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I'm trying to fix in inop tach and was hoping someone had some ideas of what usually goes wrong. Things I do know is that the timing valve is working normally. I adjusted the sensor multiple times with no reaction from the tach. Any ideas?

The problem with mine was the obc board. I had to reflow the board, replace a couple traces and the batteries. That also fixed my flakey coolant temp and fuel gauges.

Try to unplug the tach plug that sits just above the 5th cylinder on that metal mounting bracket. Clean the contact points with rubbing alcohol and plug it back in. Be careful unplugging it because the plastic housing can be very brittle from age.