[Rich T]

If you can unplug the meter, you can start with the primary operation test below... The tach needle should swing full scale and back to 0.

If that works, then I'd inspect your wiring to the meter (if it does not work, then I'd either open the meter and look for failed solder connections, or order a new meter).

 

[Rich T]

And the very first thing I'd try is just reseating the connector (and possibly adding a bit of dielectric grease if you tend to see rain).

 

[Rich T]

Yup, you moved something -- you are getting close to the source of the problem when you get things intermittent... It could be the connector itself, or inside the meter unit...

The other thing you can do is measure the voltage on the main harness at those (corresponding) pins -- you'd want to measure the voltage across some kind of load (say 100 ohms to draw 120mA) and see if it's solid, or if it might be intermittent as you tug on the wires and such.

Or if you have a sharp probe, you can even pierce the harness insulation and measure the voltage on the wires as the circuit is live (I'm often hesitant to do this unless the wires are well protected from weather, so maybe tape or heat shrink them afterwards?).

Of course, check the obvious things such as fuses as well (like reseating them) -- it seems unlikely to have an intermittent contact there (especially since everything else is working, like ignition?), but I have seen crazier

 

[Rich T]

I will usually smear a tad bit of liquid tape over the spot.

Nice, I will need to try this!