Temperature sending unit problem.

The temperature sending unit on my Valiant does not seem to be working. I have replaced it twice and it hasn’t worked either time. I have tested the temperature sending wire and the gauge and both appear to work fine. I ground the temperature sending wire on the alternator and the gauge will jump up to hot but fairly slowly. And once I take it off of the ground it falls slowly back down to cold and then goes below cold and sometimes points below the dash. When I have it connected to the temperature sending unit it just sits below or at cold. I’m not real sure what this could be. Any advice is appreciated.