Diagnosis And Testing - Rear Window Defogger System

For circuit descriptions and diagrams, (Refer to SYSTEM WIRING DIAGRAMS ). 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 run position.
2. Set the defogger switch in the run 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.
3. Using a 12-volt DC voltmeter, contact the rear glass heating grid terminal B (right side) with the negative lead, and terminal A (left side) with the positive lead (Fig 1 ). The voltmeter should read battery voltage.
   Fig 1: Identifying Rear Window Components

   Courtesy of DAIMLERCHRYSLER CORP.

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

If the defogger system does not operate, the problem should be isolated in the following manner:

1. Confirm that the ignition switch is in the run position.
2. Ensure that the rear glass heating grid feed and ground wires are connected to the glass. Confirm that the ground wire has continuity to ground.
3. Check the fuses in the Power Distribution Center (PDC) and in the junction block. The fuses must be tight in their receptacles and all electrical connections must be secure.

When the above steps have been completed and the rear glass or outside rear view mirror heating grid is still inoperative, one or more of the following is faulty:

• Defogger switch
• Defogger relay
• HVAC control head circuitry
• 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)
• Outside rear view mirror heating grid.

If setting the defogger switch to the On position produces a severe voltmeter deflection, check for a short circuit between the defogger relay output and the rear glass or outside rear view mirror heating grids.