Hello everyone,

I'm dealing with a escalating transmission crisis on my ML350 Bluetec (W166) with the 722.9 7G-TRONIC PLUS and need urgent advice. New, more severe symptoms have appeared.

The Core Symptoms:

1. RPM Flare: A distinct RPM flare (slippage) during shifts, especially from 2nd to 3rd gear. The engine revs jump up with a loss of power, then it "catches."
· Critical Clue: This only occurs under moderate to hard throttle. It is absent under light throttle, coasting, or if I lift off the accelerator.
2. Hard Shudder: A violent shudder or vibration when accelerating abruptly from a standstill or from a low speed. It feels like the whole drivetrain is shaking.
3. Hard Push at Stop: When coming to a complete stop, there is a distinct hard push or lurch as if the car doesn't want to disengage and still wants to move forward slightly.

The Crisis - My Current Situation:

1. Low Fluid: I initially found the transmission was low (only ~4L in the pan).
2. WRONG FLUID: A garage did a transmission service to address the low fluid BUT USED THE INCORRECT SPECIFICATION FLUID.
3. THE WRONG FLUID IS STILL IN THE CAR. I have not yet corrected their mistake.

These symptoms started before this service. I also have two engine codes for low fuel pressure and implausible rail pressure, but the transmission issues are the primary concern.

My Urgent Questions:

· Do these new symptoms—shudder under acceleration and hard push when stopping—confirm a failing torque converter?
· How critical is it to get the wrong fluid drained IMMEDIATELY? Am I actively destroying the rest of my transmission by driving it like this?
· At this point, is a simple fluid flush just a band-aid, and am I looking at a guaranteed torque converter replacement?
· Any chance the engine fuel pressure codes are related to the shudder, or are they a separate issue?

I need to know my next steps. Should I have the car towed to a specialist, or is it safe to drive to have the fluid changed? Any insight is greatly appreciated! Note : The vehicle is in limp mode and has been so before the flare started, it will not rev past 3000rpm. Sometimes it comes out of limp mode and goes back again randomly.

I'm no expert but by no means would I ever drive my car without the right fluids. The wrong viscocity or other factors can be detrimental to the transmission and can cause permanent damage. Just spend a 100 bucks on a tow, it'll save you a lot in the long run.

Additionally I don't think your supposed to "flush" these transmissions in the first place. You're supposed to do a transmission service. Flushing is advised against by many specialist shops, maybe someone else can chime in for the exact reason. But in your case I would absolutely flush the incorrect fluid.

Mileage and transmission service history would also be helpful info.
Also, if the "mechanic" put in the wrong fluid, I'd think it highly likely that the wrong volume of fluid was also installed

Changing the fluid (drain, swap filter, refill properly) isn't hard but it is messy without a lift and catch pan but it can be done. Plenty of videos out there to show you how, and in the end you'll know if it is done correctly.

Thanks ! Bought the vehicle with no service history (I know that was a s***id thing to do) 5 months ago. The transmission was functioning perfectly until about a month ago when the issues began. The vehicle has 170,000 km on the odometer.

There's no decision to make. You need to put the right quantity of the right fluid in there. The challenge will be to get rid of "all" the wrong fluid. You'll need to drain the torque converter and if you're really **** you'll want to drain, refill, drive a short distance and then do it all over again. MB doesn't make it easy on these cars but its possible to DIY. The tricky part is pumping in the new fluid because of MB's ridiculous design. Your options for doing that will depend entirely on how well equipped your shop is but YouTube, Google or ChatGPT can help. Personally I pretty much don't let anybody do any task on my vehicles if it is even slightly possible that i can do it myself. That creates some stress but it avoids the situation you find yourself in where you have paid an idiot to screw up a job. I can still screw up my own work but at least its only me to blame.

Once you have the right quantity of the right fluid in then you can proceed to assess what further work needs to be done but until you do that you're just guessing.

100% change the fluid out to the correct fluid + what @bobofthenorth mentioned.

After that, do you have a CEL on and can you read the codes, that would be a start. Also, since buying the vehicle, what type of maintenance have you done? This will just give more context to your situation.