CONTINUOUSWAVE

On a 2000 Mercury 90-HP FourStroke with Yamaha powerhead:

The tachometer is dead—no reading at all—nothing at all.

During the last couple years, the tachometer would spring to life for a few seconds before dying. Now it doesn't even do that.

The gray wire is properly and securely attached at the back of the housing. Before I spend money on a new tachometer and crawl under the dash to swap it out, is there a way I can test the gray wire to make sure I'm getting a proper tachometer signal?

This is our 1998 Alert 17 (special service classic Montauk)

Yes, this engine is the carburetor FourStroke with the bad reputation. But it still runs great, so what can I say?

-Peter

Do you have or can you borrow an oscilloscope?

Does the gray wire originate from the engine control unit (ECU)?

Does the gray wire originate from a rectifier-regulator assembly?

Is your engine charging the battery? Usually a dead tach that is still in working order is due to a bad rectifier. Usually if rectifier is dead it stops charging and battery goes dead. I had a 93 Ocean Runner that loved to eat rectifiers.

I wondered about that but it’s hard to say, because at the end of each day, I put the boat battery on a maintainer so it’s always charged at the beginning of a day anyway. I suppose I can watch the volt meter. If it gets up to the mid-upper 13V range while cruising at say 4000+rpm, can I assume it’s charging?

-Peter

Yup....anything over 12.5 and it will be charging. Actually also look to see if it is overcharging too. Outboards usually have a low charge rate so if you are over 14+ it might be overcharging and a sign it is bad rectifier. I had one that did that and was charging like 16+ and the tach would start spiking and die, spike and die.