Three wires go into the tank. One is the ground, one is the fuel pump power and the last one (purple) is the fuel level. It connects to one end of the fuel level sender with the other end connecting to the ground. The fuel level sender is a small "card" variable resistor that has the a float with a "brush" that touches the variable resistor at a point dependent on the level of fuel. The purple wire is connected to one end of the resistor, the ground wire connects to the fuel pump and the brush that is moved by the float.