07.5 LMM Electrical Issue

[Charcoalmax]

It all started a couple days ago, i was driving and all of a sudden all of my gauges went down, ac was stuck on and i couldnt control it and my radio restarted. The ABS light, Air bag and one other light came on as well. The DIC read service air bag, service trailer brake system. Then 5-8 seconds later it all went back to normal, no messages or lights and the radio and AC went back to normal operation. The truck didnt die or surge or anything unusual. I havent had any no start issues either. Knock on wood. I disconnected and separately load tested batteries, checked ground on frame under drivers side door, and i checked all plugs going into the BCM. Has anybody every heard of this happening? Id prefer to not throw parts at it, but ill have a new BCM in hand tomorrow. If nobody has any other suggestions i guess ill give that a try.

I have a 45 second video of what im talking about if anyone would like me to text it to them. For some reason my email wont send it to post it on here.

 

[DAVe3283]

Was it a one time thing, or is it still doing it?

If it was a one time thing, could have been a loose connection somewhere. If it is still doing it, should be fairly easy to track down. Get a wiring diagram, and find a common power/ground source for all the failed modules in the dash (ignore the DIC messages, it says a lot of that stuff when it loses comms).

I am really only familiar with the GMT-800 (OBS), and can't provide you with diagrams for the NBS trucks, sorry. Good luck!

 

[Noah]

Hey, I just found this feed and I have the EXACT same problem. I’ve been throwing money at it for about a year now and still have the problem. No one seemed to know what it was. Did the BCM end up solving your problem? If not did you ever figure out what it was? Please let me know!!

 

[gmduramax]

Do you have anything plugged into the obd II

 

[Noah]

I had a CTS2 in there but after I thought I had the issue fixed, I plugged it back in and it went crazy again and I unplugged it. So to answer, no I have the obd ll empty.

 

[gmduramax]

If for some reason the connection gets interrupted at the obd II I have seen all the gauges go dead on the dash

 

[Noah]

I have checked most connections/ports/grounds/fuses but I will definitely go check that again. Just wiggle it around with the truck on?

 

[KRODMAX/KS]

I had the same issue. Your CTS-2 needs to be replaced. You will have to send them the old unit so you may get credit for a new one. They will sell you a new unit at a discounted price.

 

[Noah]

I had the same issue. Your CTS-2 needs to be replaced. You will have to send them the old unit so you may get credit for a new one. They will sell you a new unit at a discounted price.

Did you have the same issue as OP? If so, when you got the new unit did it fix all the issues?

 

[KRODMAX/KS]

Yes, yes

 

[Noah]

You da man!! Looks like I’ll be doing some emailing here soon.

 

[Dean E]

Here is the issue. Take a look at this video.

Source: https://www.youtube.com/watch?v=cZuZrXmuy7Q


What is happening here is the LMM series of trucks have two electronic busses for the integration of the communication of the different modules. (This is pre CAN buss stuff. I believe CAN buss stuff started with the LML seriers but I could be mistaken.) These are all controlled by the BCM (Body Control Module) There is a low speed GMLAN buss which is a single wire buss with a 7 VDC reference and the GMLAN high speed buss which is a two wire buss. All the problems here are related to the low speed buss. Here is a list of most of the modules on this buss. The IPC (instrument cluster), BCM, security, remote (key fob and TPMS), HVAC, ABS, SRS, transfer case (4 wheel drive), radio, On Star, and factory trailer brake module.

What you will find is either a wiring issue or a corrupt module in the system buss. In my case I have had this happen twice since owning my LMM. The first time was from an aftermarket radio system that I installed. The factory radio is part of the DIC alert system. It give you the chimes to alert you if there is a DIC message or alert. Because of that you need a couple aftermarket modules to make everything work. A steering wheel control interface module and a radio interface module. I had one of those modules go bad and it dragged the whole buss down with it. The second time was the SRS (air bag) module. In both cases the problem was very intermittent which makes in maddening to try to troubleshoot and isolate. I had to remove one module at a time and drive the truck for a week or two to isolate the issue. So yes, I had warning lights on while driving with the various modules disconnected.

Many people want to go right after the BCM because it is the heart of all this. In some cases they are correct. The 2007.5 to 2010 years did not have very reliable BCMs. They now have later part number units that still will have to be programmed with your VIN that improve the reliability of the BCM but there are a few problem areas I would suggest first. While there have been other modules that have caused this issue there are three that cover many of the problem. The SRS (air bag) module, the aftermarket radio modules, and the remote module. The two factory modules both have GM service letters on them. The SRS module had an issue were water that would get on the carpet and work into the carpet insulation. This would then wick to the area under the driver seat where the SRS module is. This would cause corrosion of the module and connector and eventually would cause failures of the SRS or GMLAN low speed buss. Second is the remote module. The letter on this related to the factory third brake light. The original brake light seal would start to crack and fail, when it rained the water would leak in and travel down a body channel on the crew cabs. On the drivers side aft cab close out about half way up is a little black box that is called the remote module. It has two connectors, one for antenna and another for the module that included the low speed buss. The water would run down that channel and drip directly on top of that remote module and would cause it to corrode and possibly short out that module. Both modules are fairly easy to check. My SRS had no corrosion (the insulation had already been removed years ago and I use WeatherGuard floor mats) but it turns out was breaking down internally. The only way I found it was to unplug the connector and drive around for a couple weeks. My buss issues cleared up so I replaced that module and cleared things up.

