1986 Temperature guage sits on "cold"

Had this truck a year, noticed the temp guage showing colder and colder. Normally ran about 1/4 way up, but now, at operating temp it stays at cold, as if not reading the engine temp at all.

I replaced the thermostat, drained and refilled the anti-freeze... Throw me a clue anybody?

Temperature sender??

Either your temerature sender units stuffed or you have a cable join of increasing resisitance. Check to see if theres a 0Ohm path from the unit to where it hits the loom. Sometimes theres a plug in join around the engine bay that may be the culprit. To check the sender remove and use an Ohm meter and place the leads on the probe and one on the terminal at the top. Increase temperature with either hot water or hair drier or better still hot air gun. The resisitance should decrease as it gets hotter. (If Im right that is, but Im pretty sure thats the right way round)

I wouldnt imagine theres much else unless you have a blockage somewhere. Temp sender units are usually close to the head not on the heater circuit. If its on the heater circuit it may be a blocked hose but unlikely. Id be picking the sender unit.

Or maybe it is your temp gauge, Thanks Jamie and good luck