The temperature sensors are negative coefficient (NTC) style units. As the temperature rises, the resistance falls. You should be able to check your sensor resistance with an ohm meter to see if all the units you have are reading alike. If so, you may have the incorrect one. I have a 261 in my 1949 GMC with the electric sensor in the thermostat housing and it never has read correctly. Always reads low and never gets to 180. I tried resistors inline rated from 1 ohm to 10K ohm and nothing worked. The gauge would either read low or peg high. So, until someone comes up with a correct unit that'll read, I'll just watch for steam.
