Bobman,
I have the exact same scenario (touching clock 12v lead and blowing the tach).
I've replaced every fuse, tried running a new 12v lead, and even installed a new tach. None of which worked.
I've come to the conclusion that its either the OVP or the pickup for the tach (although i'm not sure how it could've gotten affected by the 12v lead).

I'll let you know if I'm able to fix our mysterious tach problem

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1987 300e

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