Hey guys, I'm having an issue that hopefully some of you knowledgeable people on here might be able to help me with (or at least point me in the most probable direction)...

As the thread title indicates, I'm having an issue where my rear defroster stops turning on/off, and it appears to be directly related to the engine temperature (which makes no sense to me). By "stops turning on/off", I mean that the light no longer toggles on/off, I do not hear the beep from pushing the button, nor do I hear the click of the defroster actually turning on/off.

On a cold start, the rear defroster works like a champ, so I know the grid/connectors on the back glass are good, and if it was a fuse, I wouldn't expect the defroster to ever work at all (I have checked them as well).

Once the car heats up however, the button stops working. If the rear defroster is on when this happens, I am unable to shut it off (turning the car off/on will fix this), and if the defroster is off when this happens, I'm unable to turn it on (the bigger problem).

If I've been driving the car, and its heated up enough for the button to stop working, I pretty much have to let the car sit for 3+ hours before the button starts working again. I've tried pushing the button a billion times, wiggling it left/right to see if it's just a bad connection, etc.. nothing brings it back to life aside from leaving the car off for an extended period.

I live in Syracuse, NY and we get a ton of snow, so this is a fairly big issue for me, as I rely heavily on rear defrosters all winter long. I just bought this car as a "winter rat", and I'm sort of regretting my decision at this point.

Thanks in advance for any ideas!

 

I'm assuming that I can't edit for a while on these forums.. not sure if it matters, but the car is the 2.7 liter V6.

 

Sounds like the relay is going bad. Look in the fuse box located at the left end of the dash. There's a legend of what's what on the underside of the cover. In the middle of the panel (it's actually called the Junction Block) there's a large relay. That's for the rear window defogger. It probably needs replacement.

 

Thanks for the suggestion, I will definitely take a look when I get home from work today. I know that you can visually see if a fuse is blown, but is there any indication if a relay is bad, or do you just swap them out and see what happens?

Also, does it make sense that this appears to be heat related if it's a bad relay? Do they overheat or something?

My knowledge of cars (beyond driving them) is quite weak, although I am trying to learn as much as I can, so I would love to hear the reason you think it might be the relay if you don't mind.

Thanks again, I really appreciate you taking the time to reply!

 

The purpose of a relay is to allow sending a large amount of current to some part that needs it by sending a small amount of current to the relay. It reduces the distance the large amount of current needs to travel and reduces the need for switches and wiring that can handle the large current.

The rear defogger is supposed to turn off by itself after a reasonable interval, 10 minutes or so. That's why yours turns off by itself. The fact that the car has warmed up is not the cause of it turning off. But you should be able to turn it back on for another 10 minutes even if the car is warmed up. I'm guessing that once the car is warm, for some reason the relay won't stay engaged. Anyway, I would start with the relay because it's cheap and a possible solution to the problem. The body computer is also involved in the circuit and could be the source of the problem but it's not cheap to replace. When in doubt, always start with the cheapest thing first.

 

Thanks again for the reply, and I 100% agree with starting with the cheapest thing first!

Just to clarify, the issue isn't that the defroster turns off when the car is warmed up (I know they are supposed to turn off after the 10-15 minute interval), it's that the button stops working completely (no beep, light, relay click, etc).

I can successfully turn the defroster on/off repeatedly for the first 10-15 minutes while the car is warming up. Once the temperature gauge comes up to what I assume is normal operating temperature for this car however (somewhere between 1/4 and 1/2 on the gauge), it stops working, and will not work again until I shut the car off for an extended period of time.

NOTE:
I did notice that the temp gauge has to be basically flat lined (cold) for the button to start working again. The defroster works fine while the car is warming up, but doesn't start working again until it's completely cold.

I will definitely swap out the relay tonight, but do you still think this could be the issue based on the above info?

 

As I said, it could also be the body computer, which also controls the gauges by the way, but if it's the relay, which would make it impossible to control the defogger with the switch, and you can fix the problem for $20, then why spend hundreds on a body computer? And if it is the relay, spending hundreds on the body computer still wouldn't fix the problem. To even have the body computer diagnosed at the dealership using their diagnostic computer would cost you a hundred bucks before they spend anything to actually fix the problem.