Hi Bill,
No similar problem here but a quick review of the Volvo Penta Workshop Manual (link on this website home page) shows that the tachometer works via a speed sending unit located on the starboard side of the engine (see page 8 ). Wires run to the tachometer (see wiring diagram page 35). The wiring diagram also provides wire color code to help find the wires.
My understanding is that the sending unit sends a current (or pulsed current) to the meter. So a multi meter test at the sending unit while the engine is running should show some current. If it does, then test again at the meter to see if the same current reading is seen there. If not it is probably a wiring or connection problem.
My first instinct was to tell you to confirm that the alternator belt and alternator were working properly. But then I remembered that when my alternator was inoperable the tach still worked. I'd start by confirming good connections at both ends of the wiring, then do the current test with the engine running.
Hope this helps,
Richard