I think the correct term is Ammeter. Back in the '50's the Ammeter was a useful tool. By the late '70's the Volt meter became a more useful tool (which didn't come out with Chrysler until early '90's).
The way to do a quick test is when in car - watch the Ammeter for any movement when headlights are turned on (engine off). it should move a smidge (to the Left) and move back once headlights are turned off.
The Temp gauge and fuel gauge (and oil pressure gauge if equipped) uses a 5 volt signal. Both (or all three) gauges work identically.
The Ohm readings for both (or all three) sensors/senders are:
Full tank/temp gauge to max reading (oil pressure gauge to max reading) = 10 Ohm
Mid range position (all gauges) = 23 Ohm
Empty tank, cold engine (or no engine oil pressure) = 74 Ohm
At one time, I had an old fuel sending unit with a couple of gater clips attached, to use to test the gauges with - and for that, it worked great (testing fuel, temp (or if equipped, oil pressure)). If the tester was used, and the gauges worked according to low (mid if used) and high readings, then the harness and gauge are OK. IF the gauges do perform using the above mentioned resistance values - then the sensor/sender is at fault. If they were not correct, then a problem with gauges, wiring or the 5-volt regulator.
Note: the 5-volt regulator will be OK if one of the other gauges work fine (fuel, temp and/or oil pressure). The oil light and Ammeter are on separate circuits.
In your case, I suspect the temp sensor wiring might be disconnected from the sensor or has another harness problem somewhere.
The general location of the temp gauge sensor is in the white circle (which is under a whole lotta of ____).
This is a better picture:
The sending unit is bottom arrow and wire is top arrow. Why both (and hose clamp) are painted blue - IDK.
BudW