99 Cavalier rear defroster question

I don't have the diagram for the 99 but the on the 96 when you press the defog button it picks a timing relay that supplies power to the rear defogger. The timing relay is probably bad. On the 96 it is in the Heater-AC and rear defogger control assembly. If you want a decent manual then get the Helm manual as it has the electrical diagrams.

I'm going on the assumption that the 96 and 99 are essentially the same, because I don't recall there being any major design changes.

Anyway, the timing relay - is that the long rectangular box right behind the defogger button on the hvac controls in the center dash? If so, I thought I tested it ok, but I'll have another look at it.

Thanks for the tip!

You won't find voltage if your measuring across the window. One side is grounded and the other side is fed battery voltage. When you push the button it activates a relay (if the light in the button comes on the relay is working) from the relay there is line running to the rear window. On any of the vehicles with a movable rear window the common problem is the wire breaking at the body flex point. On vehicles with fixed windows you want to look real close at the ground end. A little rust where they connected it to the body will make it stop working. On both look VERY close at the connectors on the glass itself. If they get hit or flex much they can break free of the glass enough to break the grid. Then your looking at some real problems to repair them.

If the glass has soldered on connectors it is possible to solder it back together BUT you need to do it FAST. No small iron and waiting, that WILL break the glass. You want a big soldering gun and you want to CLEAN the contact on the glass with a stainless brush or scraper, then clean and tin the connector. Now heat the connector until the solder is hot and put it in place and let it bond. as soon as it bonds remove the heat and let it cool slowly.

It is a very basic timed circuit.

Oh, I didn't know that. Anyway, yeah you hear a "click" and the light comes on when you push the button.

I haven't taken off the interior panels to see where each wire is going yet, since it's a coupe, it's a pain in the butt to work back there. However, I went back out and tested between ground and each wire running to the back windows and got no voltage - using this method, I should have got voltage on one of the leads no?

That leads me to believe there might be a break in the wire somewhere.

Both leads are firmly attached to the back window and there is no rust or corrosion on the window its self.

If you didn't get voltage when testing from ground to the glass you have a bad wire or a bad connection in the harness. Because of the current involved, if a connection starts to get corroded they start to heat up and can melt the connector or even burn off the wires. They can be a real pain to find as well.

Ugh, I'm in for a treat I'm sure. Well, one thing that will make it easier - which side of the window is power? Drivers or passengers side? That'll save me from tearing up the wrong side of the car :)

Thanks again you two for all the help!

Sparky

That one I cannot say for sure I don't have a chassis wiring diagram handy for that car. You should be able to find the wire color for the feed on the back of the relay and follow it to one side or the other. At least you can rule out the ground side though. The power feed I have usually seen is a heavier purple wire. The grounds have been orange or black.

I assume your voltmeter can also measure resistance? To test which side is ground; Set it to resistance (a very low value) and put one lead on the battery negative terminal and one on either side of the demister. If it registers 0 resistance this is the ground side. If neither register 0 it is safe to say the one which registers less resistance to ground is the ground side. Since neither sides register a positive voltage we can assume it is the positive end that is screwing up and therefore you will still be able to detect the ground.

Once you know which end is positive you can use a similar method to test the wiring in the car. Put one lead on the relay positive, and one on the demister positive, there should be zero resistance. If not, you will probably have to rewire the interior. Shouldnt be too difficult a job, due to age and corrosion i had to rewire the interior lighting, power windows, alternator charge circuit, sound system and fuse box in my car. Hope this helps