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Discussion Starter · #1 ·
So I have a 2008 gmc seirra 2500 lmm. Problem started out as a no start, no crank. After 4 days of looking for broken wires and cleaning up grounds, I finally found a bad ground wire. I replaced the ground wire , in process I also put in new batteries, as mine were getting bad. After replacing ground and batteries now anti theft light is on. Comes on at key turn stays on for 15-30 seconds then goes off. Truck will not Start after light goes off. Had a friend come clear the codes and truck starts right up like nothing was wrong. Runs in the garage for 15 minutes no issues, starts up 7-8 times after that no issues. After the truck sits for a couple hours light comes back on and truck is back to not starting. The guy came back the next day with laptop and gm software and reset anti theft codes and relearned my keys. Again truck starts right up and runs good. After it sits for 4-5 hours light is back on and won't start again. Kind of stumped any insight would be great thanks guys!
 

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Do you have access to a code reader that can show codes for all the various modules in the GMLAN low speed buss. Basically the security module, BCM, air bag, radio, HVAC, ABS and so on? Might be having a buss communication issue were the BCM is not communicating with the security module. There was a service letter that came out about the remote module for the crew cab trucks. The third brake light would leak during rain and run down the cab channel and drip right above the little remote module for TPMS and key fob remote functions. This will take down the GMLAN data buss and cause a loss of communication with the BCM and security module. So if rain does play into this I would also look into that. Dean
 

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Discussion Starter · #3 ·
Not sure if it makes any difference but my truck is extended cab. But I did have to replace my third brake light because it was cracked and leaking water into cab . So that could very well be possible. Not sure where any of that is located . Any tips where to look for damage ?
 

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Here is a YouTube video that talks about it. As you can see the outside of the little module box looked fine, the moisture got inside and caused some corrosion. On a crew cab truck it is located half way up the driver side back corner closeout. It is a little 1.5 by 1.5 inch plastic box with two connectors on it. One for the databuss and another with an antenna connection. The video shows what he did to isolate the problem. Like I said, having one of the better code readers that can show you the codes for the individual module can help here. This is much simpler for one that hard fails, my issue is intermittent so I have to disconnect one module at a time and drive to see if the issue reappears. If this helps you might need to take me out on a pheasant hunt. Dean

 

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Not sure if it makes any difference but my truck is extended cab. But I did have to replace my third brake light because it was cracked and leaking water into cab . So that could very well be possible. Not sure where any of that is located . Any tips where to look for damage ?
Another tip, when you get the no crank no start do you have the _ under the transmission position? You should see a _ under the P for park. If the GMLAN low speed buss in shorted out you likely will have no _ under any of the transmission position letters. In the video he refers to this as the CANBUS, those did not come out until the LML series of trucks. These trucks have a GMLAN low speed bus and a two wire high speed bus. Your issue with the security module is all low speed single wire stuff. D
 

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Discussion Starter · #6 ·
If that works and solves my issues I'd be happy to take you on a pheasant hunt. I will check it out and report back.
 

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I see your airbag light is on. What are the codes for that? That might be the source of your trouble.
 

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I would have started by asking about the keys, but if they relearned ok, then they are not likely an issue.

I would start with looking at other grounds, checking each battery separately (voltage). A lot of these types of issues tend to be electrical issues, looser connectors, bad grounds, etc.

What broke the ground you found that was bad? Could there is another issue in that area?

Also, if you had a failing module increasing your CAN voltage that could create issues on other systems, I've seen trucks not start and an ECM not communicate due to a glow plug module failing. I don't think that is the issue here since you can do everything with the ECM, but might be worth checking the voltage. I don't recall the range, but there is range it should be.
 

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In your picture post #7 that your battery light is on and your gauge shows something in the 10 volt range. Fully charge your batteries and see if the problem changes or continues. Amazing how many problems are caused by or made worse by undercharged battery.

That leads me to ask another question - was your battery charge that low when the reprogramming/relearning was done?
 

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I see your airbag light is on. What are the codes for that? That might be the source of your trouble.
Absolutely, the air bag module is part of the low speed buss. Most likely if get a code reader that shows module codes it will show a loss of communication but to drop the who low speed buss many more things should be effected. The instrument panel should be going all zeros, you would have ABS, 4 wheel drive, TPMS and trailer brake service DIC messages. You do see the BRAKE indication which is tied into the anti lock brake module. A diagnostic reader that looks at modules is needed here. Also the radio can go blank. Dean
 

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Discussion Starter · #13 ·
Those lights only display when I first turn the key then go off right away. The anti theft light stays on . I took apart the back pillar and found the module . Everything look pretty good.
 

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Discussion Starter · #17 ·
In your picture post #7 that your battery light is on and your gauge shows something in the 10 volt range. Fully charge your batteries and see if the problem changes or continues. Amazing how many problems are caused by or made worse by undercharged battery.

That leads me to ask another question - was your battery charge that low when the reprogramming/relearning was done?
When we did the relearn we had the battery charger hooked up to the truck !
 

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Discussion Starter · #18 ·
You definitely have a databus issue. You are showing air bag, anti lock brake, and security issues. Do you see different service messages on the DIC other than air bag? Dean
The only dic message that appears in the service air bag
 

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The only dic message that appears in the service air bag
Try this then. Pull the driver's seat and under the carpet there is the air bag module. Disconnect the connector and then see if you can start the truck. Just for safety I would disconnect both batteries before reconnecting it to make sure the data bus is dead before connecting it. There were GM service messages about this connector. Being under the seat with the carpet there have been reports of moisture getting to the connector and corroding some of the pins. If that does not do it then go to the anti lock brake module, it is inside the driver truck frame in front of the gas tank. You will see the brake module, it has all the small brake lines going to the anti skid block. Remove that connector and see if the truck starts. Good luck, Dean
 
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