Flickering lights then dead BCM issue

[ssthomas]

Hey all first time poster here. Backstory is that I got this 2006 Hummer H3 with a manual transmission from my uncle for 500 bucks. I thought it was too good to be true as it needed some work primarily upper control arms and things of that nature but the big one was something happened to the electrical and he had no idea. Basically the rear brake light driver side was always on and the brake lights did not work anymore. Along with this some other electrical glitches like the interior lights didn't work most of the time and the turn signal flasher did not meet the clicking noise. Somebody told him it may have a bad BCM. Well I traced that to a shorted BCM and when I mean shorted there's a semiconductor that went up in smoke on the board. Apparently from what I can tell this was caused by the chafed wire harness for the trailer package as so many others have had problems with before. There are no signs of corrosion on the BCM or the harness connectors at all it doesn't look like it's ever been leaked on before I long since repaired it after repairing it I replaced the BCM with new programming. Which brings me to my new problem with the new BCM installed the truck started up and ran perfect with brake lights and everything about 2 minutes and you running all the lights started flickering meaning the tail lights and the interior lights and then followed by the instrument cluster completely died as well. I shut the truck off to restart it and find nothing works the only thing I have lighting up is the airbag lights and a check engine light but that's it nothing else no brake lights no instrument cluster no interior lights. As I started wiggling the connector on the BCM the interior lights briefly came back on along with the instrument cluster. I am trying to pinpoint this to a bad connector on the BCM harness but I cannot seem to replicate this problem by wiggling around the harness and connectors it just comes and goes at will where sometimes the lights want to come on and then they die out. Has anyone else had this problem before? It almost acts like a bad B+ wire or a bad ground but I cannot find the actual wire pinout locations on each of the two connectors I found on this forum the BCM labels for the wire pinouts but not where they go to the connectors. I have checked all fuses and replace the usual failed trailer wire harness and brake light fuses due to a failed harness. I also repaired the harness as well

 

[rascole]

Start with the battery connections. Factory ones are a common problem and will cause numerous random problems. Lots of info here on it.

 

[alrock]

PM sent

 

[ssthomas]

Thank you for the electrical diagram. I'm looking at a 2008 diagram in the BCM connections like they are the same I am running into a brick wall with this thing. If I leave the if I leave the battery or the body control module disconnected for more than an hour, if I reconnect it I get all lights and dashboard for about 5 Seconds and then they Flicker and die out. The only thing I have after that if I turn the key to on is an engine light and two airbag lights and that's it nothing else no power to anything. I did notice I always have door locks turn on the headlight switch manually I have parking lamps but nothing else no windows power no nothing but the radio always seems to be on which is interesting

 

[Teahead]

Get new battery terminals asap. Stinger makes good ones.

 

[ssthomas]

Have good battery terminals. This goes beyond the norm. I believe what I have is a Class 2 Data serial problem on the BCM. You see when I first bought this truck from my uncle it had a shorted trailer wiring harness which took out a fuse multiple times and then when he replaced the fuse again the rear tail light was on all the time well it was a brake light. What ended up happening is it shorted out a mosfet in the BCM. I replaced the BCM but swapped the prom over to the new BCM. I'm thinking the prom is not fast enough or something else is going on. The class 2 serial data has a 7 volt potential and I can read the class 2 Data on an oscilloscope using a 2-volt peak to Peak at 50 milliseconds. So I can communicate with all the modules which tells me it's not a shorted or an open Class 2 but the other day when it started up finally which it does if I leave the BCM disconnected overnight, I had 12 volts at the class 2 Data pin two at the DLC. I'm really stumped I might try putting the original eprom onto the replaced BCM and without starting it see if I have interior lights which I can tell you I did the other day with a new BCM but I didn't dare start it or turn the key on because I didn't want to reset the instrument cluster.

 

[ssthomas]

My fault. I figured it out. Apparently I can't take the eprom out of my bad BCM and replace it into another module. Basically I took my eprom out and swapped it into another module that came out of another vehicle even though the part number of that BCM was said to be a compatible model online it is not I took my original e promise opted into my original BCM and the problem is solved however I still have no brake lights. One brake light is on all the time no matter what even if the car is off from a short that it had with the harness rubbing on the frame but that's been since repaired

 

[Korby7]

Great troubleshooting. Glad you figured it out.

