............1986 F150 6cyl 4sp manual 2wd, no AC, no cruise, no pretty.
Started up the truck last month and had no heat and temp gauge did not move from cold. I changed out the thermostat and flushed the heater core. Heat works fine now but still no movement on temp gauge. Figure it's got to be the gauge or a sensor. What is the safest way to test the sensor? I've read that the gauge can be damaged if not careful.
First off - location of the sensor. Is this it on the thermostat housing? Is there another one or just this one?

 

I have the same truck and same options as yours. I believe that is it. I am not sure if the early Fords computers used the coolant temp for anything as it is not fuel injected nor does it have E-fans. With the truck off and cold, remove that pigtail and put an ohmmeter on the leads. Shouldn't matter which side is positive. Check value. The number should go up or down as the coolant gets warmer.

 

No, that's the ECT, for the EEC only. The sender for the gauge is at the RHR of the block, nearly impossible to see or reach from above or below. Click this & read the caption:

(phone app link)

 

Just getting back to this. Thanks for the input Steve83; I will start looking for the sender now. Looks like it will be impossible to see.