My transmission would slip in reverse, and not even move the car if there was an up hill grade. This was a 1991 300SE. Don't know if this is the same transmission. 120,000 miles. Great transmission shop (Vancouver Transmission) had to rebuild the whole thing- $2,000. There was a hole in some little rubber seal that was causing the reverse slippage, but at that mileage the whole thing needed refreshing.

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1994 E320 Cabriolet
1992 300CE (Sold)
1991 300SE (Sold)
1986 560SEL (Sold)

that sounds exactly like what's wrong with mine. bummer that it's probably going to be an expensive fix.