Sorry, no help here, just commenting because I'm in the same boat.

I'm having issues with the tach in my '92 Dakota. What I've noticed with mine though is when it 'flat-lines' (doesn't read anything) it almost feels like the brakes are dragging, the truck looses its 'snap'. It's a subtle difference, but there is a difference.

I'm not sure how to diagnose it though. Yours might be easier being OBDII , but I don't know.

I've asked about the tach in my truck a few times, but no one seems to know how to diagnose it. A member here offered a known good tach to me and I swapped it in but no real difference was seen.