Parasitic voltage drain; a new diagnostic method
 

Hello,
I found one of my car's battery would run down in a few days if I did not keep a battery maintainer on it every night after driving. The battery is brand new. I decided to measure the no-load, standby current through the car with every device shut off except clock, ECU's, radio memory, alarm system and other background, legitimate power consumers. I disconnected the negative cable and inserted a 10A digital ammeter between it and the chassis ground. It read 0.8A with the trunk open and 0.4A when the trunk lights went off by closing the lid. An acceptable value would be 0.030A to 0.050A. No wonder my battery was running down.
One method to trace the source of the current draw is to pull fuses, one at a time, until the current drops off. Re-insert the offending fuse and the parasitic draw should return. This localizes the faulty circuit.
I came across another method which can be used on modern blade type fuses. It simply measures the voltage drop across each fuse. Circuits with no load will have 0mV across the two blades of the fuse. This is measured with the fuse in place and by touching a sharp electrode to each bare spot of the fuse blade on top of the fuse on either side of the amperage number. I got 0mV on all fuses except the clock fuse and the passenger side seat heat and lumbar motor. I pulled this fuse and got a passive standby current of 0.030A, which is excellent. The article on this method is:

http://motorage.search-autopar...rticle/detail/660025

I really don't need the faulty circuit on my car, but if I did, I would look for sticking relays or switches or faulty wiring in that circuit.

I found this very fast and easy. Best of luck.

Dave

Hi Dave:

The link got truncated before you cut and pasted it.

If your meter has a probe end that will make contact with the fuse blade, that's a good way, but pulling them seems to be easy enough to me. EDIT: Ignore the foregoing sentence.

Oh, and I saw a car that had a parasitic drain in the same seat circuit. I worked the lumbar switch a few times and the drain went away. :) Good ole use it or lose it.

That is an excellent way to check these things out... Betting money, based on your experience, that it is my seat heater circuit on my '92 that causes my drain on that car. It takes a week or so, but it does drain WAY faster than any of my other 928's...

I've got a slow drain myself. I think my red door lights are staying on.

Very common for the connectors on those lights to be a rusty glob and cause drains (I know). Just remove the door insert, reach back and pop them out and splice on the new ones.

This one?

http://motorage.search-autoparts.com.../660025?ref=25

http://motorage.search-autoparts.com...310_scope1.jpg

http://motorage.search-autoparts.com...cope2-copy.gif

I found an old car phone unit hooked up still sucking power in my car. Removed the unit no more drain.
I mention this because I was thinking of all kinds of things it could be. Look for things that were added on by a previous owner first.

Yep. That's the one. It's much easier than pulling fuses and looking at the ammeter at the battery cable area. Once
you find an unexpected voltage drop, you can confirm it at the battery by measuring current drain.

Given your diagrams, I'd like a little help with the math on this. What resistance are we assuming, or assigning for the fuse terminals?

In the middle box of the text jig you show 0.7mV with a 1 amp draw. Given V = IR, this would be R = 0.0007/1 or R = .0007 ohms(0.7 mOhms). in the lower box we have 4.5mV with a 650mA draw. So R = 0.0045/0.650 or R = .069 ohms(69 mOhms) which is a 100 fold delta in fuse resistance. My actual measurements of fuse resistances is hampered by not having an Ohms scale that goes down that low.

Oh, silly me. I never noticed before that you could put a probe in the TOP of the fuse. Now I see. That is MUCH simpler. Thanks.

That's one of the reason for my post. I never knew either. BTW forget about math on the milivolts readings. I varies by the fuse rating. It's either 0 or it's 4,5 7,mV when you expect zero. That indicates your problem. Check your clock fuse or your interior lights fuse to get an idea on voltage drop. Anything but zero that you can't explain, is a problem.

Thank you Dave. Very valuable information. I can hardly wait to try this method.

Yep, that's the way we do it on Volkswagen. It eliminates the problem of say a sticking relay. If you pull the fuse and cut power the relay might work normally again for a while only to start sticking again. Troubleshooting starts all over again and can be a real pain. The method does work very well. Joe

On my '87 I have one of these parasitic drains. Since I only drive the car one a week, if I leave the battery connected, it'll be dead by the time I get around to driving it again.

I tried this method last night, but I'm not sure if I was doing it right. So I'd like to have some of you experts walk me through to make sure I was doing it right.

First off, let's talk equipment. I have a Harbor-Freight $3 DMM. Look familiar?

http://www.electricrider.com/accesso...mages/dmm1.jpg

So, which of the 3 plugs do we use? I assume black goes to COM and Red goes to VΩmA, right?

Next which setting should we use on the DMM? I think I used 20m (this is the setting at about 3:00 on the dial - kind of hard to see in the pic.) This stands for 20miliamps?

When I measured the interior light fuse, I believe it read 1.2. So is that 1.2 miliamps? And is that appropriate or not?

There is an after market alarm system on this car, and I have a feeling that might be the culprit, but don't see it wired into the CE panel. They might have taken it directly to the battery.

Use the red in V-ohms-mA and black in COM. You would use the DCV 200m position. You are measuring voltage drop across the fuse terminals.