I was driving my 1996 Ford Bronco 302 the other day and my temp started to read towards the hot side, and then came down, back up, then down. Thought my thermostat was sticking or I was over heating, but the temp to the computer was well in range from the other switch. On the one that goes to the temp gauge, I have a tool that fools the gauge that it is a sending unit, and the gauge stayed in the normal range as it should...gauge good! switch right? replaced with three different after market sending units, and same problem, and all of them had around 350 ohms resistance, and Ford tech manual said this needs to be 74ohms, but the motorcraft sender was out of stock at Ford.

 

Before I put it in it read 256 ohms, but put it in anyway. and did the same thing. Engine is not overheating and computer reads normal.

 

What in the world am I missing ? I'd appreciate any help ...

 

Are you pretty confident there is not trapped air in your cooling system? The description of how the gauge was behaving sounds pretty consistent with an air pocket at the gauge.

-Rod

 

You are missing the part about NORMAL OPERATION
A properly operating thermostat will cycle open and closed as the engine warms up
You WILL see that on the gauge by the temp going up and down
Once warmed up, you no longer see the fluctuation

 

I disagree. Small movements as the thermostat opens and closes is normal but the needle should not be swinging from hot to cold and back as described below….

Maybe I'm a little slow today but I’m not understanding what you’re talking about. How do you make the gauge think it’s a sending unit? How does the gauge not know it’s still a gauge?

I also lean towards a failing thermostat.