Module confusion and issues on my 2008 LMM

[Dean E]

Good day everyone, i"m waving my white flag on trying to troubleshoot my intermittant problem on my GMLAN low speed buss. I have been having intermittant problems in losing communcation on my low speed buss. IPC comes alive with lights and DIC messages SERVICE ABS, SERVICE TPMS, SERVICE 4 WHEEL DRIVE, and SERVICE AIRBAG. The dash will go to all zeros, the radio (aftermarket with interface modules) will go dark, and the AC controls will freeze. Sometimes it clears on its own or sometimes I need to shut the truck down for about a minute or two. The interesting thing about when I have to shut the truck down is the HVAC control will still show in the display for about 30 seconds and then go out. Normally it goes out once I open the drivers door with the engine turned off just like the radio. I have unplugged one module at a time to see which one might be having an effect on this. Radio modules, HVAC, trailer brakes, remote, ABS, 4 wheel drive and lastly the SRS airbags. I thought I had this locked down, the SRS unplugged the truck ran for over a month with no issues other than the AIR Bag Service. Installed a new SRS and had the dealer program it. Had no issues for over 3 months and bang it started happening again! I also changed out the BCM and put in the latest and greatest -number from Spartan Autoworxs. Still the problem remained.

I then saw a Youtube video where the On Star module which I believe is the VICM causes simular problems. Doing some research it is just below the HVAC controls in the dash close out panel. I unplugged it. When I power the truck up I get a couple codes U0137 and U0198 both are BCM communication faults but the weirdest thing is I get a SERVICE 4 WHEEL DRIVE and SERVICE TRAILER BRAKE SYSTEM DIC messages. No codes for those in the download. I then went back and plugged in the VICM and everything cleared. HOW does the VICM cause trailer brake and 4 wheel drive messages?

 

[2004LB7]

the data lines go through the modules so unplugged them will brake connection with the remaining modules. you will need to jump the connection to keep the gmlan intact if you disconnect any modules. see attached

 

[Ron Nielson]

More info

 

[Dean E]

the data lines go through the modules so unplugged them will brake connection with the remaining modules. you will need to jump the connection to keep the gmlan intact if you disconnect any modules. see attached

That may be true for the GMLAN high speed buss but from everything I see the low speed buss is all wired in parallel and tied together thru the comb connector. I have already disconnected most of the modules individually on the low speed buss and still had communication thru the rest of the single wire low speed buss. That is how I have been eliminating modules that maybe corrupting my low speed buss. Dean

 

[Ron Nielson]

Dean
Did you find the comb where the low speed com wires all attach at one point? That's the easiest point to remove each of the modules.. I don't know if I have something that gives that location, I will post again.

https://www.justanswer.com/chevy/2u8ux-back-lights-2004-chevy-silverado-fuse.html
This is not an LMM locfation but yours will be pretty close to this.

 

[Ron Nielson]

here is the electrical manual download for 2008 lmm https://www.google.com/url?sa=t&rct...ssisCabs.pdf&usg=AOvVaw0N7QoZRarYgQo9OUzEK73B

 

[Dean E]

Dean
Did you find the comb where the low speed com wires all attach at one point? That's the easiest point to remove each of the modules.. I don't know if I have something that gives that location, I will post again.

https://www.justanswer.com/chevy/2u8ux-back-lights-2004-chevy-silverado-fuse.html
This is not an LMM locfation but yours will be pretty close to this.

I found the comb. Only problem is making a test harness to remove one at a time. If this was a hard fail it would be simple. Plenty of videos that show that and how it looks on a oscilloscope. My problem is that this whole issue is very intermittant. Connected my enclosed trailer with my bike inside and drive down to AR and back. 9 hours each way and the truck ran perfectly. The the next time I took the truck to work the whole thing after up on the way home. No difference in weather or anything else to help isolate. Thought I might have been on to something removing the On Star module. I need to go back to the prints but hard to understand the trailer brakes and ABS messages just from disconnecting the On Star module. Can't imagine any high speed stuff on either of those circuits. Very frustrating. Dean

 

[Mikey52]

But the trailer circuit could have been providing a ground that is intermittent without it.

 

[Dean E]

But the trailer circuit could have been providing a ground that is intermittent without it.

The factory trailer module was disconnected during my attempts to isolate this problem and the issue still appeared. I just used this last trip to show just how intermittent the issue can be. Dean

 

[Ron Nielson]

Intermittent electrical problems are the most difficult things to deal with. And the further apart the occurrances are, that makes it even more so. The use of a scope is quite helpful, but then you have to have the problem happening right then or you will still have to wait. .

