Temperature gauge does not work on my 1970 340 Duster ?

RedFish said
There is only 1 temp sensor listed. The only difference is aftermarket quality.
The most important factor is water temperature. You must confirm that the thermostat is working and the water is in fact hot. Of all the gauges I serviced over a 8 year period only 1 was returned. That owner claimed the gauge reads waaaay too high. He didn't want to believe his recently rebuilt engine was running too hot. I proved again the gauge to be correct and sent it back to him
Is yours the opposite condition?
A digital volts meter on the mechanical limiter shows a lot of changing numbers. 2.XX to 3.XX that should average 5.XX volts. Most of us choose to replace that antiquated mechanical limiter with a modern solid state regulator. Several to choose from ( ebay ) starting at about 30 USD. Those are universal in design, thus requiring some alternate mounting and connecting by the end user. The only one that is nearly exact replacement for this application is about 55 USD ( RTE ). Plugs right in like OEM.
Any shop ( myself included ) that would service/repair your gauge(s) would demand a solid state regulator be installed. That steady stable output voltage that can be confirmed with a digital meter means fewer questions and extended gauge life.

Great insight RedFish. I will look for the solid state limiter. Please message me if you have a link to the one you used. Although my original still works I would like to eliminate any issue going forward. Back of my mind I've marveled at the success of the simple mechanical design. If it were to get stuck on 12v I would assume it to be a $250 trip at the auto part store. Like I noted earlier, the limiter was too short of a cycle to get an accurate reading with my multimeter so I only confirmed with a lightbulb that the cycle has a constant cadence.

I'm pulling the thermostat today to see what is happening in the vicinity of the sensor, and will test the sensor in a can of hot water to see if I can get a reading. DartLarry's message above has me thinking I'm getting air in the system causing an air pocket at the thermostat. I can't thank you gentlemen enough for your generous help. At 20 years old I only have the internet and people like you to get these types of things solved.
I'm at the 'shinny new tool box' part of my life and the best source is still the 'slightly rusty Craftsman tool box with super smooth roller drawers' kind of guys like you make it happen.