Wacky tach readings

[Weathersbytaylor]

I’ve got a tachometer issue. As long as I’ve had the boat the tach has never really worked. When running it would stay at about 900 rpm and randomly jump to 4K and stay there regardless of true engine speed. I assumed it was an issue with the tach itself so I replaced it earlier this week. Well the new one does the same thing. At or around idle it stays at 900, anything that isn’t wide open throttle is 2700, and occasionally it will read 4000 exactly. I ran a new signal wire from the grey wire on the coil and no change. Does this indicate a bad coil or does the electronic advance in the distributor contribute to the tach readings? Coil is less than a year old. Any pointers? Thanks guys

 

[Maclin]

Almost certainly not the coil affecting the tach. Coils get a signal from the distributor that controls when the eventual spark gets generated for each cylinder. The tach signal just tags along. More likely that a wacky tach can affect the coil signals though, and the engine either cuts out or won't start.

 

[Bondo]

Ayuh,..... I'd suspect a bad dash ground,......