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928 Owners Club Forum :: Topic: Rear defroster relay location (1/1)
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TOPIC: Rear defroster relay location

Rear defroster relay location 24 Mar 2016 02:33 #18877

Hello, everyone. I'm a newbie to the forum. Glad to have found it! I had googled for answers to why my 928 battery is constantly draining and I came across this forum in a web search. So here I am. :) I hope I can eventually contribute to the site as my experience with my 928 grows.

Meanwhile, I read that one potential problem with the electrical drain is in the rear defroster relay. I'd like to yank it and see if that doesn't help solve the problem of my battery draining within a week or two if I don't drive the car. Can anyone please tell me where this relay is found—or if anyone has figured out a magic bullet of sorts for the '84s and battery drainage problems. I have a new alternator and battery, and I'm out of business if I don't start and run her pretty regularly. My mechanic, meanwhile, can't seem to find the problem.

Thanks so much!
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Rear defroster relay location 24 Mar 2016 10:00 #18879


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I would advise that you disconnect the battery ground strap while the car isn't in use until the parasitic drain is resolved. There are many likely sources for drains. Diagnosing them can be a challenge as often times the drains are on circuits that aren't fuse protected.

The normal current draw is between 20-30mA.

Get a helper. Disconnect the passenger door so that you can access the CE board for diag without disrupting the accuracy of your diagnostics. Disconnect the battery ground strap. Get a multimeter and set it to Amps. Place one probe to the negative battery terminal and the other to the ground point for the battery ground strap. This will tell you what your draw is. Now, have the helper remove the fuses in the CE board one-by-one until the data changes on the multimeter. Note the circuits that the draw is coming from if it is from one of the fuse protected circuits.

This greatly assists to isolate the reason for the excess current draw.
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Rear defroster relay location 07 Apr 2016 01:51 #18934

Sorry for the late reply; I've been out of town. In any event, this is good to know, though I don't quite understand everything you said.

I'm going to keep this solution in my back pocket. Meanwhile, while I was out of town, my mechanic was working on an oil leak for me and I asked him to check for electrical "leak". He found that an ABS relay (odd; my 928 has no ABS) had failed and it was evidently drawing excessively. I don't know exactly what the "savings" is as I don't really understand these electrical matters, but it appears that the battery is holding a charge considerably longer—though I'm still testing how long that is. I'm hoping for a couple of weeks minimum, which is where it was before this problem cropped up. Seems to me any car should hold a charge for at least a month or more...no?

If it continues, I'll take your lead (pun intended!) and try what you suggest. However, when you say, "Disconnect the passenger door so that you can access the CE board for diag without disrupting the accuracy of your diagnostics," what do you mean? Again, sorry: I don't know what the CE board is, and not quite sure what "disconnecting the passenger door" means. What does the passenger door have to do with diagnosing the problems? :/

Thanks again for your reply; it's much appreciated. And apologies again for the late reply.
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