Tachometer not working??
I recently had that problem on my Westerbeke 21. Hour meter worked & tach did not. Prior to my tach stopping, I had removed the alternator in order to do some engine work. The wiring that ran to my alternator ran thru a corrigated plastic tube. When I put everything together, I had not tucked the tube all the way back to its proper position. When I started the engine, everything worked fine. After about 1 hour underway, the tach stopped. What happened is that the plastic tube came in contact with the manifold & melted. This in turn damaged some of the wiring it contained. I replaced the damage sections of wiring & solved the problem. Your wiring may be similarly damaged or they may have some corrosion. Check that out 1st. Otherwise, the problem may be in the alternator itself.