I have a 1993 Buick LeSabre with 535,000 km's and for some reason the rear window defogger no longer works. When i engage the switch for operation it lights up as if ON ...but none of lines in the rear window show any signs of warming up. What could the problem be. ???
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Could be a bad relay !
The rear defogger circuit is protected by a 40A fuse. Voltage is supplied through the fuse to the rear defogger relay. When you press the rear defogger switch, the heater and A/C programmer sends a class 2 message to the body control module (BCM), to ground the rear defogger relay coil output circuit. This in turn closes the relay contacts allowing current to the rear window grid.
Do you know what a class 2 message is ? What a BCM is ?
HTD BK relay in rear fuse box ,location In the center right rear of the passenger compartment, under the rear seat cushion
Watch this video on testing from relay base ( where it plug's into )
Do VEHICLE RELAYS Testing from the relay base
I get that you can't turn it off, but what do you mean you cant "set" it? It's either on or off. When it's on (which sounds like all the time) does it work - defog/defrost the back window. If you can't turn it off, your choices are to either leave it on all the time (which may run your battery flat eventually) or replace an obviously bad switch. . .
Are you sure that is not the AC compressor relay? I don't know your car, but not aware of any cars having a defroster relay, a defogger relay for the rear window with the electric grid in the window, yes. Never heard of a defroster relay.
This must be the fuse and is found under the dash within the car. If the fuse blows out then you must check the wiring to the front washer motor and to the rear.
Disconnect and check the rear and the front washers.
Either your control module is burned out or your window contacts have become detached. Permatex offers an electrically conductive adhesive for the latter (less than $10). For the module, it's a combo radio antenna and defrost control. It's located on passenger side in back at the pillar. You have to take off the fabric panel to get to it.