I don't know if this will help or not. I have had a similar problem with mine but I have a 4 cyl Yanmar so don't know if they're the same but like another post said, it's cheaper to try than buy. Here's what I found:
- On mine the tacho is driven by a magnetic pulse sensor on the flywheel. Check around the bell housing and see if there is a unit that has two wires from it on the bell housing. They may have come off.
- If they are still on, you can put an ohm meter across the sensor and should get a reading. The technicians who eventually declared my tacho DOA said that any reading is good news (they're either right or knackered) and no reading is obviously not good news.
- On mine we had a voltage spike and the tacho itself got toasted. If you had a similar spike, my condolences, your tacho is history.
- The cost of a new Yanmar tacho (mine has a built-in hour meter) is about $290