Anybody knows what MB changed on the S500 coupe on 1996-99 models,versus the 1994-95 models of the same car.It got better? or just cheeper...

If you do a search, you'll find an earlier posting I did on exactly this issue.
In my opinion, the changes made in 96 on were for the better.

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KenP

1996 S500 W140 Coupe

It seems to me the early cars had less power assist in the steering and a little better road feel. Through the 95 MY it was an LH engine management system - distributor caps and rotors, replaced by coils in 96. The caps and rotors are about $200 every 60k miles or so - might seem expensive but bulletproof and easy to do. The newer system is theoretically more reliable but also more expensive to fix. But the big difference is the transmission - the troubled 722.6 started in 96 - seems to have half the life and twice the rebuild cost. I personally subscribe to the "never buy a Mercedes after 1995" theory but also subscribe to the "never buy a car with ASR" theory, and all S500 coupes had ASR so to me it's a moot point.