Have you checked the tranny fluid? (while the vehicle is running)
If so, is the fluid red or dirty brown?
Many on this board will recommend you buy some Trans-X and try it in your transmission (do a search for Trans-X on this board)
When you got the vehicle did it shift through all the gears properly?
Best of luck in your diagnosis and solution!

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1985 300 D T - Rauchsilber (Smoke Sliver Metallic) with MB Tex Quartz 155 = Cream interior - 320K and counting!

1999 E300 TD - Silver - Just got her 3-13-15 !

the 2nd '83 300SD - Brown Metallic - traded away

1985 Limo Diesel aka "Stretchy" - Sliver - Blue Int - Sold but still in Athens

1983 300SD - Brown Metallic - Sold April 2008

1982 300 D T - Royal Blue with Tan interior - Sold