91 Ranger 2.3 coolent temp sensor not working right

Dosconnect the wire at the sending unit. Use a jumper wire, connect the wire to a clean ground. Turn the ignition on without starting the engine. If the gauge reads hot the sending unit is bad. If the gauge doesn't work, the circuit may be open, may even be a fuse, or the gauge itself is bad.

Sorry, I was looking at an older wiring diagram, one wire going to the sensor. I believe there should be two wires going to the sensor, a light green/red and a gray/red. Turn the key on, engine off, disconnect the wires, look at gauge. It may read all the way hot or cold. Jumper the two wires in the plug together and check the gauge, it should have swung one way or the other. Ground one wire in the plug, check gauge, and do the same to the other wire. You should see movement of the gauge. When jumpering wires together and to ground do it briefly. If gauge doesn't respond it's bad. Also, do you think the thermostat is sticking. If it's stuck open, that would make the temperature gauge read low all the time.