your sending unit works off of ground, not power. the gauge is basically a volt gauge calibrated for fuel level, You should have 12V at teh positive terminal on the back of the gauge, then the s wire(usually pink) goes to the sender, Teh sender needs to be grounded, so if you have a poly tank, make sure you run a ground wire to the tank, if you have an aluminum tank, make sure the tank is grounded, if you are unsure about how well its grounded, run a dround wire to one of the screws holding the sender in place. With teh key on, the guage should read approx whats in the tank. Pull the sender, leave the wires hooked to it(including the ground) raise and lower the sender, the gauge should move with the sender. If it doesn't, ground out the pink wire, the gauge should peg beyond full, if it doesn't, you're gauge is bad. remeebr to double check you test leads when checking the system, a weak ground on your lead could scew your results