what causes my battery to drain

[clemenshess97]

I was thinking about a new BCM for some time as my truck is a EU version it came with some things deleted in the BCM like the exterior lock feedback (blinker flashing and horn honking when locking the car with the fob) and in order to have these features you’ll need a new standard US BCM. Does the truck need any programming done after a new BCM is installed ? Or is it plug&play ? Anyway to figure out if it is the BCM going bad ?


I just ordered a new voltmeter, my old one broke recently, I‘ll then start by wiring it to the battery and pulling some fuses…maybe that‘ll give me some idea what is draining the battery.
Is there any thread in particular you remember that has a great step by step instruction ?

 

[Fless]

I would recommend a standard parasitic draw test to be certain it's causing the battery drain. Don't pull fuses -- use the voltage drop method. If you need some YouTube links, try South Main Auto; Eric has a few good ones. Shoot for 50mA or less when fully asleep. Use the search function on TYF to research other threads.

What I've seen are the most common causes for parasitic draw in these are (not in any particular order):
1. Silver migration (Google it) on the instrument cluster circuit board
2. Defective door latch not triggering the RAP timeout.
3. Water migration into PCM connectors (remove connectors and inspect/clean, test for pin fitment)
4. A module not going to sleep. If you have the AUTO HVAC head, it can take 4 or 5 hours to go to sleep, and that's within spec.

 

[MassHoe04]

I would recommend a standard parasitic draw test to be certain it's causing the battery drain. Don't pull fuses -- use the voltage drop method. If you need some YouTube links, try South Main Auto; Eric has a few good ones. Shoot for 50mA or less when fully asleep. Use the search function on TYF to research other threads.

What I've seen are the most common causes for parasitic draw in these are (not in any particular order):
1. Silver migration (Google it) on the instrument cluster circuit board
2. Defective door latch not triggering the RAP timeout.
3. Water migration into PCM connectors (remove connectors and inspect/clean, test for pin fitment)
4. A module not going to sleep. If you have the AUTO HVAC head, it can take 4 or 5 hours to go to sleep, and that's within spec.

@Fless That was exactly the guidance I was talking about! Sorry, I forgot who mentioned what... I knew this came up recently and you had some great advice. All good stuff to look into!

 

[Fless]

I sure learned a lot when I tackled mine. Took me a while but it's great now, and I bought an inductive ammeter that would read DC mA (not easy to find one of those). I ended up replacing the door latch with an OE part from my dealer. I had also had my instrument cluster re-done prior to that but the IC wasn't the cause. I wonder, too, if disconnecting and reconnecting the PCM connectors might have helped, but they were clean from what I could tell.

 

[clemenshess97]

I sure learned a lot when I tackled mine. Took me a while but it's great now, and I bought an inductive ammeter that would read DC mA (not easy to find one of those). I ended up replacing the door latch with an OE part from my dealer. I had also had my instrument cluster re-done prior to that but the IC wasn't the cause. I wonder, too, if disconnecting and reconnecting the PCM connectors might have helped, but they were clean from what I could tell.

the battery should be disconnected before disconnecting the PCM, right ?

 

[S33k3r]

If that is the issue, the switch mentioned by @S33k3r is part of the latch mechanism. I tells the computer when the door is open or closed, turns interior lights on/off.

The interior lights on my 04 shut themselves off after a while, it does that so they don't drain the battery if doors were left open for too long. It is like 10-15 minutes IIRC.

I am not saying @S33k3r is wrong. But, I kind of think the body control module would shut lights down, if the door latch switch was indicating door open while the door was actually shut. Not sure that switch would do that, if everything else was working properly.

The body control module (BCM) can do weird things, especially if ground connections to the body and frame are not 100%. The BCM is supposed to keep some things "awake" for a period of time, but then put everything to "sleep" after a period of inactivity. If grounds are not good, sometimes the BCM won't let anything sleep and drain the battery down if left too long.

Again, I am not saying @S33k3r is wrong. He is definitely one of our go-to advisors. I am just thinking out loud and wondering if there is a chance it might be the BCM having bad grounds.

Oh, I am wrong often -- just ask my wife. Since I really don't have enough information to really trouble shoot, I am offering my best guesses based on any experience I have, plus the shared experience of folks in multiple forums. This kind of crap happens to all the trucks on the GMT800 platform. Yay. On the flipside, not too shabby for 20 year old vehicles, to be fair.

 

[MassHoe04]

Oh, I am wrong often -- just ask my wife. Since I really don't have enough information to really trouble shoot, I am offering my best guesses based on any experience I have, plus the shared experience of folks in multiple forums. This kind of crap happens to all the trucks on the GMT800 platform. Yay. On the flipside, not too shabby for 20 year old vehicles, to be fair.

What I like about the folks on this forum is this... I have been on other forums (not Tahoe and rhymes with mangler forum), where 100 people throw out 100 random solutions, then argue about who is right or wrong. Then, the name calling begins and the whole thing becomes toxic and not helping anyone solve their vehicle issues.

Big difference when I came over to the TYF! Folks on TYF seem to be really nice, aside from really knowledgeable and experienced.

I really like how everyone does their best to walk through the steps needed to get information to help solve a problem. And 100 people are throwing out 100 random answers... You all do a great job pulling information out of people, who might be less experienced, the walk through next steps of the diagnosis.

Even better, folks will look at other responses and just talk through the "if (part X) is doing this, then that other part must be working (or not)..." I like the friendly discussions on the what-ifs. It is like having a human field service manual helping figure things out.

The process you all have going on here seems to have a VERY high success rate. Thanks to you all at TYF and keep it going!

 

[MassHoe04]

the battery should be disconnected before disconnecting the PCM, right ?

That would be a good precautionary measure to take.

 

[robhotdog]

Battery came new in July 2020.
Alternator seems to work fine judging by the charging voltage (14.1V-14.5V).

I noticed that maybe 1 out of 20 times the small display in the cluster (the one that usually shows the mileage, Driver 1 or Driver 2, or any other message like „Check Engine Oil“) stays on after locking the car. It doesn‘t say anything, it‘s just completely lit up. I then usually unlock the car, open and close the driver door and lock it back up, it then goes away.
I check it everytime though (if it‘s lit up or not) and it didn‘t light up last time I parked the truck.

There are several possibilities. As retired 30 yr GM electrical tech. There is no good GUESS..Testing and proper diagnostic methods are required. First you test the battery internally! New ones can go bad!! Next..testing alternator...voltage and loaded amperage. If all shows good do a parasitic draw test..This is difficult for the DIY GUYS. You need a scanner with the ability to Power Down Now all the awake modules monitoring systems on your truck..wasting time and money GUESSING! acceptable draw is arround 30 ma for modern vehicles. You never said what yr or milage of truck..Your money ahead taking it to your dealer for things like this! You can screw up quick having to go to blog sites for YOUR perceived problem. Batterys usually last 3 to 4 years then cold usually kills them the most. You cannot fully charge a discharged battery by driving it. Having it internally tested is a must! They go Bad! Sitting for 3 weeks is ok on a Fully charged battery but if it's not will reflect in time you can sit it. My suggestion is buy a maintainer..Battery Tender..will keep you up and ready to go assuming your battery is good.

 

[S33k3r]

Did you ever figure it out?