Window Defogger System Test

Rear Window Defogger System Schematic:

1. Start the engine. With the engine running, push the Rear Window Defogger button on the ATC or MTC head. The button LED indicator should light. If the EBL indicator does not light on the MTC HEAD, then there may be a wiring problem. This may also indicate that the BCM will not complete the circuit to ground that is required to energize the EBL relay coil (non-ATC vehicles only). (Refer to REAR WINDOW DEFOGGER RELAY - DIAGNOSIS AND TESTING) On ATC vehicles, the LED indicator may light when the BCM failed to complete the circuit to ground required to energize the EBL relay coil.
2. Using a DC voltmeter with a 0 - 15 volt range, connect the negative lead to a known good vehicle electrical ground.
3. Touch the positive bus bar and check voltage. The voltage should be above 11 volts. Push the Rear Window Defogger button to turn OFF the indicator. The measured voltage should be 0 volts. If these voltages are correct then the problem is with the Rear Window Grid or Grid ground electrical connection. (Refer to REAR WINDOW DEFOGGER GRID - DIAGNOSIS AND TESTING) If there is no voltage to the rear window defogger grid when the system is ON, (Refer to HEATED GLASS - DIAGNOSIS AND TESTING - FUSE AND ELECTRICAL SHORT TEST).