Hmmm, well you have to have the proper temp sending unit first of all. That's very important because not every sender will work with every gauge. If you already know what I'm about to say next please ignore it. It is easier for me to explain it all than just bits of it.
On the back of the gauge, one post will be 12v (positive if you have a negative ground system or negative if you have a positive ground system). That voltage will be present when you turn on the ignition. And that voltage will only understand one thing...it wants to flow straight to the ground connection as fast as it can.
Your sending unit (the part that screws into your block or your head or where your radiator hose mounts...could be any of these) is a temperature sensitive variable resistor. The one below was actually removed from a 1956 235 engine. When it is cold, resistance will be high (could be something like 6k ohms). The hotter it gets, the more the resistance will drop until at some point there will be very little resistance. And it is sensitive...if you hold it in your hand while your ohmmeter is connected to it, you can actually watch it working. Think back to the voltage wanting to get to the ground connection. Your sending unit is resisting that voltage in varying degrees...like linemen on a football team. The hotter the weather gets, the more of them say "forget this nonsense" and head off to the bench and the easier it is for the other team to advance the ball. The opposing team being voltage and the goal line being ground.
So the best way to measure your sending unit...to see if it is working...is to take a resistance reading from the very tip (where the wire connects) to a known good ground...maybe an exposed part of the frame, engine, generator case...something like that. When the engine is cold, expect a high resistance and when it is hot expect that resistance to drop considerably. Hope this is helpful. Good luck!
~ Jon 1952 1/2 ton with 1959 235 | T5 with 3.07 rear end
