Temperature gauge does not work on my 1970 340 Duster ?

Yeah, you’re right. I have forgotten a lot of the details of this puzzle.

One thing that I remember from my youth (I think) is that the temp sensors can be damaged by too much heat. If you are trying to simulate engine temp with a torch or a lighter, the sensor can be damaged and will go open I think. Don’t know if you have done this but worth mentioning.