The tach is just reading a square wave from your ignition. And converting it to voltage to drive your tach needle.
Hook it up to a signal generator you can test the tach. The other side of the equation would be to hook an oscilloscope to the negative terminal of your coil and see the pattern and frequency. That would be the same as your Sun meter reading it, except with the Sun meter you don’t see the pattern. Since your Sun meter is reading correctly there’s a component issue in your tach. An older capacitor that’s leaking voltage?
Here’s a circuit diagram.
If you had a complete failure if, let’s say, the leads were reversed would blow the 1N914 Diode. I’d guess your C1 or C2 capacitor is going out on you. Maybe your R1 resistor has gone of spec. Capacitors are usually the culprit.
