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Rear Window Defogger System Components
The rear window defogger system consist of the following components:
• HVAC control module
• Body control module (BCM)
• DEFOG relay
• Rear window defogger grid
Rear Window Defogger Operation
When you turn the ignition to the ON position, ignition voltage is supplied to the DEFOG relay coil. Battery positive voltage is supplied at all times to the DEFOG relay switched input. When the rear window defogger switch is depressed, the HVAC control module illuminates the rear window defogger indicator and sends a class 2 message to the BCM. The BCM energizes the DEFOG relay by grounding the control circuit of the relay. This allows battery positive voltage from the relay switched input through the switch contacts and out the relay switched output to the rear window defogger. Ground for the rear window defogger is provided by G306 or G390 (crew cab).
When you turn ON the ignition and press the rear window defogger switch for the first time, the defogger cycle lasts 10 minutes. Further operation results in 5 minute defogger cycles.
Do you know what your messing with ? What's the problem ?
check your coil pack (they can test at most parts stores) if good Check the oil sending unit, probably have to replace it, don't know how to test it. If that doesn't get it It might be the crank sensor. All of those will make your engine not spark. Assuming, you are getting fuel that is.
There are 3 fuses that control the rear defogger, and 1 relay
The fuses are: 10 amp HEATER, 10 amp MIRROR-HEATER; under the dash
The 30 amp DEF fuse is in the fuse box under the hood
The relay is the DEFOGGER relay, which is on the back side of the fuse panel under the dash
Also, check for a loose wire on the defroster grid (on the back glass), this is the most common problem.