Brian is correct, the resistance of a sender lowers as the temperature goes higher. The two common sender resistances are 43-51 ohms and 11.5-13.5 ohms (NOT VOLTS). Both are measured at 220 Degrees F.

To test your gauge, you can go to Radio Shack and buy a resistor that is about 50 ohms. Put the resistor between the gauge and the 12 volt supply and connect the negative side of the gauge to ground. You should see a deflection.

A temperature gauge is really a volt meter, measuring the voltage across the resistor. As the sender is cold, the resistance is high and the voltage is low. As the sender warms, the resistance lowers and the voltage increases