1940 Ford temp sending unit troubleshooting
 

The problem I'm having is that the temp gauge's behavior is very erratic, for example; at times the needle seems to indicate temperature correctly by moving from the HOT side of the gauge when the ignition switch is in the OFF position and advancing slowly to the COLD side of the gauge when the ignition is switched ON and proceeding to register temperature accordingly as the engine warms up. At other times, when I turn the ignition switch to ON, the needle advances RAPIDLY to the COLD side of the gauge and remains there even after the engine warms up. I don't understand what is causing this. Any ideas?

Re: 1940 Ford temp sending unit troubleshooting
 

Check to make sure all connections are tight and that there are no internal breaks in the wires. Plus, your temperature senders may have internal corrosion. Peruse this info:
https://www.fordbarn.com/forum/searc...rchid=27314425
and
https://www.fordbarn.com/forum/searc...rchid=27314431
and look here:
https://www.google.com/search?q=repa...hrome&ie=UTF-8
and
https://www.fordbarn.com/forum/searc...rchid=27314521
and
https://www.fordbarn.com/forum/showthread.php?t=311533

Do you have any spare temp. sender units you could install to see if they correct
the situation?

Re: 1940 Ford temp sending unit troubleshooting
 

You should be able to read some kind of small resistance on the sensor with an ohmmeter. There is a heater coil inside the sensor which goes through a set of points which are controlled by a bi-metalic strip of metal. My photo is too big for this web site.

Re: 1940 Ford temp sending unit troubleshooting
 

You should be able to read some kind of small resistance on the sensor with an ohmmeter. There is a heater coil inside the sensor which goes through a set of points which are controlled by a bi-metalic strip of metal.