tacho needle flickers on starting but doesnt work with engine running

[onespeed11]

Hi, Im new to boats but fairly good with cars...

Bought a 1989 Searay with sterndrive mercruiser 3.0 4 cylinder, just replaced the ring gear and retapped a starter motor thread...
Now trying to get the tacho to work. The needle flickers on starting but once the engine starts it doesnt move.
Ive checked that both earths to the tacho are good, the tacho has power and the grey signal wire has voltage to it. Tacho switch is set to 4 cylinder

My question is, is this simply that I need a new tacho or anything else to check first?
Tacho is a Faria.

Thanks!