Diagnosis and testing

Defogger system

For circuit descriptions and diagrams, refer to 8W-48 - Rear Window Defogger in Group 8W - 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 On position.

While monitoring the instrument panel voltmeter, set the defogger switch in the On position. When the defogger switch is turned On, a distinct voltmeter needle deflection should be noted.

2. Turn the ignition switch to the On position. Set the defogger switch in 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.

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 (Fig. 1). The voltmeter should read battery voltage.

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 rear window defogger logic and timer circuitry, but does not confirm that the electrical current is reaching the rear glass heating grid lines.

Fig. 1 Rear Window Glass Grid Test

1 - TERMINAL "A"

2 - TERMINAL "B"

3 - FEED WIRE

4 - MID-POINT "C" (TYPICAL)

5 - HEATED REAR WINDOW DEFOGGER GRID

6 - GROUND WIRE

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 On 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 heating grid is still inoperative, one or more of the following is faulty:

• Rear window switch pod
• 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).

If setting the defogger switch to the On position produces a severe voltmeter deflection, check for a short circuit between the rear window switch pod defogger relay output and the rear glass heating grid.

Rear glass heating grid

For circuit descriptions and diagrams, refer to 8W-48 - Rear Window Defogger in Group 8W - Wiring Diagrams. To detect breaks in the grid lines, the following procedure is required: (1) Turn the ignition switch to the On position. Set the defogger switch in the On position. The indicator lamp should light. If OK, go to Step 2. If not OK, see Defogger Switch in the Diagnosis and Testing section of this group.

(2) Using a 12-volt DC voltmeter, contact the vertical bus bar on the right side of the vehicle with the negative lead. With the positive lead, contact the vertical bus bar on the left side of the vehicle. The voltmeter should read battery voltage. If OK, go to Step 3. If not OK, repair the open circuit to the defogger relay as required.

(3) With the negative lead of the voltmeter, contact a good body ground point. The voltage reading should not change. If OK, go to Step 4. If not OK, repair the circuit to ground as required.

(4) Connect the negative lead of the voltmeter to the right side bus bar and touch each grid line at midpoint C with the positive lead. A reading of approximately six volts indicates a line is good. A reading of zero volts indicates a break in the grid line between midpoint C and the left side bus bar. A reading of ten to fourteen volts indicates a break between midpoint C and the right side bus bar. Move the positive lead on the grid line towards the break and the voltage reading will change as soon as the break is crossed.

Defogger switch

For circuit descriptions and diagrams, refer to 8W-48 - Rear Window Defogger in Group 8W - Wiring Diagrams.

WARNING: ON VEHICLES EQUIPPED WITH AIRBAGS, REFER TO GROUP 8M - PASSIVE RESTRAINT SYSTEMS BEFORE ATTEMPTING ANY STEERING WHEEL, STEERING COLUMN, OR INSTRUMENT PANEL COMPONENT DIAGNOSIS OR SERVICE. FAILURE TO TAKE THE PROPER PRECAUTIONS COULD RESULT IN ACCIDENTAL AIRBAG DEPLOYMENT AND POSSIBLE PERSONAL INJURY.

(1) Disconnect and isolate the battery negative cable. Remove the lower bezel from the instrument panel and unplug the rear window switch pod wire harness connector.

(2) Check for continuity between the ground circuit cavity of the rear window switch pod wire harness connector and a good ground. There should be continuity. If OK, go to Step 3. If not OK, repair the open circuit as required.

(3) Connect the battery negative cable. Check for battery voltage at the fused B(+) circuit cavity of the rear window switch pod wire harness connector. If OK, go to Step 4. If not OK, repair the circuit to the Power Distribution Center (PDC) fuse as required.

(4) Turn the ignition switch to the On position.

Check for battery voltage at the fused ignition switch output (run) circuit cavity of the rear window switch pod wire harness connector. If OK, go to Step 5. If not OK, repair the circuit to the junction block fuse as required.

(5) Turn the ignition switch to the Off position.

Disconnect and isolate the battery negative cable.

Plug the wire harness connector into the rear window switch pod. Connect the battery negative cable.

Turn the ignition switch to the On position. Back probe the rear window defogger relay output circuit cavity of the rear window switch pod wire harness connector. Depress and release the rear window defogger switch. There should be battery voltage.

Depress and release the rear window defogger switch again. There should be zero volts. If OK, go to Step 6.

If not OK, replace the faulty rear window switch pod.

(6) Depress and release the rear window defogger switch. The rear window defogger indicator lamp should light. If the indicator lamp does not light, replace the bulb with a known good unit and test again. If the lamp is still inoperative, replace the faulty rear window switch pod.