I am new here so I hope I do things right. I have a 2005 W203 4Matic with 133K on the odometer. Within the last 2K miles a noticeable "thunk" and jerk has developed when coming to a gradual stop. I just had the transfer case and trans flushed and filled, but no improvement. The "thunk" with the accompanying jerk only happens when the car makes the low speed downshift from 2-1 (in S) or from 3-2 (in C). The odd thing is that none of this happens when the engine is not yet at operating temp (like at the first couple of stops when leaving home in the morning). Once at normal operating temp the problem shows up.

If I shift to neutral and coast to a stop, no "thunk" so probably not brakes.

Anybody have an idea what may be going on? Oil in the trans control module? Transmission going bad? Maybe a differential problem (seems unlikely since it doesn't happen when the car is cold), but what do I know?

Any insight would be appreciated. Thanks

A little update. This does not appear to be a transmission problem since when i put the trans in 1st gear and slow to a stop I still get the "thunk." So no downshift occurs and the problem persists.

 

First things to do are check rear transmission mount, flex discs for cracks, center propshaft bearing for play, grease the slide & check rear CV joints.

Good luck!

 

Thanks so much Glyn. These all seem like much easier fixes than a rear dif! I'll post what I find.
Steve

 

So my MB mechanic can't figure out what is wrong. The "thunk" can be heard and felt but no out of the ordinary movement can be detected at the CV joints, center support bearing, or flex joints. He says he has never encountered anything like this. His "guess" is worn planetary gear or chain in the transfer case? But I sure don't want to replace the transfer case on a guess. Can anybody confirm or challenge his "guess"?

 

Maybe tq. converter goes bad or converter screws get loose, did you check that....??

ZAYED,,

 

Something is taking up play somewhere. The TC will make a loud howling noise so I doubt it's that.

We recently had a similar problem on a friends E Class & the centre nut on the transmission output flange had come loose.

You need to get the car on a hoist & diagnose properly.