Acta_Non_Verba

Thought I would share a recent experience with my car in hopes of helping someone else one day with similar issues.

My car has been running fine with no dash faults appearing and no adverse symptoms in operation and this is with a variety driving conditions - normal stop/go city stuff, to some pretty aggressive driving. I do a deep scan of my cars at least 4 times a year. The deep scan I do reports on 40 modules in the car and will give details for any faults found. The most recent scan I did was April 28th and these were the results:




With a variety of codes like this and in these categories, and without seeing the next level details, I know there will be multiple faults in Powertrain (P-codes), Chassis (C-codes), Body (B-codes) and CAN BUS (U-codes). Experience has taught me that when this happens, prioritize the U-CODES!

I am not a believer in erasing codes to see if they come back W/O first at least doing an analysis about what could cause the fault, EXCEPT when U-codes are involved. In this case, I will do a global delete as I know once CAN errors are detected, errors along the BUS where all the other module interact will often cause operational errors (because of the CAN fault) and likely there is nothing wrong with those individual modules.

So, with that, I did a global delete, started the engine, ran it for a few minutes, then did a fresh deep scan. Here are those results:




What I am left with are the REAL fault(s). Notice all of the "nonsense" that went away such as transmission errors, keyless go errors, door modules, etc., and notice what is left. I did a detail of these faults and they appear above at the end of the string. There are a total of 4 CAN BUS errors:

1. U103888 - Diagnostic CAN bus is faulty.
2. U103812 - CAN bus faulty. A short circuit to battery +
3. U118300 - Diagnostic CAN network unstable
4. U103100 - Diagnostic CAN bus off.

So, now, before lifting a wrench, I needed to do a soul search - what did I do (or possibly the dealer do) since the last deep scan I did which was back in January, and where the scan came back clean - no faults?

1. I did an oil change and reset the SERVICE A counter.
2. I had the center console out to make a couple modifications to the glove box, including the spring pins to assist in opening the glovebox door.
3. I had the front bumper off doing some detail work on the front grilles.
4. The dealer had the center console out for a minor switch replacement under CPO.
5. Serviced the transaxle (oil changes)
6. Bled the brakes
7. Changed spark plugs
8. Various other really little stuff like air filter cabin filter changes, etc.

Something about the CAN BUS. It is an incredibly complex structure and equally incredibly boring topic, enough to put even the super Analytical Poindexter to sleep, so I want to keep this at the highest level and avoid deep dives; otherwise, no one will ever get to the end of this post, including me. Regarding diagnosing and finding a fault, the odds are in your favor the fault will be pretty obvious, usually a shorted out wire(s) on a chassis somewhere, sometimes a blown CAN module, or something stupid such as, in older MB models, gateway modules placed near the running boards where water ingress eventually takes out the module. At least on this platform, MB did an OK job placing the gateway modules out of the rainwater (LOL).

Mercedes does a terrible job IMO on how it terminates CAN BUS Hi/Lo lines. The lines in and out of the bus terminals look like a high school shop project. It is pretty disappointing. Here is one example of another member with similar faults, that actually caught up with him and the resulting error came from the bus terminal at the Rear SAM Module with a shorted out pair of CAN Bus wires that wrapped themselves around a metal grounded chassis bracket.

https://mbworld.org/forums/coupe-roa...n-t-towed.html

In his case (and eventually my case if I didn't catch it soon enough) more and more modules continued to fail when they were using the CAN BUS, and eventually, the "no start" fault got him.

So, back to these Bus terminals. One is located back by the Rear SAM, another under the console, another at the Front SAM, and still another at the AMG gateway module. Since I wanted to satisfy myself I didn't suffer the same actual short the other guy did I spent a "few hours" examining those bus terminals in the trunk, under the console (since both the dealer and I were there recently), the Front SAM where I've never been, and I took a hard look at the AMG Gateway module. All of this took some "disassembly/reassembly" time, but I had the free time, so...free.

