I was working on my 1989 Chevrolet C2500 truck. The temperature gauge was not working and so I figured the temperature sensor needed to be replaced, Since the gauge worked when the gauges all ran through their test when you start the truck. I also notice a small leak where the thermostat is. Today I replaced the thermostat (even though it was fine), replaced the sensor and tested the truck. WTF it still does not work. I also notice I still had a tiny leak and re-tightened the bolts on the thermostat, that's when I noticed a wire that had a connector dangling near something that look kind of like the sending unit I replaced but with a different connector.
OH COME ON REALLY!
was a second temperature sending unit down by the exhaust, the connector
is just clipped on, and had come off. I reconnected it, zip tied it in
place, and.... now the temperature gauge works! DOH!