Tach Not Working

My tachs on both stations stop

My tachs on both stations stopped reading for the port motor. Is there some sensor that may need to be replaced. Where does the tach signal come from? The motor is a 1983 Mercruiser 230HP.
Thanks

"sometimes the coil

get a s

"sometimes the coil

get a shop manual, trust me it is well worth it"

the tach signal comes from the

the tach signal comes from the voltage regualtor wich comes from the alternator

"Tach signal on Inboard Outboa

"Tach signal on Inboard Outboards comes from the negative side of the coil. Two grey wires on the neg side if its Thunderbolt IV. Older models you will find two brown or one brown and one black. If some one has been messing with the wiring you find any number or colors.
I bought a cheepo auto store tach and wired it to test the signal to the dash it's easy with aligator clips.
1: Delete the light wire.
2: hook the signal wire to the S post on the back of the tach
3: Hook the power and ground wires to the power and ground on the dash.
start the engine. If the test tach reads ok the replace the tach. If not trace the wire and repair. The tach on a boat if bad can also cause the engine to run poorly or not at all. This system can also test the accuracy of you tach, which I have found to hit or miss if it is more than 5 years old."