This is a 300D Turbo Diesel, with a 617.xxx engine and 722.xxx transmission. Maybe someone can fill in the blanks, but I do not believe it really matters much.

If the engine is not making enough power, the shifts will be hard.

First thing you need to do, is find out what's wrong with the engine. Can you be more descriptive? Does it smoke? Does it take off OK, but eventually slow down? Does it surge or pulse?

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1976 BMW 2002- 100k
1995 BMW 325i 175k
1984 Mercedes Benz 300D Turbo Diesel- 270,000 mi

1981 Mercedes Benz 240D- Estimated 300,000+ mi Returned to the earth