[Dave at Pelican Parts]

The sensor for the gauge is an NTC thermistor--a "thingie" that has more resistance at low temperature than at high temperature.

To test the gauge itself, get a few resistors or a potentiometer. Probably a 3,000 ohm pot will work well. Or a 10, a100, a 1000, and a 3000 ohm resistor. A potentiometer (pot) is effectively a resistor whose resistance you change by turning a knob.

Hook one end of the resistor to ground, and the other to the "sender" connection on the gauge. Supply power to the power connection on the gauge and ground to the ground connection (if present). Note the needle reading. Swap out another resistor--or turn the knob on the pot. You should see the needle move a different amount as you go from 10 to 3000 ohms of resistance. If not, the gauge is dead or you aren't supplying +12V and ground to it.

--DD