What type key are you using, the original MB steel key? Are the teeth on the key worn out? If you dont have the original key, i recommend you buy the proper key from your local MB dealer first.

Sometimes its a worn key rather than a worn ignition tumbler.

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1985 300TD Turbo Euro-wagon
1979 280CE 225,200 miles
1985 300D Turbo 264,000 miles
1976 240D 190,000 miles
1979 300TD 220,000

GONE but not forgotten
1976 300D 195,300 miles
1983 300D Turbo 175,000 miles

http://i42.photobucket.com/albums/e3...e485-1-2-1.jpg