Rear Window Defogger

Electrically Heated Rear Window Defogger operation can be easily checked in three steps:

1. Use ATC or MTC self diagnostics, or the DRB III(R) scan tool to check fault codes and control button operation. Note, this does not check the EBL system. There are no fault codes that can check for the EBL grid, relay, or wiring. Refer to steps 2 and 3 for system tests.
2. Perform a system test with a voltmeter.
3. Check the voltages at the rear grid relay.

SYSTEM TEST

ATC Control Head:

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). 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. If there is no voltage to the rear window defogger grid when the system is ON .

FUSE AND ELECTRICAL SHORT TEST
1. Stop the engine.
2. Check the fuse supplying power to the rear glass grid relay. If the fuse is good . If the fuse is open, then go to Step 3.
3. Locate the ground lug that connects the negative lead of the rear window grid to ground. Remove the screw holding the ground lug and isolate it from any vehicle ground.
4. Connect one lead of an ohmmeter to the negative ground lug of the rear window grid (with the lug still not connected to a vehicle ground), and check the resistance to a known good vehicle ground. If the ohmmeter indicates less than 10 ohms, then the rear window defogger grid is shorting to the vehicle ground, then go to Step 5. If the ohmmeter indicates more than 100 ohms of resistance, then go to Step 6.
5. Disconnect the rear window defogger grid voltage feed wire from the vehicle harness. If the ohmmeter still indicates less than 10 ohms, then the rear window defogger grid is shorting to the vehicle ground and must be replaced. If the ohmmeter indicates more than 100 ohms of resistance with the grid voltage feed wire disconnected, then go to Step 6.
6. There is a problem with the circuits that connect to the rear window defogger relay. Refer to the appropriate wiring information for circuit descriptions. Check wiring with an ohmmeter to verify proper wiring connections to the rear window defogger relay. If no problems are found, then replace the fuse and try running the system.