If I'm reading correctly there may be an instrument voltage limiter in the temperature gage.
There's some guys here who were or are techs that know more about the different digitial meters.
My suggestion for now is to start checking voltage to ground starting at the battery.
In other words.
Disconnect the battery and check its voltage. This will should either restore confidence in the meter readings or not.
Then reconnect the positive and ground cables and check the voltage at
the big starter stud
then at the starter relay
then probe the firewall disconnect terminal for Q1
Now you can jump ahead while your there and check the terminal for R6 as well.
Both should be at battery voltage.
If there's voltage at Q1 and not at R6, there's break inside. Find the next terminal that's exposed and reasonably reachable and see it has voltage. Probbaly inside of the bulkhead connector, then each of the ammeter studs or key switch if that's easier.
