The sensor reads TDC for #1 cylinder. The tach signal is derived from the ECM (I believe). In any case, the easiest approach is to rule out the tach head itself. Pull it and have it tested. I'd bet on that being the source of your problem.

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.