Problem Solved!
Thanks Mike,
After rotating the selector switch as you suggested I disconnected all the wires and in the process the hot lead ring terminal came off. It was a bad crimp held on with shrink tube. Put on a new ring terminal the tach works fine!
However, the tach has a couple of extra wires on it not mentioned in the Faria "Installation Instructions". They are small, maybe 22 ga. on the signal and ground terminals. I don't know which electronics applications uses the tach signals, but I have a fish finder, radar, autopilot, chart plotter and VHF. Anybody have ideas on this?
By the way the Faria trouble shooting guide says about erratic tachs..."This symptom is caused 99% of the time by an intermittent connection between the wire and the ring or spade connector". They were right this time.