I didn't see anything that would suggest the CAN failed from a short or an open, but the MB protocol on this network will report ANY CAN error as a short or an open, sometimes both. I did spend some time at the gateway module BC, if all else is OK, then probably the module itself failed. The gateway module is located on the driver's side footwell, actually, far above the footwell between the edge of the dash and the front fender. It is a real physical exercise, trust me.



Here is a pic of the Gateway module. It is unplugged in this pic, but ignore that for now. I'll explain it shortly.

With some "due diligence" done now and no physical faults being obvious, my last resort was to do a hard reset of the gateway module...in fact, a hard reset of all the modules in the entire car. These central gateway modules are similar to the router (or modem) you probably have in your house. They operate perfectly fine most of the time, once in a while (a great while in the case of this router) they can get scrambled.

Long story short, I did a complete hard reset of the entire system. I won't go through the gory details of that. It has the propensity to sicken some people whom I've describe this to in the past. While the car was "Anesthetized" I unplugged the gateway module and used a contact cleaner on the connectors and the module (almost NEVER the problem, but it was convenient to check the box).

With all the modules now hard reset, I woke the car back up, started it, took it for a long drive with everything warmed back up to op temps. Came back, shut it down and did another deep scan. Here are the results, and what I am used to seeing with deep scans of my cars:





The conclusion to all of this -

When I did the oil change and reset the service A indicator, mysteriously, a service B indicator came up immediately afterward and showing some ridiculous overdue mileage, mileage that went far before my ownership mileage. It took another week of driving before the system let me reset that indicator. Researching it, I found that is a rather common fault and a number of owners have had it happen in various MB models, not just the AMG platform. I am concluding the "culprit" that caused the Can errors was this. It upset the AMG gateway module (which acts just like the router in your house). With that module upset it started to cause other systems using the CAN BUS to fail intermittently (even though they continued to function normally), until it was "reset" similar to the way you would reboot your router at home. BTW, one of the primary functions of the gateway module - it manages the service intervals.

Another conclusion - reading the actual description of U-code faults, sometimes can be misleading, but it's how the designer of the EMS chose to message faults. With MB, it defaults to the same 3 or 4 U-codes as in my title for almost any CAN BUS error. These faults can indicate shorts and opens that only really exist BC certain firmware code couldn't exercise fully, so the simple flow chart concludes a short or an open must exist....frustrating!

Since then I've driven the car and rerun 3 more deep scans all of which are showing no faults.

Hendrik Koster

Nice work and a great write up, for any DIY type owner it's good to see how you go about staying on top of things. Working on these cars has thought me that any procedure not executed properly can over time generate lots of side effects, your method of periodically checking prevents these so called "gremlins" from escalating to the point that for some owners and MB service advisors it becomes a guessing game of what is really the originating issue.

I appreciate you taking the time to give us some insight of how you go about keeping your car in tip top shape!

Well done - enjoy

thebishman

AVN: you are such a great resource for the forum! Thanks for all you do. Vic needs to make you our ‘Technical Moderator’! lol

BTW: where in the hell has Vic been lately? lol

Acta_Non_Verba

Thanks for the very kind words, for such a title though I would nominate

Hendrik Koster
Stenzel

Maybe both!

Stenzel-Germany

Hi Steve,

no false modesty.Just like Hendrik, you dare to do everything.
I really appreciate the knowledge and the ability to think outside the box.
Your sharing and documenting for others is still important.
This makes the forum interesting, worth reading and, above all, a real help.

Great respect and thank you for the great effort

Best regards

Stenzel

P.S.

Diagnosis is also part of maintenance for me.

I'm in Switzerland right now (until the beginning of July) After 2 days, in the evening, the GTR was electrically "dead".(Undervoltage protection had triggered.)
It's difficult to be there, without a workshop, tools, WIS and Xentry.(Didn't even had internet. :-) )
That's probably how it is for most people here.

Luckily, a borrowed multimeter was enough.(It was the accessory BT module for the steering wheel, it works, but now, no shut down)


Best Regards

Stenzel