[prairieschooner]

Clarke,
Not sure about this since I haven't had to repair it yet but typically these Tachometers were driven off of the AC Tap on the Alternator.
OK, we don't have a computer and our Diesels are basically Marine Grade unlike versions that followed. Now when my Alternator quit working so did the Tachometer, not saying that this is the case with you, simply stating what I experienced.
I suggest that you start by having the Alternator Tested so that you can eliminate it. If all is well then take a Great look at the wiring (most likely use an Ohm Meter to check for resistance). Otherwise the Tachometer itself could be the problem.
I don't like changing parts to solve a problem so that is why I am suggesting that you have the Alternator tested. Maybe Richard could chime in her and help with another way to test the Signal from the Alternator.

---

PS my YANMAR Diesel in our boat had a Magnetic Sensor similar to what Jimmy had posted. Don't think that we have one of these but I have been wrong before.