That's the route I was planning to go before reading this thread. I have the sensor on the harmonic balancer and a round receptacle on which to put a tach amp...but no amp. I think someone said its a diagnostic plug...either way, the wire from the crack sensor leads to it. I just need the amp...and to run wires from the amp to the tach.

One more component and I'm there!

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now