I don't drive my rig very much as a frequent traveler and teleworker, so it's mostly a date night vehicle. On a recent trip, cruising along a main thoroughfare at 45mph, the transmission got a bit herky jerky, RPM surging accordingly, and I pulled over into a parking lot. I can't remember, but believe I shifted to neutral, but subsequent attempts to shift into D or R would not work and I thought I'd have to push it into a parking spot and call for a tow. There were NO engine lights or other Caution/Warnings. Using the good 'ol Power Cycle method of turning the ignition off, waiting a few potatoes, then restarting...everything was back to normal. I kept accelerations to a minimum (ginger on the gas at stoplights), but drove another 20 minutes without issue. This was on the way OUT. On the 30 minute drive home, there were no issues. The vehicle has 32,000 miles, so I know the tranny is not bent. Google search did point to an "electrics plate" in the tranny or perhaps speed sensors. Now, incidentally, over the past two years, both (original) front wheel speed sensors have failed, putting the car in limp mode, but each time the incident came with a Christmas tree of caution/warning lights. This transmission deal was totally silent.

 

That sounds like a torque converter lockup issue. Typically this will trigger an error code that can be read in XENTRY. Something like P074100
Though every type of transmission issue has its set of symptoms, so there could always be exceptions….
I’m willing to bet you probably didn’t put it in neutral, what can typically happen in this type of failure is the transmission feels like it’s in neutral and won’t re-engage until you shut car off and restart,(exactly how you described) though sometimes it will engage if you slow down enough.

Your car sounds like mileage isn’t necessarily the issue, but rather the fact that it’s more of a weekend car makes it a prime candidate for battery issues. Especially in winter….
Hopefully you keep it on a tender. I can tell you low voltage conditions cause all kinds of nonsense on these cars including transmissions. If your batteries are in questionable condition due to age, or have not been verified good, via resistance or load testing, I would definitely start there. Without XENTRY you're kind of flying blind, so that’s the easiest thing to start with. Then I would have the codes read, recorded, and cleared in XENRTY. I will tell you unfortunately many generic scanners will not be able to read the codes nor be able to access testing diagnostics of the transmission.


Good Luck!

 

Thank you, sir! However, I just made a trip today with similar distance with NO issues. Since you mention batteries...this past month before the anomaly, I replaced the Aux Battery due to cockpit indications that are gone now. I'm in a high rise and keep the car in the underground garage and I don't use a tender. The battery continues to sound strong on every start, but you bring up an interesting point. The trip where we had the anomaly was right after a lengthy period of not starting the car (maybe three weeks). I purchased this rig in Apr 2021 from a broker, so it's 3.5 years in my hands. I should go pop the boot and check the battery date and replace if getting beyond normal life. It only had 29k on purchase. I do have a Foxwell code reader, but haven't had the time to go pull codes -- it has a Mercedes module, but not sure if it's XENTRY or some knockoff.

 

if your keeping it - you should consider getting the diagnostic tools and support you need.... just in time for Christmas

https://mbworld.org/forums/s-class-w...a-s-n-1-a.html

 

It sounds very similar to a problem I had with my 2017 SL550. Unfortunately, my entire transmission had to be replaced at my expense of $12,500. When I purchased it CPO from Mercedes there were only 2,750 miles on the clock. When the transmission failed, there were 18,500 miles on this car. I'm 77 years old and drove the car as you would expect a 77 year old to drive; I NEVER abused it. The good news is OEM replacement transmissions were available for my repair; the bad news is OEM transmissions are now on a lengthy backorder. Hopefully, you do not need a replacement.

 

Is yours a 9-speed? The OP's car is a 7G+ tranny. It does sound like the conductor plate, which is not a terribly big deal other than you can only get a new one from the dealership and they MUST install it. But as @crconsulting said, scan the transmission for codes first.

 

This aligns with some of my research. My rig has a 7-speed transmission. I've read of issues related to this conductor plate as well as issues related to an ISM. Looking for some videos leads to rather harrowing, deep maintenance efforts to repair as I believe the conductor plate lies in a wet area within the transmission fluid. I hope this was a spurious anomaly and not something recurring.

 

LOL, Don’t go down the wormhole, if you start listening to everyone on forums, and delving down in other peoples experiences, you’ll soon need a new engine, transmission and find out you have cancer too 😀

While we are all trying to help, me included. You should start with the easy stuff first (since you're kind of working blind without XENTRY) and work your way to the more involved repairs progressively….

The fact you don’t drive the car often and car is not on tender, indeed can create low voltage conditions if battery is getting weak or old.
I’ve seen this transmission voltage issue happen many times. One of my family members was diagnosed with needing a new transmission. She had a dead aux battery, which dealer was of course going to replace too in the repair estimate. I just replaced aux battery first, since it tested bad in XENTRY. Transmission issue never came back. She had 34k at the time, now she has over 125k on car, never had any issues return.

Do you drive the car in eco mode? Sometimes you can use ECO mode to spot a weak battery…..
Without any electrical components on, will the car stay off at a long red light or does it restart?

If you don’t have a tender, you can try to double click the lock button on your key fob. That will put the car in a “sleep” mode. You won’t be able to open the car by touching the locks and will need to use the key to unlock it. But it will use less battery power in sleep mode. As a side benefit, no one can clone your cars key signal.

It may be something deeper, but with that kind of mileage it probably is something maintenance related….

Good Luck!

 

Appreicate the Voice of Reason, @crconsulting !

ECO Mode. Kind of hate it, but mostly because I believe it does more harm than good. All those cycles on the starter, and the fuel/ignition process repeating...I don't believe it's beneficial. [And my daughter is an Enviornmental Scientist, so I do my best to live green despite driving a 14mpg car...occasionally.] But since you mention it, I would say that it does kick on sporadically not just when lifting foot off the brake. Honestly, I think it doesn't shut the vehicle down routinely at traffic stops. The day of the incident, I got caught at the Intracoastal drawbridge and sat idling for 10 minutes, I'm pretty sure it was in ECO mode (I hadn't secured it), and it remained idling. I typically don't punch off ECO until it cycles and reminds me that it's on...which is not very frequent, so perhaps you are on to something with the main battery voltage. I'm going to get thru the Holidays and then see if my man at MB will pull codes for me...after I try with the Foxworth. I also need to dive into the boot and check the main battery date. The Aux battery is brand new.

Thanks again for the sage advice, sir!