Hi Mc,
Just guessing of course but
this sounds like it could be a bad ground connection to me.
The wiring diagram shows the tach and angle meter side by side on the IP. They share two common ground connections in the dash. One for the lights and one for the meters themselves.
Both those ground wires are color coded black.The meter ground ultimately goes through the main relay and to what looks like a major common ground for many other components. That is probably the main ground lug in the engine compartment.
The loose connection or bad section of wire could be anywhere of course but the place I would start
....if you can get access and still be able to run the engine....
Is where the instrument harness plugs into the control box.
The wiring diagram shows 5 bullet style plug in connectors. Find the one for the black wire and give it a twist and a gentle tug while watching your tach. It's also a good idea to wiggle any section of the black wire too. You'll likely need a helper to try this.
Next connection point should be the 14 pin black connector under the hood. You can wiggle the connector as well as the.wire but I would first...with the engine OFF...unplug it and look for signs of corrosion or maybe a loose socket for the black wire pin.
Using DE-OXIT or other good quality contact cleaner before plugging back in (engine OFF) is good practice here.
Next stop is the 6 pin connector for the main relay.
Again, engine OFF. Unplug the connector and inspect for corrosion or a loose socket. Contact cleaner then plug back (IGNITION OFF) in.
Anywhere you can wiggle the black wire a little bit, do so.
It's a tedious exercise, I know, but doing searches like this can work and you always learn something about your rig when you dig in like this.
Even if you don't find anything wrong you might still get lucky and find that you "accidentally" fixed it by unplugging and reconnecting. ALWAYS, IGNITION OFF!
Hope you find it and
Good luck!
