Hello Oldsouth,

I own a 93 S500, and I had that problem also.
If that is like you describe, believe me that you have a damaged transmission shaft. If you fell the vibration on the steering wheels it is probably from the lower arm bushings, but if the vibration is felled all over the car the problem is definitely the trans. shaft.
The reason is that the “cardan” in the middle of the shaft becomes damaged with miles.
In my case I had to replace the complete trans. shaft US$ 1000 without man work.
YES it solved the problem but the W140 will always have some vibration at low speeds, but of course nothing comparable to this.

I hope this may help you.

Folgado
S500

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Folgado
S 500