Parasitic battery drain. Help!

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30 May 2013 06:32 #14913 by paul928
Parasitic battery drain. Help! was created by paul928
Would really appreciate any advice regarding this frustrating problem. I have an 85 928S that drains the battery over a 2 week period. Battery is new, tests fine and is fully charged. Voltmeter shows .5mA draw at rest. Two weeks takes the battery from fully charged to around 5 volts. It seems as though something is turning on intermittently? I have read other posts and I guess a battery tender would be a solution but I would really like to solve the mystery and correct the problem. Thanks for any/all advice.

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30 May 2013 10:05 #14917 by Kiln_Red
Replied by Kiln_Red on topic Parasitic battery drain. Help!
Here is a prior post by Brad with instructions for chasing current draws

The way to check is to disable the passenger side door switch by unplugging it so that you can pull fuses when you get a meter hooked up. set your meter on amps and plug the positive probe at the meter into its 10 amp hole. Disconnect the negative cable and hook the leads between the cable and the battery. read the draw and start pulling fuses until you find the circuit that kills the draw. That will give you a direction to go

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30 May 2013 10:48 #14918 by xstepson
Replied by xstepson on topic Parasitic battery drain. Help!
That post is absolutely the best way to find a parasitic drain.

I have found that several of my drains were from the rear defrost relay. I have removed all those rear defrost relays from my cars and amazingly have no more battery drains. You might get lucky. YMMV

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30 May 2013 11:57 #14919 by paul928
Replied by paul928 on topic Parasitic battery drain. Help!
Thank you both for the quick replies and the good suggestions. I will remove the rear defrost relay immediately and proceed from there with my fingers crossed!

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30 May 2013 12:12 #14920 by martinss
Replied by martinss on topic Parasitic battery drain. Help!
I found that the alarm was drawing quite a bit of current, especially with the hood open - the alarm is connected to the hood switch that also serves the underhood light. Since I tend to have the hood open when i am working on the car I leave a battery tender attached.

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30 May 2013 15:22 #14921 by paul928
Replied by paul928 on topic Parasitic battery drain. Help!
Thanks Martin. I will check that, too. Really appreciate all the suggestions. It's great that you all are willing to take the time to help! I'm optimistic now!

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30 May 2013 15:46 #14922 by wallyp
Replied by wallyp on topic Parasitic battery drain. Help!
One of the first things to check: Do all interior lights work exactly as designed?

All of the lights have a three-position switch - OFF, ON, AUTO. With the switch in AUTO, the light must come on when a door or the hatch is open, go off when it is shut. If the interior lights do not work like that, start with the hatch switch, then the door switches, then a shorted wire in the light near the interior mirror.

Check the glovebox light and under-hood light.

Wally Plumley
928 Specialists
www.928gt.com

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30 May 2013 16:18 #14924 by paul928
Replied by paul928 on topic Parasitic battery drain. Help!
Thankyou Wally. Will do. I'll post the result of my efforts in a week or two as I monitor the battery. I'm going to make one change at a time to see if the problem can be isolated. The drain can only be measured at .5mA. Is that in the realm of "normal"? I thought the clock and maybe computer memory would draw that much? The battery is stone dead in a week or two of "sitting" and that's what led me to think something was turning on itself on while the car was idle?

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31 May 2013 10:33 #14934 by wallyp
Replied by wallyp on topic Parasitic battery drain. Help!
I think that you have slipped a decimal point - the "normal, expected" drain on a late model 928 is 20 mA. A reading of 0.5 mA (0.005 Amp) is virtually unmeasurable. If the measurement is actually 50 mA, that is high, but the battery will last one or two weeks. If the measurement is actually 0.5 Amps, the battery will last one or two days.

Wally Plumley
928 Specialists
www.928gt.com

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01 Jun 2013 15:47 #14940 by paul928
Replied by paul928 on topic Parasitic battery drain. Help!
Thanks Wally,
You must be correct. It must be 50mA. The battery is definately dead in one or two weeks not one or two days. This past Thursday it was 12.20 V and today (Sat.) it's 12.06 V. I pulled the rear defroster relay on Thursday. If the drain is 50mA would pulling the fuses one at a time enable me to diagnose the source of the drain?
Thanks again, Bill

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01 Jun 2013 23:47 #14942 by emmanuelli
Replied by emmanuelli on topic Parasitic battery drain. Help!
Bill, that's the idea. usually, when you pull the offending circuit's fuse or relay, you should see a drop in the amp meter. that would give you a good idea of where to look.

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02 Jun 2013 07:55 #14943 by wallyp
Replied by wallyp on topic Parasitic battery drain. Help!
After extensive searching, I finally found that a similar drain on my '90 GT was due to a faulty SCR in the fan amp on the panel forward of the radiator. Only easy check is to unplug the amp and see if the current draw goes down. Remove one screw, pull the plug.

Wally Plumley
928 Specialists
www.928gt.com

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02 Jun 2013 16:01 #14949 by paul928
Replied by paul928 on topic Parasitic battery drain. Help!
Thanks Wally. I'll try it.

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