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periodic parasitic drain.

posted May 22, 2016 20:23:13 by Inge_Jeppesen
No spesific case here. Just wanted to air your methods on how you go about troubleshooting parasitic drain on newer cars with plenty of modules, Each fuse you pull and the data bus is live again and so on.....

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7 replies
Tyler said May 22, 2016 21:37:23
I (thankfully) don't see a ton of this, but voltage dropping fuses is my usual go-to test. After verifying the presence of a draw in the first place, of course.

Always trying to avoid disconnecting the battery to install a DVOM, so I'll usually hook up a low amp probe first, to attempt to gauge the severity of the draw. I've found that draws below 200mA are difficult for the low amp probe to measure accurately, so the ammeter still comes out at times.
Inge_Jeppesen said May 22, 2016 22:05:24
Often what I see is around 700 mA.

Very ofte radio/bluetooth modules that won't "sleep"

Can you really measure voltage drop over these small current draws?

Often what we end up doing is report it to engineering and wait for feedback in form of a sw update or a tsb.

We have very wet climate here and put salt on the roads in the winter. It eats up all the outside wiring...

I will try the fuse v.drop test next time. If it works it will save a lot of time.

Tyler said May 22, 2016 23:55:10
Can you really measure voltage drop over these small current draws?

Yep! Well, according to the charts that are available, anyway. Check it out:

I personally have some reservations about using the voltage drop to make a determination about the specific amount of a draw. The chart suggests you can make accurate measurements this way, but I believe live circuits are too dynamic to be charted like this. More often, I'll use the voltage drop method to find suspect circuits, and switch to other methods from there.
juergen scholl said May 23, 2016 02:56:26
Inge ,

the fuse voltage drop measurement really works. It is even the recommended method by some manufactorers like VW, Audi etc.
You might wanna build a test circuit with a couple of LED's, install a fuse into the circuit and measure current flow and voltage drop over the fuse.
Another option to find small drains once you installed an ampmeter in series and the modules went a sleep is to pull a fuse and NOT to reinstall if the current drain doesn't change. Proceed with the next fuse(s) til the ampmeter readingchanges.
Inge_Jeppesen said May 23, 2016 09:04:56
My voltmeter only goes down to 1 mV.

The first section on these tables start of from 0,1 mV to 1 mV.

What to do there?

MattWhite said May 23, 2016 12:25:21
I'm yet to try one and I don't seem to get lumbered with these much either but I've always had an amp hound in mind. Just a thought.
AndyMacFadyen said May 23, 2016 20:35:41
The most difficult to trace I ever had was down to the antenna for the key transponder on a Lucas 5AS imobilizer system. As on this system the transponder was optional the cure was simply to unplug the antenna, the downside being the driver has to press the unlock button on the fob imediately before starting the engine and to restart if the engine stalled.

The heaviest drain I ever encountered was through the alternator. The alternator was still managing to charge around 13.4 to 13.7 volts but when the engine was stopped about 15 to 25 amps was leaking back through the alternator.

The fuse voltage drop would never work with my UNI-T UT203 meter but my new Owon B35T meter claims to have a resolution of 0.01mv +or- 5% +2 digits so 0.1mv or even 0.05mv should be just about measurable.

[Last edited May 23, 2016 20:41:32]
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