A VERY common problem in the 560SEL is failure of the rear nitrogen-filled accumulators, which I believe are part of the rear shock system. When they go bad, you get a very rough ride.

These fail so frequently that from what I've read, if a 560SEL has ride problems and the accumulators haven't been replaced, that's the first thing many folks do.

If you do a search there are likely to be more posts on this. I believe this is a reasonable DIY project.

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Steve
'93 400E