This past Saturday I drained the transmission and torque converter, dropped the pan, and replaced the filter. The old fluid was somewhat dark (not bad, but darker than new) and there were some metal flakes present, but not terrible. Not enough to really concern me at the time, anyway. I did not notice any unusual odor.

I refilled the transmission with the engine at operating temperature, running in park, and filled it to the max line. Went for a test drive and everything felt fine. Shifts felt crisper than normal, and the car chirped the tires between 2nd and 3rd (never done that before!)

The car then sat for 48 hours. No leaks or anything else unusual developed.

Last night we got in to go to the grocery store, shifted into reverse, and nothing. Revved it up a little and I get a faint "clunk" from the rear end and the car moves forward slightly, then moves back again when I release the accelerator.

Shifting into forward gears produces the same result. Faint clunk from the rear, and the car won't go anywhere at all, regardless of RPM. But in forward gear, if I wind it up past about 2000 RPM, the speedometer starts to jump up to about 40 mph, then back to zero when I let off the accelerator.

I siphoned about half a quart of ATF out of the dipstick hole and poured a bottle of Trans-X high-mileage in, then ran the engine until it was at operating temperature again, and verified proper trans fluid level in park.

The car still won't go anywhere in any gear.

So ... it seems amazing to me, but did I break something simply by changing the fluid and filter?

I've got a "spare" transmission in a parts 300SD that I'm consdering dropping and putting in this car. It's got ~80k fewer miles on it and seems to work fine, but I haven't had a look at the pan.

Is there anything more I can do to troubleshoot before I just drop the transmission?

 

I have seen you say that before. Why do you believe Trans-X will cause damage? From what I've read, lots of Mercedes owners use it, and it's certainly been around a long time.

I'll get under the car tonight and see if I can see flex disc damage. I just ordered a new set of them this morning, since I was planning on replacing them during the transmission swap anyway. Hopefully that's all it is.

 

Ok -- I got under the car this evening and looked at both flex discs; they're both intact. I didn't even see any cracks in either one.

I verified the vacuum line is connected to the transmission.

I started up the car with the rear wheels up on jackstands. When I put it in reverse, the wheels turn in reverse. When I put it in drive, the wheels turn forward. But nothing when the wheels are on the ground and under load.

Driveshaft looks intact and normal.

I dropped the pan and looked at the fluid (which only has about 5 miles on it) and compared the filter I installed to the one I took off the car.

Filters are identical.

The fluid has metal particles and again looks burned -- has black swirls throughout -- but does not smell bad.

After replacing the filter and refilling with ATF, still the car will not go anywhere.

 

Yep, sure did. Drained the TC and the pan.

Yes, the car holds its place when put in park, but park doesn't engage (after being in gear) until I move the car back and forth by hand. It seems to stay in gear until rocked back and forth.

The engine revs freely. It doesn't act as though it's under load at all.

 

ForcedInduction: the driveline is intact and the vacuum line is connected properly.

Do you have any more ideas? Do you think I should swap in my other transmission at this point, or do you still think the problem lies somewhere other than the transmission?

Thanks for your help.