Hi everyone! Long time listener, first time caller...
So I have been working on my Dad's '56 Mercury Montclair. Ever since the car had been rescued from the junkyard in '80, the factory temperature gauge has never worked so we relied on an aftermarket mechanical gauge under the dash. I decided to get the dash gauge working again, so I bought a new sending unit from a local parts supplier and hooked up the sender wire, nothing happened. I checked for voltage at the backside of the gauge and it showed 12v, so I determined the gauge itself was bad. I sent in the temp gauge to a gauge/speedometer repair shop, and they replaced the internals and returned it back to me.
Note: 1956 was the first year of a 12v electrical system in the Mercury, and their gauges ran off of a full 12 volts that year. Correct functionality for the temp gauge is for the needle to rest on "H" when the key is shut off. When the key is turned on, the needle moves to "C" and rises as the engine warms up.
 Once I received my gauge, I reinstalled it and turned the key on. The needle moved to "C", I ran the motor to operating temp and the gauge quickly pegged to Hot, which hasn't ever been an issue with this car and no modifications to the cooling system had been made. I allowed the motor to cool down and tried again the next day. This time, I turned the key on and the needle didn't return all the way to "C". I suspected either a faulty sending unit, incorrect resistance calibration in the gauge, or a combination of both. I purchased a NOS temperature sending unit and sent it, along with the gauge back to the repair shop to diagnose. The technician advised that the gauge was bad again, and the temp sender I included had the wrong resistance values. How could that be? It's NOS for a '56 Merc and it's 12volt as indicated on the box. What am I missing here?  I've never had such a problem with getting a gauge to work. Any insight would be greatly appreciated. Does anyone have the correct ohm resistance for a '56 Mercury temperature sender? Thanks!