DTC U0137 00: Lost Communication With Trailer Brake Control Module
DTC U0198 00: Lost Communication With Telematic Control Module

 

[Chevy1925]

it sounds like you have a rubbed through wire issue that is providing dirty signals to the low speed side. i dont think this is a module issue.

if chaning the SRS gave you the longest run time before issues came back, have you tried tracing the wiring back looking for any pinch points? wires may have moved enough when unplugging and re-plugging back in that the bad wires separated untill vibrations moved them back.

your diag overall has kinda ruled out easier options. you almost need a scope on the wires but that wont really help eithers.

Can GM low speed command things like the HVAC to stay on during RAP or is power turned on to it via an actual power feed? in otherwords, does the hvac have constant power to it even with the door open and the low speed or high speed LAN are what trigger it on or off?

 

[2004LB7]

it sounds like you have a rubbed through wire issue that is providing dirty signals to the low speed side. i dont think this is a module issue.

if chaning the SRS gave you the longest run time before issues came back, have you tried tracing the wiring back looking for any pinch points? wires may have moved enough when unplugging and re-plugging back in that the bad wires separated untill vibrations moved them back.

your diag overall has kinda ruled out easier options. you almost need a scope on the wires but that wont really help eithers.

Can GM low speed command things like the HVAC to stay on during RAP or is power turned on to it via an actual power feed? in otherwords, does the hvac have constant power to it even with the door open and the low speed or high speed LAN are what trigger it on or off?

I think HVAC and radio, probably others too are constant power and turned on via the communication wire

 

[Ron Nielson]

HOW does the VICM cause trailer brake and 4 wheel drive messages?

Dean
The ABS fault (if ABS light is on in the dash display) is most likely the cause of these 2 messages. Both systems need to know information from the ABS in order to operate correctly. If ABS problem goes away, very likely both of these messages go away without any other intervention.

VCIM is part of the infotainment system. You don't happen to have an aftermarket radio do you?

Vehicle Communication Interface Module (VCIM) (UE1 -YE9)

Center of the I/P, below the center I/P Panel

The high speed GMLAN serial data allows communication between the BCM,
ECM, Transmission Control Module (TCM), Vehicle Communication Interface Module (VCIM), 4WD control module, EBCM, and the suspension
control module depending on RPO.

 

[Dean E]

Dean
The ABS fault (if ABS light is on in the dash display) is most likely the cause of these 2 messages. Both systems need to know information from the ABS in order to operate correctly. If ABS problem goes away, very likely both of these messages go away without any other intervention.

VCIM is part of the infotainment system. You don't happen to have an aftermarket radio do you?

Vehicle Communication Interface Module (VCIM) (UE1 -YE9)

Center of the I/P, below the center I/P Panel

The high speed GMLAN serial data allows communication between the BCM,
ECM, Transmission Control Module (TCM), Vehicle Communication Interface Module (VCIM), 4WD control module, EBCM, and the suspension
control module depending on RPO.

The ABS and trailer service messages cleared when I went back and plugged in the VICM. I knew something was up since I had no faults on either system on the diagnostics, just the communication codes. Who would have known, did not imagine that there was high speed data for the ABS or trailer brake system. So I left the cover off of the VICM so when everything acts up again (IPC goes all zeros) I can reach over and unplug the VICM to see if the IPC comes back. Do you have the wiring print that shows the connection to the VICM? I'm guessing that I can disconnect the GMLAN low speed wire and not effect any communication to the rest of the system. Dean

 

[Dean E]

Dean
The ABS fault (if ABS light is on in the dash display) is most likely the cause of these 2 messages. Both systems need to know information from the ABS in order to operate correctly. If ABS problem goes away, very likely both of these messages go away without any other intervention.

VCIM is part of the infotainment system. You don't happen to have an aftermarket radio do you?

Vehicle Communication Interface Module (VCIM) (UE1 -YE9)

Center of the I/P, below the center I/P Panel

The high speed GMLAN serial data allows communication between the BCM,
ECM, Transmission Control Module (TCM), Vehicle Communication Interface Module (VCIM), 4WD control module, EBCM, and the suspension
control module depending on RPO.

BTW, I do have an aftermarket radio in the system. That includes two modules, the one that is the interface for the radio (chimes and warnings) and another for the steering wheel inputs. I thought that was my fix the first time I troubleshot this. Got a replacement from Crutchfield and things were wonderful for about 3 months. Then when things acted up again I disconnected both of those modules and still got the issue. If this is a wiring issue it won't be from wiring the radio modules. Those were done with heat shrink solder joints.

