Hey guys! I seem to have an issue going on with my rear window defogger on my 91 Olds 88. I had to replace the original defog switch a few years ago because it no longer would turn on"no light", so I found a used one at the junk yard off an 87 Olds, hooked it up and sure enough everything worked fine. However recently I have noticed that it shuts off after 3 minutes and I usually have to wait a good 15 minutes or so before it will even turn on again. The grid lines all heat up evenly and clear about quarter inch of frost before it shuts off, I am positive that there are no breaks in the grids. So I went and checked the circuit breaker and it was HOT!! which surprised me because the damn thing only ran for 3 minutes and I have already tried swapping breakers and it does the same thing. I am thinking that the relay may be to blame here because my power seat and windows work fine all the time. I would like to hear some of your thoughts on what I should check next, and I do have a digital multi meter and test light to use if needed. And by the way I have had the alternator and battery replaced this year and the battery cables have been tested and cleaned.

Hopefully one of our team can find an All Data electrical diagram for your car.
There is a timer that limits the time on, for the defogger.
I am not sure how many minutes that should be.
They are a fairly high amp draw and ya stuff can get hot, and mine cooked the radio/defogger module wires. Now I wish mine was limited to 3 minutes.
Hopefully soon more members chime in to help you out...

Thanks Soft Ride! I am amazed that your module wires got burnt! I will say that GM had some very odd ball wiring in these cars and they can be a total nightmare since they are so hard to get at. I do have the 2 inch thick factory service manual for this car but it seems to have more info about the grid lines and the digital climate control systems but nothing for the manual style control, like what I have. I know it mentions a timer, but is the timer actually inside the switch module itself, or is it located elsewhere?? According to the manual: The defogger will stay on for 10 minutes, and if you switch it on again then its 5 minutes every time after that. The funny thing is that when I use it now, it stays on for less than 5 minutes and it simply wont let me turn it on again for a good 15 minutes. So would it be possible that they changed something in the control switch between 1987 to 1991? I did however swap the relays behind the glove box as well as the circuit breaker today and I took it for drive and it still shuts off too soon. I know I can get another one at the junk yard, but I figure that I should find out if something else is at play here.

There often are changes or modifications made between model years, IDK if any effect the defroster though.
Is your FSM book specific to 1991? Do you have the electrical schematics section?
If not GM dealer should have reference material available to help you with the defogger wiring.
In Pk Ave they use the rear window grid as the antenna as well.
It has one shared module for antenna and defrost grid. Common to have this design fried.
In my case it was hmmm..what'* that smell....oh the defrost module is cooking!

Hi Soft Ride. Yep my book is for the 1991 Olds 88 Royale. I bought it used earlier this year off Ebay and it does have the wiring schematics. As for the antenna and defroster module your mentioned, did they really use the grid as the antenna? Because I know we had an old Chevy Blazer that had the antenna in the middle of the windshield, but I never knew that GM also used the defroster grid too. I have an aftermarket power antenna that I installed 3 years ago to replace the original one, and it works perfectly. Would that possibly have anything to do with the defroster because I think that was around the the same time that the defog switch went out but I am not positive? I am going to study the FSM and see if I missed something. I have had the car for over 7 years and my grandmother had it before me ever since it was new, so its always been very well maintained and now at 188000 miles, I am hoping to get to the 200000 mile mark if I play my cards right lol. Thanks for your suggestions! I will post updates soon.

If you have a body mounted antenna, the grid will not be part of the antenna system.
Anywhere in your book does in mention where the timer is? Is it part of the switch?

The 97-05 was the new designed Pk Ave. They smoothed the lines out and a body mounted radio antenna was not in their plans I guess.
So instead of a windshield antenna, they used the rear defrost grid.
I think the Lesabres are the same. I wish they would have made my system more beefy in it'* design. The modules are expensive for the PA and they dont last long unless you improve the gauge of wires going into the module. If and when & get around to mine, I will be attempting to take the new module apart, and install thicker wires going in.

Hey Slow Ride! I have been looking thru my FSM and it does not mention where the time is. However I am betting that it is in the module switch itself, because I saved the old switch and I decided to take it apart and it has its own relay inside, so I then took the other switch thats in the car now and compared them and there are absolutely no differences in design or anything. I am going to see what happens when I use it and I ll keep up udated. Thanks!

Here is an update on in regards to the rear window defogger issue. I had to make a quick trip to the store tonight and its -5 degrees here in Montana, as usual the defogger shuts off after 3 minutes, so as I was heading home, I thought to myself about how the defogger is also on the same circuit as the power door locks. About 6 or so years ago I installed a Bulldog keyless entry system in my car and it has always worked great, so while I was driving I pressed the lock button on the key fob and I could hear each of the doors make a click sound, but they didn't lock! So I wait a few minutes and then press the lock button again, and this time they did lock. I know the power locks are tied to the same circuit breaker as the rear defogger, so I am thinking that since the circuit breaker gets hot and stays hot even though the defogger stays on for less than 3 minutes at a time, so something has to be wrong in the wiring. Does this issue I saw with the door locks give me somewhere to start tracking down the culprit???

the fuse that circuit or the relay socket could be loose or corroded, this causes only a less amount of power available

That'* what I am thinking, as of right now I am studying the FSM for the location of all the relays,fuses and connectors. I will post an update later today as to what I find out.