If you have issues like these please feel free to reach out to me on this. Letting the dealership shop work on this is ridiculously expensive even if you are lucky enough to have them find the issue. Also there are some specifics on what needs to be done if you replace the SRS module. Good luck, Dean

 

[Ron Nielson]

Thanks for your posting. When our trucks have problem with their LMM GMLAN systems, it is indeed very difficult to accurately diagnose the problem. Biggest problem is that few people have any understanding of the GMLAN, its components, and operation. Next is that few people have a scope capable of displaying the waveform from the GMLAN to tell if the communication is working or not. Last is a time effective method to do the actual testing of the GMLAN so that you don't have to go to individual modules, disconnect them, and drive around for a while to see if the problem is fixed (the GMLAN commnicates).

The attachment comes from GMUPFITTER.com and gives a good description of the system and it's basic operation. There are additional learning aids with Youtube videos. I found many of them are well meaning, get some helpful information and techniques communicated, but lack a systematic, coherent, easily understandable method for exploring GMLAN and it's quirks. It takes a lot of time to view the existing videos but they can provide information and the more you learn, the more you understand. I know that the Youtube presenters and doing the best they can and I appreciate what they've done.

I have not YET had a problem with my GMLAN but when I do, I will be buying a small scope, probably 2 channel. rather than taking the truck to a shop for diagnosis. For the cost of shop diagnosis, you can likely pay for the scope. Knowlegde is critical, tools are helpful, very helpful.

 

[Dean E]

Thanks for your posting. When our trucks have problem with their LMM GMLAN systems, it is indeed very difficult to accurately diagnose the problem. Biggest problem is that few people have any understanding of the GMLAN, its components, and operation. Next is that few people have a scope capable of displaying the waveform from the GMLAN to tell if the communication is working or not. Last is a time effective method to do the actual testing of the GMLAN so that you don't have to go to individual modules, disconnect them, and drive around for a while to see if the problem is fixed (the GMLAN commnicates).

The attachment comes from GMUPFITTER.com and gives a good description of the system and it's basic operation. There are additional learning aids with Youtube videos. I found many of them are well meaning, get some helpful information and techniques communicated, but lack a systematic, coherent, easily understandable method for exploring GMLAN and it's quirks. It takes a lot of time to view the existing videos but they can provide information and the more you learn, the more you understand. I know that the Youtube presenters and doing the best they can and I appreciate what they've done.

I have not YET had a problem with my GMLAN but when I do, I will be buying a small scope, probably 2 channel. rather than taking the truck to a shop for diagnosis. For the cost of shop diagnosis, you can likely pay for the scope. Knowlegde is critical, tools are helpful, very helpful.

Well, hey there stranger! Did not know you were already here. Dean

 

[Dean E]

Thanks for your posting. When our trucks have problem with their LMM GMLAN systems, it is indeed very difficult to accurately diagnose the problem. Biggest problem is that few people have any understanding of the GMLAN, its components, and operation. Next is that few people have a scope capable of displaying the waveform from the GMLAN to tell if the communication is working or not. Last is a time effective method to do the actual testing of the GMLAN so that you don't have to go to individual modules, disconnect them, and drive around for a while to see if the problem is fixed (the GMLAN commnicates).

The attachment comes from GMUPFITTER.com and gives a good description of the system and it's basic operation. There are additional learning aids with Youtube videos. I found many of them are well meaning, get some helpful information and techniques communicated, but lack a systematic, coherent, easily understandable method for exploring GMLAN and it's quirks. It takes a lot of time to view the existing videos but they can provide information and the more you learn, the more you understand. I know that the Youtube presenters and doing the best they can and I appreciate what they've done.

I have not YET had a problem with my GMLAN but when I do, I will be buying a small scope, probably 2 channel. rather than taking the truck to a shop for diagnosis. For the cost of shop diagnosis, you can likely pay for the scope. Knowlegde is critical, tools are helpful, very helpful.

Ron, one of the most effective ways to isolate the GMLAN low speed buss is thru the network comb. This is up by the BCM between the BCM and the fuse panel on the left side dash. If you have a hard fail buss you can remove that comb and start connection modules one at a time while looking at the scope to see when the buss becomes active. On my issue the problem was very intermittent. Even in the video I made the problem cleared itself while I was recording. I would only see it driving every week or two and many times the issue would end up correcting itself by the time I got home. In that case looking at a scope would likely not tell you much other than looking at the scope to see if it trailed off on the 7 volt peak showing a weak grounding issue. About the only way I could continue driving to isolate the intermittent stuff was to start going around one module at a time disconnecting it and bagging the connector then drive around for a week or two to see if the issue was still present. This takes a lot of time but a whole lot better than shot gunning modules.

That is why I made reference to the big culprits that cause many of these types of problem. I already learned something that I had not thought about and that is the active displays on monitors like the CTS-2s. These must read directly off of the buss for real time data and therefore could likely be an issue if they start acting up. I thought looking at the individual codes on the different modules would help troubleshoot this issue but it is not. I thought I had a bad IPC (instrument cluster) because I had numerous codes each time the system acted up but what I thing was happening is so much information is displayed on the IPC that when the buss is corrupted it logs many faults that is sees from the buss. for the information lost. Most everything else including the SRS from the last fault only showed a loss of communication fault. The whole thing ended up being quite the learning experience. Disconnecting modules and driving probably is not the most elegant way of going about this but it is practical and it works. I have heard of many people trading in their trucks because of issues like this. Dean

 

[Ron Nielson]

Great post Dean and glad to see you posting over here also. Yo have lots of experience with our trucks and you're willing to share. Those GMLAN problems are certainly headscratchers and few have the knowledge you do in figuring these things out.