Inside the vehicle's fuel tank is a plastic component known as a float. It is attached to the side of the tank using a tiny metal arm set up with a hinge, which allows the plastic "float" part to remain on top of the fuel. When the vehicle has a full tank, the float component is forced toward the top. The hinged arm allows for the up and down movement. Depending on what position the arm is in, it registers a different voltage level to the wire that connects to the fuel gauge itself, located in the passenger compartment's dashboard.
The electrical signals are carried to the dashboard from the sending unit using sheathed wiring. These wires are routed out of site, under the body of the vehicle. One end of the wire is attached to the sending unit, and the other end to the fuel gauge in the dashboard. Like all electrical systems, the fuel sending unit is grounded. The ground will be located close to the unit itself, because components are better protected when grounded nearby.
The gauge that tells us how full or empty the tank is will be located in the dashboard, in front of the steering wheel. Depending on the voltage sent through the wiring from the sending unit, the gauge needle will reside at a certain place in the spectrum. More voltage means the needle will lean toward the "F" (for "Full"), less means that the needle will make its way toward the "E" (for "Empty"). Digital gauges work in much the same manner, only instead of a needle they employ a series of lights to denote the amount of fuel in the tank.
If you plan to work on the fuel sending unit yourself, be very careful when messing with the wiring. Always disconnect the battery before working on any electrical system on the vehicle. Always wear safety glasses and do not smoke near the fuel tank. If you have any doubt about your mechanical talent, it is best to leave fuel tank work to a professional.
