rpm462

Ok, my rear window defrost is not working. I have a 2003 coupe. I have continuity from one side of the defrost element to the other with the electrical clips disconnected from the contacts. The climate control button lights up when I select the rear window defrost button. Fuse was good and I swapped the Rear Defogger relay (#46 in passenger fuse panel) out with an identical one used for the Bose speakers (#45) just in case it was the relay. Earlier today I would get 12v when I had a meter on the rear window contacts and turned the rear defrost on and then would go back to 0 when I turned it off so thought everything was fine. Left it out of the garage tonight until the rear window was fogged over and tried the rear defrost and let it on for about 20 minutes with car running and it wasn’t working. Put a meter back on the contacts and wasn’t getting any substantial voltage, maybe a few millivolts. Tried turning the defrost button off and on several times and still nothing. Took the cover off the passenger fuse panel and tapped on the relay with a the handle of a screwdriver and then started getting voltage back at the rear contacts. Voltage went to 0v when I turned the rear defrost off and then back to 12v when I put it back on and could tell the window was defogging. So it seems the relay is sticking even though it’s a different one then I had the original problem with. Anyone have any ideas why it would be doing this? I can’t believe I would come across 2 relays having the same identical problem. Maybe a weak battery? DIC is reading that I’m outputting about 13v when car is idling, maybe a bit low? I didn’t put a meter directly on battery tonight probably give that a shot tomorrow.

docohm

Sounds like a bum relay to me. Can you figure out by using your volt meter on the relay socket which is the relay power input and output? If so, make a small jumper and install it in those two sockets. This simulates the relay. Then put your volt meter on the input to the defroster and make sure you're getting power there, 13 volts or so with the motor at idle is about normal. Don't leave the jumper in there without the motor running! You'll run the battery down in a hurry! ha!
If the rear defroster gets warm and works OK with the jumper in there then it was the relay. Try another.
A quick and dirty way to tell if you're getting power is to wire a 12v test light across the heater wires in the window. You could have a bad ground on the heater too, be sure to check that with the ohm meter part of the voltmeter.
OR you could do like I do, don't take her out when the weather is so bad you're gonna need that rear defroster! I disconnected mine right after I got it 3 years ago because it caused interference on the radio....
DocOhm...

rpm462

Thanks for the suggestion, have already tried a couple different relays with the same result (works sometimes but not always) but will give the jumper idea a shot this weekend.

docohm

If several different relays work sometimes but not all the time and the jumper works all the time then it's in the circuit that controls the relay. The button on the dash or the wiring in between. If the jumper doesn't make it work all the time then it's in the defroster itself or the wires hooking it up. I'd start with the ground! Those seem to be the first to go bad on a C5.
Good luck...
DocOhm

Benpup

I had problems with min a year or so ago and found that the actual pint was scratched here and there. Bought a kit from a parts house and masked off the paint where I could gee gaps and painted the stuff on to close the gap. Took a while as there were a few of them where there was a scratch in the paint through to the glass. It's been working fine since then.
I checked mine with a bright beam maglite. even a minor hairline gap in the material will cause it not to work.

rpm462

Thanks for the suggestion, but I put a meter on each side and had continuity all the way across so I'm confident that I have a good connection there, plus it does work from time to time and when it doesn't if I tap on the relay it usually starts working again.