Strange issue troubleshooting a parasitic drain.

[drummertom]

I am chasing down a parasitic drain on a 1995 Mustang that someone put a 97 Cobra motor and trans in. I think they used the wiring harness from the Cobra. They also added wires for the convertible top and rear windows. Anyway, what is strange to me is, I took the Positive cable loose and put a test light between the POS terminal and POS lead and it lit up, indicating a drain, but if I put the test light between the NEG battery post and the POS battery cable, I am showing voltage. How can I have 12V on the POS cable with the battery disconnected? What in a car would have capacitance? If I short the POS cable to ground, of course it goes away, like discharging a capacitor. I don't think it has air bags or an amp with a capacitor but does have ABS and a security system. Any ideas?????

 

[MacGyverS2000]

Every electrical item in your car has caps at some point in the chain... radio (and amps), ECU, , etc.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[IRstuff]

You've not said anything to suggest there is a parasitic load. The fact that you can sense the voltage on the capacitors for any length of time says the parasitic load, if any, is quite small.

And note, almost all cars have some level of continuous loads, to include the clock, any interfaces for key fob remotes, and the battery's own internal parasitic load.

TTFN
faq731-376

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[Panther140]

Ignition coils (condensors) have capacitance. Could that potentially be it?

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

 

[drummertom]

I think I have it figured out for the most part. The taillights were wired to a "Hot All The Time"wire. Several other wiring issues have reared their ugly head, but I have just about solved all of them. Only issue right now is the PATS system (Anti-theft system) is active and will only let it run for a few seconds, but the owner has a tuner program that will eliminate that. Once we get it de-activated, and I am 100% sure that is the problem (right now I am 97%n sure) I will troubleshoot that system. Thank you all for your help in resolving this issue.

 

[drummertom]

Well, I don't have it figured out after all. Had some problems with the Anti-theft system but got that fixed. Now I have a 9.5 amp draw with nothing on, all doors closed etc. This strange issue is, if I turn on the headlights, the draw DROPS to 5 amps I would think turning on the lights would make the draw increase. What am I missing?

 

[MikeHalloran]

You have to eliminate the possibility that a previous owner has concealed a second battery and alarm somewhere.
It's not common, but it has been done, mostly to cars that are valued.

9.5 amps drain with everything off suggests a wiring error or a chafed harness.
That should produce some heat.
So get an infrared thermometer and survey every inch of the harness.
Have someone watch your drain ammeter while you are removing carpet and fiddling with harness. If the drain changes suddenly, you have found at least part of your problem.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

9.5 A is somewhat comparable to the current draw of the lighting system in a car. You might want to check if all of the lights are working; some bulbs have shorting lines in parallel with the filaments. You might try removing each bulb separately to see if one of them has their switch messed up.

TTFN
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[drummertom]

But wouldn't bulbs be isolated if the switch is off?

 

[IRstuff]

Well, obviously, your situation isn't that simple.

TTFN
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[bimr]

Anti-theft systems have a small load.

There are also some other consumers of power. For example, I have a 2001 Mazda. If you unlock the doors with the Keyfob, the doors will automatically relock if not manually opened in approximately one minute. So there is some control circuit monitoring the door operation.

On a BMW, if the battery is not fully charged, the anti-theft system will not fully engage, which then causes a very high drain on the battery.

 

[waross]

Get a cheap compass. When you put it close to a current carrying wire the needle will be deflected by the magnetic field. You should be able to follow the wiring harness to the component causing the draw.
Some auto parts stores carry current indicators that work on this principle.

Bill
--------------------
"Why not the best?"
Jimmy Carter