 

[ssthomas]

Well, now that BCM us dead too. So in turn, you cannot swap an EEPROM on these BCM units. If you do, something weird happens. The BCM quits responding and kills off all accessories it controls when powered up (battery connected). The longer it's been disconnected from power, the longer the BCM will function, but eventually quits responding (acts like a bad connection, and had been causing me to pull my hair out). I can still scan all other modules on the bus, but the BCM quits talking, then lights go dead and no crank no start happens. Basically the only true way it seems to replace the BCM is to clone it or use TECH 2 or VCS nano to rewrite the unit through the data bus. I ordered the VCX nano and will flash it soon.

 

[Jeepwalker]

That was back in the 80's a guy could swap eproms. lol

Trace back your brake wiring. Look in your bulb connectors just to be sure nothing is shorted. Could be a shorted brake switch or even turn signal switch, or flashers. It's going to require some meter work.

How is the battery to body (and engine) grounds? The above advice on the battery terminal connectors is good. Make sure yours are really good. I suppose from N CA you don't get much corrosion, huh?

 

[ssthomas]

Well I can tell you that no there's not any corrosion on any terminals or battery terminals or wiring in this vehicle at all. I was able to repair the short that happened previously. Also what I do know is that the original BCM was in fact shorted I just swapped it with another one that came from another vehicle same calibration number on the ticket of the BCM and even though it works it has issues since it came from an automatic transmission vehicle and mine is of course a 5-speed manual. The trucks wiring harness is great now as I have headlights tail lights and brake lights and turn signals everything works . But the BCM I picked up is not coded for the truck . The problem I have now is that I purchased a VX diag vcx Nano tool and I cannot seem to get it to reprogram the BCM using techline connect. I can see the tool and plug it into the diagnostic port on the truck and be able to read the truck but as soon as I try to reflash with the purchased then slot that I had to buy for 45 bucks on AC Delco TDS as soon as I try to flash it will give me a error after three tries. I know the Nano is trying to do something cuz the lights turn on and off for a moment and it resets the instrument cluster as the lights flash on and off but it can't seem to program anything. I'm faulting the vcx Nano as a problem here but I'm close I'm just trying to flash the factory firmware back to the new body control module. Has anyone had any luck using a vcx nano and the AC Delco TDS software techline connect to flash modules on a Hummer H3 yet?

 

[ssthomas]

Just to finish this one off, I fixed it. I purchased a MDI clone and put that on one laptop and a vcx nano on another. When trying to reflash the replaced BCM with the same calibration number that I got online, I couldn't get it to go through don't fail at a third every time with some stupid code. For some reason it tried to get all the way to the end on tech line connect only with the ignition off which was really weird . The ultimate eye opener was when I attempted to read the PCM with Tech 2 win. It said that it couldn't communicate then when I look back on tech line connect it told me that the calibrations were no on the PCM. I immediately flashed the PCM suddenly the ABS light turned off and I heard some energization of the fuel pump. At this time I went back and retried The Flash the BCM and it took mostly however it told me there is an error and it retry twice but it did get all the way to the end of The Flash. Even though tech line connect is reporting an error flashing the BCM, surprisingly the BCM took the Flash. After turning the key off and back on I immediately was presented with a running engine and at that point I cleared all dtcs on every module. Had to do the crank relearn then everything was back to normal.

So the takeaways here, you cannot and I repeat cannot remove the eprom and swap it onto another BCM this doesn't work. I mean it'll work right away but then everything dies out within a minute like it acts like there's a bad connection but what's actually happening is the BCM shutting down. This must be performed with tech line connect. And can only be performed using an MDI module as a vcx nano would not work flashing the BCM on the Hummer H3. I was also told by the seller that vcx Nano is not compatible with Hummer which sounds like a load of crap because it's a GM product and basically if he was to tell me that then that would mean it's not compatible with the Colorado or Canyon either. Anyhow back to normal no engine lights and now I can go register this pig.