I might be wiring, I did check the harness after the SRS module replacement and found no chaffing or difference when I wiggled the harness around. One thing I have noticed is there have been a few times where the issue comes up when I turn to the right in an uphill turn. Not everytime of course but there have been a few. I can go down some pretty rough roads and never see the issue, last time I was driving down the highway, smooth as glass and the problem appeared.

I did watch this video

Source: https://www.youtube.com/watch?v=TlM81swpjf4&t=118s


Not sure why you would need to remove the connection on the PCB when you can just removed the 10 amp fuse for INFO. I did that and no messages appeared and truck runs fine. Dean

 

[Ron Nielson]

I thought I posted this previously - link to complete electrical manual for our trucks. Lots of good info in addition to the actual wiring diagrams.

https://www.google.com/url?sa=t&rct...ssisCabs.pdf&usg=AOvVaw0N7QoZRarYgQo9OUzEK73B

Normally, I would post the link direct from GMUPFITTER Body Builder manuals, but they have really screwed up all the links for anything prior to 2013

 

[Dean E]

I thought I posted this previously - link to complete electrical manual for our trucks. Lots of good info in addition to the actual wiring diagrams.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwi_6KDWjoP5AhXMKkQIHc3vDIAQFnoECAYQAQ&url=https://www.gmupfitter.com/wp-content/uploads/2021/05/2008_LD_ElectricalPickupsChassisCabs.pdf&usg=AOvVaw0N7QoZRarYgQo9OUzEK73B

Normally, I would post the link direct from GMUPFITTER Body Builder manuals, but they have really screwed up all the links for anything prior to 2013

Well, I was about to give in and take it down to a specialty shop to bite the bullet and get this fixed. Then I came across this video.

Source: https://www.youtube.com/watch?v=Rwsfe4KT330


He does a great job explaining the GMLAN low speed (otherwise known as GM class 2) with the breakout test harness for the comb and the small single channel hand held scope. Ordered everything thru www.aeswave.com. At that point I can watch the scope and move harnesses around to see what changes I get to the scope (if there is a chaffing wire) or when the buss goes high or low I can remove one circuit at a time to isolate my problem. I would have spent more just to have a shop look at it. Thanks for the prints Ron. They will be very helpful in isolating the problem. If I can sort this out I might do a thread here explaining the process. Dean

 

[Ron Nielson]

Pretty good video. He keeps it simple and doesn't get into the more complex stuff . I had not previously seen the multi-connector he used to plug into the comb connection, which makes diagnosis much simpler. Our lmm's have 2 communication systems, a low speed, and a high speed. The low speed is like what is shown in the video. The high speed does not use that comb to tie everything together and makes diagnosing a little more difficult.

Very good that you bought the tool to get the job done. Many people just won't spend the money, although they will everntually spend it anyway at the dealer.

Once you figure out what the problem is, please let everyone know what you found.-

 

[Chevy1925]

Well, I was about to give in and take it down to a specialty shop to bite the bullet and get this fixed. Then I came across this video.

Source: https://www.youtube.com/watch?v=Rwsfe4KT330


He does a great job explaining the GMLAN low speed (otherwise known as GM class 2) with the breakout test harness for the comb and the small single channel hand held scope. Ordered everything thru www.aeswave.com. At that point I can watch the scope and move harnesses around to see what changes I get to the scope (if there is a chaffing wire) or when the buss goes high or low I can remove one circuit at a time to isolate my problem. I would have spent more just to have a shop look at it. Thanks for the prints Ron. They will be very helpful in isolating the problem. If I can sort this out I might do a thread here explaining the process. Dean

absolutely post what you find and how you did it. This kind of stuff has always fascinated me and where there is nothing on worth watching at home, ill usually cruise through youtube vids of guys working on electronics in trucks/cars. Thats always been a weak side of me when wrenching.

 

[Dean E]

Well I wish I could give a good progress report on the GMLAN class 2 buss issue I have been having but nothing has happened since I pulled the 10 amp INFO fuse out. This is the one that powers the OnStar module. I have a OBS2 breakout box and used my new oscilloscope on it and naturally the buss looks good. I will say this new oscilloscope is quite the tool. Smaller than most cell phones, a high definition color screen and many features to check ignition, fuel pump motors, alternator rectifying bridge output and more. Here is a overview of what it can do. Quite a bit for a $160 investment! Dean

Source: https://www.youtube.com/watch?v=aazmvhgZpy8&list=PL-Rl7bcueUT1YdOGGMXw3LgNxLFGi1EZt