IF all the other gauges work properly, we can assume we have good power and ground to the tach. Disconnect tach and place a test light at the white wire with black tracer. Start the engine the test lamp should blink rapidly. If ok you have a bad tach. If not, you have an open in the wire between the coil and tach.
Conner

Over the hill but still climbing.
88 Montero 2.6L auto, manual hubs, cooling fan conversion gps,recurved distributor,LSD Diff.