It looks like you can connect an external tachometer to the harness of the tack signal output to see if it's the gauge. My tach signal looks to be taken off the power transistor. The tach also has a ground connection which you might try by-passing to a new ground to test for good ground. There is also a power wire from the ignition switch to the tach. I think you'll need a wiring diagram, but with input from other members you should be able to trace this.

92 Montero LS 3.0L V6 Auto, Stock, Original owner, 185,800K miles