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post #1 of (permalink) Old 01-13-2019 Thread Starter
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battery parasitic drain

hi everyone
I have a battery draining problem on my 2007 impala.
By measuring voltage drop across fuses,, i determined the 10amp bcm fused circuit is problem.
Measures .6mv across fuse ,,, according to charts drawing around 75ma constantly.
??? any electrical experts on forum have any idea where my problem may be ?????
thanks for your help

Last edited by deadimpala; 01-13-2019 at 12:31 PM.
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Originally Posted by deadimpala View Post
hi everyone
I'm have a battery draining problem on my 2007 impala.
By measuring voltage drop across fuses,, i determined the 10amp bcm fused circuit is problem.
Measures .6mv across fuse ,,, according to charts drawing around 75ma constantly.
??? any electrical experts on forum have any idea where my problem may be ?????
thanks for your help
I'm not an expert on BCMs, but I know there are several inputs to it which can keep it awake thereby causing the high current draw.

When it goes into the standby mode, the current should be much lower. One thing that can keep it from going into standby is the ignition switch.

Is the key getting turned fully off when removed? Old lock cylinders will allow the key to be pulled out without rotating the cylinder all the way to off. I've seen dead batteries caused by that. So be sure the cylinder gets rotated all the way to the left.

There are other inputs to the BCM that might also keep it from getting to standby, but they will normally show up in the instrument cluster. For example: door ajar or trunk open or hood open. Do you have any indicators like these?

Doug

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excellent ,,, will check ignition switch carefully,, thanks Doug
if that checks OK not problem,,, any other advice ?? ,,, no warning lights on dash (door ajar etc).
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I changed my cargo light in the trunk to led, I just happen to notice after the courtesy lights time out that the led is still illuminating. Sorry for the random tip but something simple like that could happen to drain the battery.
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post #5 of (permalink) Old 01-14-2019
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Originally Posted by deadimpala View Post
excellent ,,, will check ignition switch carefully,, thanks Doug
if that checks OK not problem,,, any other advice ?? ,,, no warning lights on dash (door ajar etc).
If the ignition switch does not appear to be the problem, I suggest back tracking from probing the BCM fuse for voltage drop - I'm not sure how accurate that it - at least, I've never used that technique.

What I've done is to connect an ammeter in series with the battery, and then use a removable jumper across the ammeter.

Most ammeters cannot handle more than 10 amps, so you cannot crank or run the car using one, but you can measure the current AFTER the key is turned to OFF. (BTW, jumper wires cannot usually handle crank current, either.)

With the jumper in place across the ammeter, turn the key to ON, wait a few secs, then turn it to OFF, wait a few seconds, then remove the jumper from the ammeter and watch the current for a few minutes and see what it does.

Ultimately, you want to see the current go down to the 10 or 15mA range. If it stays well above that, that confirms the parasitic draw. At that point, I would begin removing fuses from the fuse boxes one at a time, and see which fuse drops the current down.

Depending on what happens when a fuse is removed and replaced, you may then need to re-connect the jumper and cycle the key back ON and OFF to get the system back in the same state it was before you removed the previous fuse. It's important that the system be in the same state each time the next fuse is removed in order to be sure the parasitic draw is present.

As you can see, it can be a painfully slow and tedious process. In your case, since you already suspect the BCM, I would start with that fuse and fuse box.

After that, if it is indeed the BCM, the next step would be to get a drawing with the all the inputs to the BCM and to probe them one at a time looking for one that's not correct, and that can be difficult. Furthermore, it may not be readily apparent what the proper level should be. The other thing to keep in mind is that, when probing signals, you need to have a high impedance probe such as a typical voltmeter (DMM) has. Otherwise, such as with a probe light, you can load the signal thereby affecting its level thus getting an inaccurate reading. That could also cause the BCM to see a different level causing its behavior to change.

Another thought, there are four big fuses (as I recall) just downstream of the battery which feed the various fuse boxes in the car. By pulling those (one at a time), you can isolate which fuse box has the parasitic draw (in case it's not the one feeding the BCM). That is, you want to be sure you have the right fuse box isolated, then isolate which fuse in that box is the culprit.

All that said, if I had to roll dice on this, with the parasitic draw established, I'd pull the On-Star fuse first.

Anyway, those are some thoughts I have on it. HTH.

Doug

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thank you,,,, does my (all) 07 impalas have on-star ???
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thank you,,,, does my (all) 07 impalas have on-star ???
Not sure about fleet cars, but I think most other Impalas had the On-Star. My 07 SS does, for example.

There are a couple reasons I might suspect the On-Star. They have both a battery (lithium coin cell) and an antenna. A dead cell or a bad antenna connection might could keep it from powering down properly resulting in a parasitic draw. But that's mostly speculation rather than learned observation.

HTH.

Doug

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so,,,, all somebody has to do is pull on-star fuse and no more on-star ???
will car still run with on-star fuse removed ?? seems too easy ,,,,

,,,, or when fuse removed >> on-star module activates ???

Last edited by deadimpala; 01-14-2019 at 09:57 AM.
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so,,,, all somebody has to do is pull on-star fuse and no more on-star ???
will car still run with on-star fuse removed ?? seems too easy ,,,,

,,,, or when fuse removed >> on-star module activates ???
As far as I know, yes, the car should still work with the On-Star fuse removed. And the On-Star will be disabled. I could be wrong, but that's my understanding.


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thank you again,, i did remove on-star fuse,, no change in my parasitic drain on the BCM fused circuit,, around 80ma.
Ignition switch seems ok, no visible lights/indicators, or anything "on" or activated.
Wiring diagram shows BCM fused circuit feeding >> ignition switch, BCM module and theft module?

Not sure how to proceed from here
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Originally Posted by deadimpala View Post
thank you again,, i did remove on-star fuse,, no change in my parasitic drain on the BCM fused circuit,, around 80ma.
Ignition switch seems ok, no visible lights/indicators, or anything "on" or activated.
Wiring diagram shows BCM fused circuit feeding >> ignition switch, BCM module and theft module?

Not sure how to proceed from here
This where you need to start pulling fuses one at a time until that 80mA goes away.

Doug

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Doug,,,, the only fuse underhood that showed current flow was the BCM fuse when car off.

No other fuses showed current flow (as measured by voltage drop across fuse),,, that is how i identified the BCM circuit.

Are you suggesting i try to pull other circuit fuses that showed zero current flow ??
thanks again
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I didn't read through the whole thread, but I know people have been having issues with LED replacment bulbs keeping the BCM "awake" in some of the 9th gens - is it possible that is happening here as well? Have you recently replaced any bulbs with LED bulbs? Just throwing it out there....
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Ignore post. BCM goes to a bunch of fuses.
Tried to upload bcm pdf schematic but didn't work, sorry.

Last edited by Bill58; 01-15-2019 at 03:46 PM. Reason: no upload
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Originally Posted by deadimpala View Post
thank you again,, i did remove on-star fuse,, no change in my parasitic drain on the BCM fused circuit,, around 80ma.
Ignition switch seems ok, no visible lights/indicators, or anything "on" or activated.
Wiring diagram shows BCM fused circuit feeding >> ignition switch, BCM module and theft module?

Not sure how to proceed from here
Let me look for some drawings of the BCM, and I'll post something tomorrow.

Sorry about the bum steer earlier - brain fart

Doug

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