding rear window assembly)
• Rear window defrost grid
• HVAC module
• FCIM