OK. 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. Got it yesterday, but 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 ?

 

Wow...not one response. I thought for sure a Ford tech would have an answer on this one LOL

 

Bump bump bump

 

Maybe you have air in your coolant system.

 

I have been watching this thread to see if someone smarter than I could answer your questions. It looks like Ford used the same temperature sender for many years. I also looked through various years' EVTM manuals to see that same value listed (74 to 9.7 ohms). From my research I believe those values are for normal engine operating temperatures, not room temperature to engine overheating.

It takes some reading in between the lines, but this thread has some real world values for a typical Ford temperature sending unit and where the temperature gauge was pointing: Ford Mustang Forums : Corral.net Mustang Forum - View Single Post - Randy, I need your help please, I think my local Ford Parts Dept is clueless.

Based on those results I would say the sender(s) are okay.

I know you stated you used a tool to simulate the sender, but what were the resistance values used throughout the gauge range?

 

Very good comments. Ford doesn't have an answer for this as these newer indicators seem to be generic for a variety of applications. They show exactly what you say, but when we test these senders, they have a higher ohm rating on them cold( around 174). What we are going to do is put a 100 ohm resister in-line...stay tuned

 

Is the resistance in the wire to the sender changing ?

 

 

I had this problem years ago with a ford car. The temp gauge would vary with the amount of electrical load, i.e. lights, A/C fan speed. I finally found the body ground was not good and there was a ground potential difference between the block and chassis. Once I cleaned the connections everything was OK.

 

Hmmmm That's interesting and makes sense...although the guage was still reading 174 ohms when it's suppose to be 74. I'm gonna still look at grounds.Thanks

 

Are you going to try adding the parallel resistor ?

 

Yes...seems like the best fix. I didn't find a bad ground