On my '53 F100 239 Flathead, the temp gauge is at Hot when the ignition is off, even with the battery disconnected. With ignition on, the gauge moves to Cold but is erratic when running. If I disconnect the lead from the gauge to the sending unit and ground the lead, the gauge goes to the left of Cold. If I ground each terminal of the two-terminal sending unit separately, no continuity. If I jump across the two terminals of the two-terminal unit, there is continuity. On the sending unit in the other head, if I ground the center lug I have continuity. Any idea whether it is the sending unit or the gauge, and which sending unit? Why would the gauge go to Hot even with the battery disconnected?

 

Sounds like it's working normally.

At least on 48-52's the gauge goes to H with the power off by default.
One sender is just an on/off switch when it reaches a pre-determined temp. The other sender is variable and sends the current the gauge shows.

 

Then the erratic behavior when running is more likely one of the thermostats than the sending units. I'll go there next. Thanks for your help.