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NBS 2007 Sierra 1500 electrical woes

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I bought a very clean and sharp looking truck about 6 weeks ago, a private sale. About 2 weeks after I bought it I had unintentionally left the stereo playing while out in the yard to the point it drained the battery. Put a battery charger on it over night and good to go... or so I thought.


First I noticed the remote start would no longer work, and would not start with the key, instrument panel said something about security. I popped off the negative cable for a few minutes and put it back on.


Started the truck with the key and traction control and stabilitrack is disabled and constantly flashing and dinging on the dash. Check engine light is on. Park assist off flashing too. Now the truck shifts really hard out of park, shift really hard 1-2, 2-3, and into reverse. Transmission now shifts into over drive really prematurely, I almost need to floor it to reach the speed limit or it shifts to OD and becomes a sluggish dog. Occasionally the speedometer will randomly dip or drop for a brief second and I can hear a clicking sound(s) from under the dash.


I tried leaving the battery off overnight, tried touching the positive and negative terminals together (and left them touching for hours). I had the truck scanned by several scanners which show 11 different "U" codes (loss of communication) along with a few others. All codes can be cleared but come right back moments later. I was told by several mechanic friends who were helping me out that I need to take it to the dealership.


Fast forward to the dealership, they take it into the shop and run their scanner on it and clear the codes, say all is fine and it just needed to be reset. Before leaving the parking lot the check engine light, park assist, traction control and stabilitrack lights come back on, now added is service 4wd. A mile down the road ALL my gauges drop to zero and back repeatedly, the indicator under the "D" is flashing randomly, the door locks are cycling up and down, shifting issues resume.. Everything it was doing before, and more, it became worse! I take it back and they do more checks, and tell me they're not sure what the issue is but it's the end of the day, bring it back next week. They also tell me my battery is low and may need replacing. I leave the dealership and go to the grocery store, truck is dead in the parking lot and jumping it doesn't help, on to the tow truck hook and into the air... went to put the transfer case into 4wd so the guy could drop my front driveshaft and the truck starts?! I drove myself home. Now driving my truck with all dash lights blinking, warnings chiming, under dash clicking, and door locks cycling, and who knows what else, it seems possessed. Once every 5 starts or so the battery voltage drops to about 11 and if left like that, it will not start. Simply pulling over and shutting off the truck for 5 seconds and restarting, the voltage goes back to 14 or better.


I spent $20k on a truck I drove problem free for 2 weeks and now it's better suited as a lawn ornament.


What have I done? At the beginning when the battery went dead I did no modifications or tinkered with anything.. it seems the battery going dead triggered these issues, but i can't believe that caused all this!


Since these issues I have replaced the battery (old one tested fine, but I put in a optima battery that I know is good). Alternator tests well too, cleaned all connections and checked belt and tensioner. I checked all positive connections from battery, alternator, fuse box, mega fuse, and starter. All connections appeared good but I cleaned them up with emery cloth and dielectric grease. I checked the negative cable from battery, block, under rad support, firewall, even the one on the back of the block by the transmission and gave the same treatment. I also added a 4awg ground from battery to firewall, firewall to engine block, block to frame, frame to firewall.


I could be totally wrong but I believe there is a short or intermittent contact somewhere in the information bus running throughout my vehicle and it is causing chaos.. I'm just at a loss of where or how to go about tracking it down.


I have done a lot of reading online, lots of videos, and this seems to be a common issue that is ongoing with many of these trucks. Some are repaired by replacing battery cables or mega fuse but they appear to be a short term fix and the problems eventually return. Some have spent thousands replacing modules, fuse boxes, switches, contacts with no resolve or none that I can find... or they are repaired and their thread/forum is not responded to.


So here is another thread and I'm hoping someone can point me in the right direction... and I WILL post a resolve, if I get one. Thanks in advance for your time reading and responding to the novel I typed above, lol.

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  • 2 years later...

Wondering if you ever found an answer to your problem?  I've got the same issue on my 07' Avalanche.  I'll get the dropping speedo, clicking door locks, hard shifting, StabiliTrak and ABS alerts.  When I get to the dealer it's always a "generic communication bus" problem.  No specific codes, so no specific module.  The last visit we decided that the suspension module was the most likely cause, as it was the one responsible for many of the alarms.  Apparently, it almost fell off in the mechanic's hand and now $700 later, it's started again (was okay for about 7 weeks or so).  Once in a while, I'd get a "check suspension" alert, but it was rare.  So the problem continues and I can't just keep replacing models as they're all coded to the ECM and not just swappable for testing (or so I'm told).  Running out of steam on this...



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