Hmm, I think those tachs have the same capacitors on them that the clocks do if I'm not mistaken, you might try replacing the cap(s) a real cheap fast fix if it works, Radio shack has them. I know they cause common failures on the clocks. If that doesn't work and SS doesn't have them, there's a place out here called Gustafson's, never been there myself, but word is he has all kinds of good things over there, MB's included, (530) 241 4246 I've been meaning to go explore the place one of these days, several people have told me its worth the look.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale