A vehicle fuel level indicator uses two components: a sending unit in the fuel tank, which measures the level of liquid through electrical resistance, and a fuel gauge that displays that level on the vehicle dashboard. Some newer vehicles use microprocessors to read and report fuel levels, typically providing a more accurate reading.
The sending unit in the fuel tank is made up of a float connected to a thin, metal rod. The other end of the rod is connected to a variable resistor, with a strip of resistive material grounded at one end, and a wiper connected by an electrical wire to the fuel gauge. The gauge sends current to the resistor. If the wiper is close to the grounded end of the strip, there is less resistance in the circuit, so a high current flows. If the wiper is farther from the grounded end of the strip, there is higher resistance in the circuit, so a lower current flows.
The current from the sender passes through a heating coil wrapped around a bimetallic strip. This strip is linked to the fuel gauge needle. As less current passes through the heating coil, the bimetallic strip cools and straightens out, pulling the gauge needle from a full reading to empty.
Newer cars use a microprocessor to read the variable resistor in the fuel tank, which communicates that reading to another microprocessor in the dashboard. Manufacturers can program these units to compensate for oddly shaped fuel tanks, giving a more accurate level reading.