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Discussion Starter #1 (Edited)
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
 

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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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Discussion Starter #3
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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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 ???
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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Discussion Starter #8 (Edited)
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 ???
 

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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.


Doug

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Discussion Starter #10
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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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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Discussion Starter #12
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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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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Discussion Starter #16
Ignore post. BCM goes to a bunch of fuses.
Tried to upload bcm pdf schematic but didn't work, sorry.
your right Bill,,, BCM module is connected to 6 other fuses,,,
i'll pull them tomorrow and remeasure parasitic drain.
JT,,, No modifications or ad-ons that i am aware of, owned for 4 years.

thanks :)
 

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Let me look for some drawings of the BCM, and I'll post something tomorrow
I pulled some BCM drawings together. The data is for 2009, but it should be the same on your 2007.

Below is a brief summary of the drawings. Start on page 1.

There are many things to consider in all this. I'd start by pulling the 2A fuse that feeds the steering wheel. It's tapped off the 10A BCM fuse. You want to see if that affects the current.

Doug


Page 1 shows the 10A BCM fuse. It feeds the BCM, the ignition switch, _and_ the theft deterrent module.

Page 2 shows several fuses feeding into the BCM.

Page 3 shows BCM X3-18 going to interior lights dimming circuit on page 8.

Pages 4-7 show the many signals connected to the BCM.

Page 8 - see page 3 above.

Page 9 shows several lights connected to BCM.

Page 10 shows the theft deterrent connections for the BCM.​
 

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Discussion Starter #18
wow,,,, looks like BCM module goes everywhere :(
thank you,,, i'll see where it takes me,,, easiest will be to pull fuses,,,

i dont think i'll be able to probe the BCM module itself,, due to it being under dash (my old body wont go there)
 

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wow,,,, looks like BCM module goes everywhere :(
thank you,,, i'll see where it takes me,,, easiest will be to pull fuses,,,

i dont think i'll be able to probe the BCM module itself,, due to it being under dash (my old body wont go there)
I got a few minutes to ponder the drawings. Here's a few more thoughts on the subject.


Things to fiddle with.

1. Fuses in underhood fuse box (page 2) feeding BCM. Appears to be 6. Not sure what effect these will have. Frankly, pulling one should cause some sort of alarm condition in the BCM - ie, if it affects the BCM current, I'd expect it to go up, not down.

So what I'd do is pull each one and ohm it out to be sure they're all good and hence feeding proper voltage to their respective BCM pins.

2. On page 3 in the upper right corner, the BCM goes to the "interior lights dimming circuits". I'd fiddle with the dimmer knob while monitoring the BCM fuse to see if the current changes any. My half-baked thought is that maybe something's wonky in the dimmer causing extra current to be drawn. That said, it's not clear if the BCM watches this pin, X3-18, or only pulls it low when it closes the switch.

3. Following that same wire, it goes thru the 2A STRG WHL fuse in the underhood fuse block to the steering wheel. Again, it's not clear how or if this path affects the BCM, but I'd pull that 2A fuse (as mentioned previously), and see if the current in the 10A BCM fuse changes.

4. Page 4, pwr_and_gnd, is mostly a rehash of page 2, but notice that the ignition switch taps the 10A BCM fuse in three places. One of those is the key detector circuit. So I wonder if that little doololly in the side of the lock cylinder could be contributing to the current draw. (I think it's a Hall effect sensor that detects the presence of the metal key, but not positive.)

What I think should also be done is to get an ammeter in series with the 10A BCM fuse in order to get a more accurate reading, then monitor that as you perform various tests.

[Edit]
Measures .6mv across fuse ,,, according to charts drawing around 75ma constantly.
Thinking about this some more, with a car battery typically having something like 80 amp-hours of capacity, at 75mA, you'd have to leave it parked for something like 44 days to run it down. Even 20Ah would take 11 days. That is, if it's only drawing 75mA, but it's killing the battery in a short time, maybe the battery is part of the problem.

That said, as suggested above, if you can get a ammeter inline with that fuse, it will give a much more accurate indication of the actual draw.

HTH.

Doug

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Discussion Starter #20
Doug,,, thank you for calculation,, yes i calculated the same rate of battery discharge,, over a month. Problem is compounded by fact car is only driven 2 miles twice a day,,,, the battery seems to lose 50% of its charge after 2 weeks ??? Battery was tested and replaced twice,, no change.
Alternator does charge at proper voltage (over [email protected]) and does charge-up battery when driven more miles. Never had this problem before with low battery!!!
Thinking further,,,, maybe i'm getting additional battery drain from (defective) alternator too ??
,,,,would a leaky diode still charge battery normally ??
but i still have 75ma constant drain on the BCM fused circuit 1.8A/dayX30=54amps/month !!!!!
and car wont start with very low battery,,, may actually be fewer days than calculated.
Had the car in shop,,, mechanic did verify, but could not identify problem causing low battery,, maybe electrical not his specialty ??? ,,, told me hes been working on cars for 25 years :)
 
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