Rear defroster question

[mechanizeddeath]

I'm not entirely sure if there's a problem, or if this is just how it works, but I don't think my rear defroster is working right. It turns off after just a few minutes, I haven't timed it or anything but it only works for 2-3 minutes or so. That's enough to get rid of interior fog but not enough to take care of any ice and snow buildup on the outside of the glass.

Of course I chisel the ice off the windows before I start driving, the issue I have is more when I'm in the car but not moving fast, snow sticks to the window and after a few minutes I can't see out of it. I have to remember to mash the button every few minutes or pull over and brush the back window off by hand.

Is this normal? Or should it be staying on longer than 2-3 minutes?

[Pontiac6ksteawd]

Its not normal. Sounds like the thermal timer needs to be replaced. But I dont know if that is located in the switch, or in a relay type, in the fuse panal.

[mechanizeddeath]

Hmm, well that's a start at least. I have a '96, if that makes any difference.

I hope it's in the fuse box, I really don't want to take the dash apart again, lol.

[mechanizeddeath]

What would I ask for down at NAPA? Thermal timer for a rear defroster? I looked on Rockauto and all I can find is the switch itself, but I might be looking in the wrong spot.

I won't be doing anything until it warms up. It's about 10 degrees out right now. :O

[86euro]

If it's like the older ones, it's built into the switch.

Also, it should be good for maybe 10 minutes on the first go around. Then if you switch it on again, it should only stay on a couple minutes. After the ignition is turned off and then on again, it will reset and it will stay on for 10 minutes or so again.

[mechanizeddeath]

Interesting. Yeah I don't think it stays on for 10 minutes at all, but then again I haven't timed it on the first try. I did notice it seemed to vary though, that the more often I pressed it, the shorter it worked.

Any idea why it works that way? I guess I was spoiled by the Subaru, not only could the defroster easily melt thick ice, it could also be left on all the time. But those cars are designed with winter in mind, so it's apples and oranges I guess. :)

[SilentWing]

My best guess is that people forget to turn the defrost off, so it stays on long enough to clear the window under light snow and ice then shuts off so it doesn't drain the battery if someone were to leave the car with it still on? I could be entirely wrong though seeing as though defrost draws its hot from ignition wiring, so it's really just purely a guess.

[86euro]

I think it's just to help the alternator live longer. The rear defog is basically just a big electrical short.

[86euro]

And for some reason, the defogger just doesn't work very well in some cars. My Celebs have all worked great, it sucks in the Beretta, and some Camaros have been better than others.

[mechanizeddeath]

Electrical system protection is certainly a valid explanation. I know when I press the button, the lights dim quite a bit, especially if I'm stopped with the brakes on. Not to mention I've got a 4 gauge power wire hanging off the battery, so my current draw is above stock anyway. :D

I probably won't bother with it then, if I'm only going to get a few more minutes of activation out of it. Thanks for the replies though. :)

[SCREECH]

The Aleros and GrandAms had a big problem with their rear defroster. They actually have a module on the right rear C-pillar as the rear window is also used for the radio antenna. The module and wire connectors that power the grid is prone to failure. It develops a weak connection, overheating the connection further due to increased resistance, melting the connector(s), further breaking the connection, until the defroster ceases operation altogether. The new module is over $100 from the dealer, and the wiring connectors are another $15-$20. The only way to fix it for good is to solder the wires directly onto the module (which I did), and I've been enjoying a clear back window all winter! :)

['84 6000]

I was just thinking about posting the very same question as the Buick has the same problem. Its what I figured though, its only normal for a thermal switch to wear over time and many cycles.

[CamoDeafie]

the 94 ciera wagon had the same thing; defogger wont always work...ditto for the 93 century....btw Maxine DOES NOT have defogger for rear window, so it can be trick getting the car out in reverse if i didnt manually clear the window lol

[mechanizeddeath]

Well at least I'm not the only one with the problem. I've been lucky enough that every car I owned has had rear defrost, even when they were optional features you were a fool not to order it on your car if you lived here. :) I remember my friends dad ordered a Dodge Omni and deleted every option on it, including the radio. Vinyl seats, manual everything. But even that car had a rear defroster!

[mechanizeddeath]

Had a chance to time it again tonight. Times are rough, but close:

First press: 2 minutes.
Second: 1 minute.
Third: 30 seconds.
Fourth: 10 seconds.
Fifth: 2 seconds.
Sixth: 1 second.
Seventh: 15 seconds, after waiting several minutes after the sixth try.

