How Does a Fuel Sending Unit Work?by Don Bowman
The fuel-sending unit is a potentiometer that is adjusted by a float attached to a rod exactly like the float in a toilet bowl. When the fuel in the tank drops in level, the arm with the attached float correspondingly drops, which changes the amount of resistance in the potentiometer. It is a simple mechanism, although all the contacts must be very clean and tight since the whole system relies on resistance metering. A bad ground can cause a difference in resistance that the gauge would indicate.
The sending unit is located in the fuel tank, and in late-model cars it is usually attached to the fuel pump carrier bracket. The bracket contains both the fuel pump and the sending unit; however, each can be replaced separately. The wires to the sending unit are intermingled with the wires for the fuel pump in a common electrical connector at the rear of the tank.
How the Fuel-Sending Gauge Works
The gauge has four electrical points. It has one terminal for battery positive, a ground, an illumination and the signal, which is the wire to the sending unit. The ground on the gauge must be good for the gauge to work right. Whenever possible, the ground should be attached to the frame.
All present-day vehicles use a printed circuit board. To check the gauge, the individual circuit lines on the board must be probed. It is not difficult, but if a fault is found in the board, it should be replaced. Check from the gauge circuit to the connector on the board for continuity to check the board for operational readiness. From the board connector to the electrical connector on the tank to the sending unit can be checked for continuity. To eliminate the gauge in a failure, just ground the signal wire at the electrical connector at the tank while the key is on. The gauge should go to full. If it moves the gauge, then check the ground for the fuel pump. The power to the sending unit comes from the gauge through the signal wire and passes through the potentiometer and to ground. There will be 12 volts going in via the signal wire but only 4 volts coming out of the sending unit potentiometer going to ground, depending on how full the tank is.
Remember, when changing a sending unit, it must be compatible with the gauge in terms of ohms. If a factory replacement is called for, there should not be a problem, but for those that are replacing both units, this applies.