OhYou_

So I had a bit of a problem a couple days ago with my S430.
I was in 3rd most likely, at 25-30mph, and I went to pass a car and accelerate because the road speed changed to 55.
So I gave it near 100% throttle, the car downshifted into first, and then almost immediately went into neutral.
I coasted down to 20 or so and it popped back into gear.
(The going back into gear at 20 I think is normal because I've tried putting the car in neutral and back into drive while moving, and it stays in neutral till about 20)

I checked the gear selector and it was definitely in drive. I also checked DAS and there was no errors with the transmission.
I haven't been in the same situation yet to see if its persistent, but I have gone into first at speed before and it held fine.

What happened?

ItalianJoe1

Conductor plate failing in the trans most likely. The speed sensors sent a bad signal and the trans went into safe mode, which at that speed was neutral, until the speed dropped enough to go back into 2nd gear. My 600 was doing that in full load 3rd gear situations (so like 75-100mph passing moves on the highway, quite unsafe), I replaced the conductor plate while doing a trans service, never had the issue again.

OhYou_

Surely it would have generated a fault code though? There's no faults related to it, just one for a 5>3 shift, but I forgot what it said. I think it was a request to downshift was ignored or something like that.

ItalianJoe1

Maybe, maybe not. The sensors are on the plate and the TCM is underhood, have you checked for oil wicking into the TCM? Common issue on these cars.

I know, for sure, it was the plate on my car, as before I could replicate the issue 100% of the time at full load, and after replacement the car has been flawless for 2 years. Just because there's not a stored code, doesn't mean you don't need a new conductor plate.

OhYou_

I have not checked for fluid leaks, but the car has already done 240,000 miles, so I'm going to assume that the common issues have already been done at least once by a previous owner.
I thought the TCM was in the passenger footwell?

ItalianJoe1

No it's in the right side fuse box underhood. Just because it was done once, doesn't mean it will never be bad again.

wallyp

Open the right SAM/fuse box under the hood, remembering that left/right is always from the viewpoint of the seated driver. Closely examine the wire harnesses in the box for any traces of oiliness. If one of the harnesses has traces of oil, carefully work it up and out of the box, removing the attached small black plastic box (TCM) as you do so. Carefully open the box - it might be full of ATF (transmission fluid). If you find fluid in the box, use aerosol brake and parts cleaner to thoroughly wash the electronics inside the box. Let it dry thoroughly, close it and reinstall it. Drive the car and see if the transmission works better.

If you did find fluid in the TCM, you will eventually need to replace the transmission electrical connector adapter on the other end of the wire harness. The part is available at a Mercedes dealer, or at a Dodge dealer (the Dodge Sprinter van is built by MB, and uses the same adapter). Replacing the adapter is a relatively simple job, other than safely supporting the car so you can get underneath to the transmission. Google and YouTube are your friends. You do not lose a lot of ATF during the replacement. You may have to eventually clean the TCM again.

OhYou_

It wasn't entirely full, less then half a shot glass came out. I couldn't really get it to pop apart, and didn't want to risk breaking it without a replacement laying around.
I just drained it for now and stuck it back in plug side down.

How do I carefully open it?

ItalianJoe1

There are snap in tabs around the edges, a small screwdriver or pick to lift them and it should slide apart. Yes, you have to be gentle. If there was fluid in it, that's not good, you need to drain and clean it, and replace the connector on the trans to stop further fluid wicking. According to the MB bulletins, if you dry the components and flush the connectors with contact cleaner or similar, and repair the leak, then the residual oil in the harness isn't a problem. Fairly simple to correct and hopefully it solves your issue, if not, you may need a new TCM