Whatever the problem, it seems to be getting worse. :( I guess I may have to replace that switch anyway.

[Prospeeder]

Replace the switch, I recall reading a Technical Service Bullitin from GM this very issue

[mechanizeddeath]

Thanks Prospeeder. A TSB? Interesting...

At least I'll only have to pull the long vent panel to get to the switch. Glad it isn't buried like some cars.

Placing an order at Rockauto right now. I'd just raid a yard but with my luck I'd end up with a faulty one anyway. Looks like it's going to be a hair over $26 shipped, which seems a small price to pay for a window that stays clear.

[93centsp]

Based on my '93 Century factory manual, 86euro correctly identified the defogger controller (switch) operation (10 minutes for first operation, 5 minutes for further operations).

Looks like a solid state timer relay within the controller determines operation time.


Mechanizeddeath, perhaps, in your spare time, you could dissect and repair the faulty controller, then report back with a how-to for all us tech hungry souls ;)

[mechanizeddeath]

Well as soon as I get it swapped I'll be taking it apart to see what's inside. If it looks repairable I may very well take a crack at it. At the very least I'll post a picture of the pieces before I toss it in the bin. :D

[mindbender]

I just had the same problem on my way to work! It was annoying the hell out of me. Please let me know if the switch fixed your problem.

[mechanizeddeath]

Still waiting on the part I'm afraid. It's only been 4 days though. A shame, since it was above freezing around noon today, and I'd have gladly installed it if it was here. :(

[mechanizeddeath]

Part is still not here. I'm a patient person so I thought little of it, until I noticed that my post about ordering it was made on the 11th, and it is the 22nd!

Logged into Rockauto, said it was shipped and tracking info indicates USPS left a notice on the 14th. I received no such notice, and this is the 2nd time they've done this. Not Rockautos fault of course, but my local post office sucks. This never happened at my apartment or my parents old house, only here.

Oh well, expect an update Monday

[mechanizeddeath]

Well I went and picked up the part today. Installed it despite the temperatures and just my luck, it still cuts out after 3 minutes. :(

Either the new (reman?) switch is bad, or there's something else amiss. Timer is definitely in the switch though, with the panel off I can hear and feel it click on and off.

Any other ideas?

[Pontiac6ksteawd]

Check the ground on the rear window..

And on the other side, make sure the positve line is fully into the plug. Mine wasnt on my STE. Didnt have rear window defrost for 2 years until I figured it out.

[mindbender]

Well I went and picked up the part today. Installed it despite the temperatures and just my luck, it still cuts out after 3 minutes. :(

Either the new (reman?) switch is bad, or there's something else amiss. Timer is definitely in the switch though, with the panel off I can hear and feel it click on and off.

Any other ideas?

Hum, I just order one too. Wonder if mine is going to have the same problem.

[Ray_McAvoy]

Installed it despite the temperatures and just my luck, it still cuts out after 3 minutes. :(

Either the new (reman?) switch is bad, or there's something else amiss. Timer is definitely in the switch though, with the panel off I can hear and feel it click on and off.

Any other ideas?

You might want to check the 30 Amp power accessory circuit breaker (located in the fuse box). According to the wiring diagram in the factory service manual, power for the rear defogger goes through that circuit breaker. I believe it's an auto-resetting thermal type circuit breaker. It might be getting weak and kicking out under the load of the defogger.

[Prospeeder]

im wondering if the switch uses the resistance of the circuit to detirmin roughly what the glass temprature is and how long to keep it going. More heat is more resistance i belive, so if the conections are corroded or any wiring has built up resistance it could be throwing the switch off.

[mechanizeddeath]

Interesting thoughts. I'll certainly be looking into this more come spring, gonna take the car apart and rewire the stereo anyway. :D

[jeffreyclay]

I have the same problem on my '94 Ciera. First activation gets about 2 maybe 3 minutes then turns off. Subsequent cycles get shorter as you go. Bought a new switch module and there was almost no difference in the times! shortly after I bought the car I discovered a nicked wire that feed the current to the glass. It was touching where it passed thru some sheet metal in the trunk and nearly killed the engine when I turned on the defogger. Wrapped the wire in tape and that fixed that. The short cycles seemed to arrive after that. I'll have to take some voltage readings to confirm the wiring harness wasn't damaged from the shorted wire. Could be a case were I only have 8-10 volts available when under a load. Does anyone know if the module relies on a specified current for timing sorta like a turn signal flasher does?