Help! Tach problems

[spareparts]

Frem is correct about the crank sensor giving the tach signal, it goes thru the ECU and the computer generates a tach signal for the tach to read. Easiest way to diagnose it is to hook another tach to it. I don't care for the yamaha digital tachs, they are expensive and seem to only last four or five years, I'm old school, I like dials and needles