It's one of two options: For example, 0-190 ohm (empty 0 ohm, full 190 ohm), or 240-33 ohm (empty 240 ohm, full 33 ohm). Most sending units are faulty out of the box, but there are only two different versions in general. So remove the wires at the unit, take it out and cycle it up and down to test it. The gauge simple reads full if you ground it as far as testing that goes.