Low-Speed GMLAN

Matt Diagnostician Red Wing, Minnesota Posted   Latest   Edited  
Case Study
Network Communications
AESwave
2015 Chevrolet Impala LTZ 3.6L (3 LFX) 6-spd (6T45)—2G1165S35F9202766
Erratic Gauges
Erratic Hvac Blower Motor
U-Codes

I realized today that I used a few different AESwave tools to diagnose an interesting vehicle, especially if you haven't seen a failure like this before.

The patient was a 2015 Chevrolet Impala with a 3.6L engine. The complaints were that the patient presented with a multitude of issues ranging from poor running and a lack of power to the instrument cluster gauges and light swinging and blinking erratically and sometimes going dead altogether. The same could be said of the HVAC blower motor. The client was convinced that the vehicle "was possessed by a demon".

I'm fairly certain she was not speaking of the Chrysler version...

I connected a scan tool, and maybe I should get the screenshot but, hopefully, you will take my word for it, it was riddled with "U" codes. I printed those out and cleared codes from all modules, not really paying all that much attention to what was coding and what wasn't. Many codes successfully cleared, but I still had a great deal of "U" codes that re-set.

I printed out a network schematic, which may have been a bit of wasted paper as I had 8 pages of them, and started marking modules that had codes associated with them. A pattern started to emerge. All the codes were associated with, of course, the first page of schematics I printed out: The Low-Speed GMLAN bus.

The next thing was to try to communicate with any of the modules on said bus. I could not. I then used my LineSpi HC (Harvey Chan version) connected to the DLC and then connected to my scan tool to allow easy access to Pin 1 of the DLC.

Using my PicoScope I connected to Pin 1 and saw this pattern.

Now, I won't lie, I've scoped these before so I knew immediately that this was incorrect. But, if you haven't scoped one before and don't have immediate access to another vehicle with Low-Speed GMLAN you would find that service information gives you a bit of a hint.

So, I'm reasonably certain, one way or another, that this signal is not as it should be. The path I chose was to access connector JX300 (Figure 5,6), which is really a splice-pack. Basically it does exactly as it implies, it splices together many wires, and in this case a bunch of Low-Speed GMLAN wires. This will effectively help me "divide" the problem. I could have gone with JX200, but I can't kid you around...experience was at play.

I removed the "comb" and saw this in my waveform.

So, I shipped the car.

Okay, just kidding.

I've now done myself a great service in drilling down the issue. I now KNOW that it can only be from that connector "back".

I decided to use yet another "toy" I picked up from AES, the uIsolate Network Isolator. These were a product suggestion by long-time AESwave friend Scott Shotton

I chose the set with the proper terminal sizes and used them very much like jumper wires. I started with Pin A because that is where Low-Speed GMLAN comes in from JX200. A watched my waveform closely as I added connections down the line. B, C, D, E, etc...until I reached K; and this is what happened to my signal.

So, now you can see from Figure 1 where I need to target, and for those of you that have seen this you can wonder why I just didn't go right to it but...what kind of war story would that make?

The Side Object Sensor Modules. The Low-Speed GMLAN bus passes through the Left module to get to the Right side.

So I disconnected the Right side first. No change to my waveform, but I had a lot of green corrosion (actually fairly normal in MN...).

Disconnecting the left side module restored my signal to normal.

I'm recommending both modules and a harness.

Thanks to AESwave products, both that they sell and a couple they design and build, I narrowed this issue down well within an hour and fairly easily.

If you look it over and look at the process, would much experience really be required to track down a similar issue even faster than me (which really isn't saying all that much as I'm pretty darn slow)?

+19
Joel Diagnostician
Soquel, California
Joel Default
 

very nice thank you for sharing

+2 Default Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Unfamiliar with Side Object Sensor Modules. Where are they and what do they look like?

0 Default Ð Bounty Awarded
Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
 

Hi Geoff: Here's a generalized description. See if it answers your question. gm-techlink​.​com//wp-content/up…​.​pdf Please note that the system described in the article may work this particular way but it doesn't mean that Matt's vehicle works the same way. HTH, Guido

+1 Default Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Almost, thanks. Can't tell where those pictures are. Under the hood or under the car? I have never had any trouble understanding wiring, electronics, theory, communication, etc. It's finding the dang parts on the car that takes up most of my time. I tend to see every problem only once out here. Sometimes I'll run a part number on eBay just to find a picture of it. Can't test what I can't find…

+1 Default Ð Bounty Awarded
Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default
   

Hi Geoff: I forgot that you'll almost be retired by the time you see them. :) Here's the layout of one GM model, behind the bumper fascia on each side. gmpartsnow​.​com/oem-parts/gm-s… Here's another view of a different vehicle. acadiaforum​.​net/9-general-tech… HTH, Guido

0 Default Ð Bounty Awarded
Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

"I forgot that you'll almost be retired by the time you see them" - LOL, Yeah, pretty much. Will B musta mentioned me. What usually happens here, is I will suddenly see ONE of something new-ish (five years old) and spend well over an hour learning all about some new (to me) system from a DTC or warning message. Then after further research and some testing I will determine some part that costs

0 Default Ð Bounty Awarded
Chris Diagnostician
Lansdale, Pennsylvania
Chris Default
   

Matt, I think you did exactly what you needed to do to get the job done. I'm never super concerned about time while I'm doing the job, but I think an hour diag isn't too bad at all. I generally keep the best records of processes and tests/results that I can while doing the job. I throw each one in a file folder. End of the day, I go through them while eating dinner and try to see what I could

+3 Default Ð Bounty Awarded
Bentley Manager
San Diego, California
Bentley Default
 

Nice write up. Thanks

+1 Default Ð Bounty Awarded
Michael Diagnostician
Champaign, Illinois
Michael Default
 

Nice job Matt. Do you think Aeswave could make a smartbob with a 9 Pin Deutch for HD?

+3 Default Ð Bounty Awarded
Chris Diagnostician
Lansdale, Pennsylvania
Chris Default
 

I'll second that, and I'll pre-order 2 of them. I'm building my own as a stop gap.

+1 Default Ð Bounty Awarded
Matt Diagnostician
Red Wing, Minnesota
Matt Default
 

I would certainly think the could. @jorge menchu would be the man to ask.

+1 Default Ð Bounty Awarded
Matt Diagnostician
Red Wing, Minnesota
Matt Default
 

I should have probably just hit it with some electrical contact cleaner.

+1 Default Ð Bounty Awarded
Chris Diagnostician
Lansdale, Pennsylvania
Chris Default
   

That's a bit much for regular old QD contact cleaner. I think stabilant 22a might be up to the task though. Looks like rock salt packed up in there.... Anyway, thanks for the update Matt. I enjoy seeing the failed parts at time. That is definitely on par with PA modules.

+2 Default Ð Bounty Awarded