Body Control Module draining batteries overnight?

KITTYBOY

MEMBER
2003 CHEVROLET SILVERADO
6.6L
V8
TURBO
2WD
AUTOMATIC
150,000 MILES

Question regarding Body Control Module (BCM). Truck was killing batteries overnight, door locks and lighting acting weird. While probing fuse block (fuse removed) I found that the fuse socket for LBEC1 had voltage on both side (12.6/11.9) which may be normal but didn't make sense to me? Found the wiring diagram for LBEC1 and here's what I did/found: at key off and all downstream fuses removed socket read 12.6v/12mV. Flasher fuse in (12.6v/15mV), add door c/b (12.6v/1.5mV), add TBC 2a (12.6v/1.8mV), add TBC 2b (12.6v/1.5mV), add 2c (12.V/11.9v). Thought it might be drivers door module since I have had issues due to poor design, unplugging DDM (12.6v/11.9v). Pulled TBC IGN under hood 12.6v/11.9v). At key on voltage goes to 12.3v/5v. Pulled LBEC2 thinking it might be back feeding through DDM, no luck. Pulled BCM, had some corrosion on pins, cleaned up, reinstalled, passed diagnostics via OBD port. Only other codes are B0159-13 which I am trying to track down (new sensor did not fix), B1259-13 (xm radio antenna which I don't use, and despite the truck never having been in a collision two B0159-13 codes. Thinking the BCM is the culprit or is the strange voltage normal?

Friday, August 8th, 2025 AT 5:17 PM

3 Replies

KEN L

MASTER CERTIFIED MECHANIC
54,137 POSTS

So I don't think the code has anything to do with the battery draw issue, the LBEC1 50 amp has a few items on the circuit such as the BCM, DDM, window switch and turn signal flasher.

B0159 - Outside Air Temperature Sensor Circuit

The fact that the BCM had corrosion could have burnt it out causing the draw, don't worry about the fuse having power on both sides this can be the BCM back feeding the fuse. I would unplug the turn signal flasher as well to see if the draw stops. Here is the location and I have included the wiring diagrams for the LBEC1 50-amp fuse so you can see what all it powers. Check out the images (below). Let us know how it goes.

Images (Click to make bigger)

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Saturday, August 9th, 2025 AT 10:55 AM

KITTYBOY

MEMBER
2 POSTS

I pulled all the downstream fuses controlled by LBEC1 including the flasher fuse and it made no difference, the back feed only occurs when I install the TBC2 fuse. Additionally having done more research at least two of my other problems seem to refer to the BCM (outside ambient air sensor and vertical seat sensor on the drivers seat). I replaced the outside air sensor and inspected the wiring to the extent that I can without breaking into the harness. I have not pinned the wire from the sensor to it's endpoint which I'm assuming is somewhere on the BCM. Although the BCM currently passes my OBD check I'm still leaning towards it being the culprit, you're probable right in that the corrosion may have damaged something internally cleaned the pins and inspected all the solder joints but obviously don't have the ability to check IC's or mounted resistors. Installed a couple of kill switches on the batteries so at least that problem is temporarily fixed.

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Sunday, August 10th, 2025 AT 12:06 PM

KEN L

MASTER CERTIFIED MECHANIC
54,137 POSTS

Yep, I believe the BCM is the culprit. I would search google and ebay for a pre-programmed unit to see what happens. Good job with the kill switches ;)

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Sunday, August 10th, 2025 AT 6:41 PM