Window Defogger System Test

WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, DISABLE THE AIRBAG SYSTEM BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. DISCONNECT AND ISOLATE THE BATTERY NEGATIVE (GROUND) CABLE, THEN WAIT TWO MINUTES FOR THE AIRBAG SYSTEM CAPACITOR TO DISCHARGE BEFORE PERFORMING FURTHER DIAGNOSIS OR SERVICE. THIS IS THE ONLY SURE WAY TO DISABLE THE AIRBAG SYSTEM. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN AN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

For complete circuit diagrams, (Refer to Appropriate Wiring Information). The operation of the electrically heated rear window defogger system can be confirmed in one of the following manners:
1. Turn the ignition switch to the ON position. While monitoring the instrument panel voltmeter, depress the rear window defogger switch to the ON position. When the rear window defogger switch is turned ON, a distinct voltmeter needle deflection should be noted.
2. Turn the ignition switch to the ON position. Depress the rear window defogger switch to the ON position. The rear window defogger operation can be checked by feeling the rear window or outside rear view mirror glass. A distinct difference in temperature between the grid lines and the adjacent clear glass or the mirror glass can be detected within three to four minutes of operation.

Rear Window Glass Grid Test:

3. Using a 12-volt DC voltmeter, contact the rear glass heating grid terminal A (right side) with the negative lead, and terminal B (left side) with the positive lead. The voltmeter should read battery voltage.

The above checks will confirm rear window defogger system operation. Illumination of the rear window defogger switch LED indicator means that there is battery current available at the output of the rear window defogger relay but does not confirm that battery current is reaching the rear glass heating grid lines.

If the rear window defogger system does not operate, the problem should be isolated in the following manner:
1. Confirm that the ignition switch is in the ON position.
2. Ensure that the rear glass heating grid feed and ground terminals are connected to the glass. Confirm that the ground wire has continuity to ground.
3. Check the fused B(+) fuse in the Power Distribution Center (PDC). The fuse must be tight in its receptacles and all electrical connections must be secure.

When the above steps have been completed and the rear glass heating grid is still inoperative, one or more of the following is faulty:
- Rear window defogger switch
- Rear window defogger relay
- Body Control Module (BCM)
- Rear window grid lines (all grid lines would have to be broken or one of the feed wires disconnected for the entire system to be inoperative).

When the above steps have been completed and the heated mirror glass heating grid is still inoperative, one or more of the following is faulty:
- Body Control Module (BCM)
- Programmable Communications Interface (PCI) data bus
- Driver Door Module (DDM) or Passenger Door Module (PDM)
- Outside rear view mirror heating grids.

If turning the rear window defogger system on produces a severe voltmeter deflection check for a short circuit between the rear window defogger relay output and the rear glass heating grid.