Re: 93 reinell 5.0L top speed
Your wiring has been corroding for over 18 years. So it likely needs work. Remove tach, clean all electrical connections, spin the big cylinder selector screw around in circles a few times to help clean a bit of internal corrosion and reset where it was, then reinstall. Usually bad grounds cause gremlins like you describe. So now go and clean ALL of your grounds on the entire boat...under the dash at the ground block, at the battery, at the engine connection for the ground cable...and as long as you're at it, might as well do the positive side too.
After all that, if tach still doesn't work, replace tach.
