Rear Defogger: Testing and Inspection

NOTE:
- Illumination of the defogger switch indicator lamp means that there is electrical current available at the output of the rear window defogger logic circuitry, but does not confirm that the electrical current is reaching the rear glass heating grid lines.
- For circuit descriptions and diagrams of the rear window defogger system, refer to Diagrams/Electrical.

Operation of the electric backlight (EBL) system can be confirmed by the following:
1. Turn the ignition switch to the On position. Set the rear window defogger switch in the On position. The rear window defogger operation can be checked by feeling the rear window glass surface. A distinct difference in temperature between the grid lines and the adjacent clear glass should be detected within three to four minutes of operation.

Grid Line Test:

2. If a temperature difference is not detected, use a 12-volt DC voltmeter and contact the rear glass heating grid terminal B with the negative lead, and terminal A with the positive lead. The voltmeter should read battery voltage. If the voltmeter does not read battery voltage, check the following:
- Confirm that the ignition switch is in the On position.
- Make sure that the rear glass heating grid feed wire and ground wire are connected to the terminals. Confirm that the ground wire has continuity to ground.
- Check that fuse 13 (40 amp) in the integrated power module (IPM) is OK. The fuse must be tight in its receptacle and all electrical connections must be secure.
3. When the above steps have been completed and the rear glass heating grid is still inoperative, one or more of the following is faulty. It may be necessary to connect a DRBIII(R) scan tool to perform further diagnostics.
- Rear window defogger switch in the A/C-heater control.
- J1850 bus communication between the A/Cheater control and the front control module (FCM).
- Rear window defogger (EBL) relay in the IPM.
- Rear window defogger control circuity in the FCM.
- Check for a loose wire connector or a wire pushed out of a connector.
- Rear window grid lines (all grid lines would have to be broken, or the power feed or ground wire not connected, for the entire heating grid to be inoperative).
4. If EBL system operation has been verified but defogger switch LED indicator does not illuminate, replace the A/C-heater control.
5. If broken defogger grid lines are suspected, use a 12-volt DC voltmeter and contact terminal B with the negative lead and each rear glass heating grid line at it's mid-point with the positive lead. The voltmeter should read approximately 6 volts at each grid line mid-point C. If the voltmeter does not read approximately 6 volts, repair the open grid line(s).