Normal Operation
WARNING: This page is about a different car, the 2008 Mercury Mariner and 2008 Ford Escape. However, it is still accessible from the selected car via links, so may be relevant.
The rear window defrost relay receives power from the battery junction box (BJB) fuse 35 (10A) on circuit CBB35 (YE/GY). When the rear window defrost switch is pressed to the ON position, the heating ventilation air conditioning (HVAC) module activates the rear window defrost relay by grounding circuit CH122 (WH/OG). When activated, the rear window defrost relay supplies 12 volts to the rear window defrost grid on circuit CRD06 (BN/YE). The rear window defrost grid is grounded through circuit GD149 (BK/GY) to ground.
The HVAC module will deactivate the rear window defrost relay when one of the following conditions is met:
- The rear window defrost switch is pressed when the feature is active
- The ignition switch state is changed from ON to OFF/LOCK
- The 15-minute timing function is complete
- DTC B12BF - Heated backlight/mirror relay drive circuit open or shorted to ground, heated backlight and heated mirrors wire failure, relay drive circuit open or shorted to ground.
This pinpoint test is intended to diagnose the following:
- Rear window defrost relay
- Wiring, terminals or connectors
- Rear window defrost switch (integral to HVAC module)
- HVAC module