OK, so I finally determined that the tacho itself is toast. Now the question is where to from here. The Yanmar replacement costs around $300 and frankly, I don't need to know the RPM that badly.
What pearls of wisdom are out there regarding non-genuine rev counters? I suspect it will be real hard to find another one that runs off the same Yanmar sensor.
I recall in my youth working with gasoline engines, rev counters could be bought that ran off the back of the alternator and my thinking is that this should work irrespective of the fuel type.
My question (probably rhetorical) is how does the rev counter know what the ratio is of alt RPM relative to engine RPM? I have a very accurate analog tacho that I can run off the crank pulley to determine the real engine RPM but if it's different to the alt RPM (and I can only assume it will be), how does it get corrected?
Or is there a better idea altogether that mere mortals like me can afford?