The level sensor in a vehicle’s fuel tank is actually a combination of three components; a float, an actuating rod and a resistor. This combination of components sends a variable signal to the fuel gauge or an electronic device -- a “little black box” -- that actuates the fuel gauge. The sensor assembly is often referred to as a sender. It is a relatively simple system once the function of each component part understood.
The float can be visualized by thinking of the ballcock in a lavatory cistern. The buoyant float -- a sealed composite or metal ellipsoid, or a foam solid -- is typically oval rather than circular and rests on the surface of the fuel. It is attached to a pivoted actuating rod.
As the level of the gasoline or diesel in the tank changes, the float moves up and down with the fuel’s surface. It is attached to a thin metal actuating rod, one end of which moves with it. The rod is pivoted at some point along its length, then the opposite end is attached to a grounded variable resistor.
12-volt power is supplied to one end of the resistor from the vehicle’s battery. A wire from the resistor runs to the fuel gauge. In some vehicles, the wire runs directly to the gauge, and in others it runs to a stepper or an electronic device that interprets the signal and actuates a mechanical gauge or a digital readout.
How It Works
Inside the resistor, a device that resembles a tiny windshield wiper is moved over a strip of resistive material by the movement of the actuating rod. The farther along that strip from the grounded end of the resistant strip the wiper is, the less electricity is conducted to it by that material. The wiper is oriented so the most resistance is encountered when the tank is at its emptiest, and the least when the tank is full.
The maximum signal -- the unmodified 12-volt current -- makes the needle in the fuel gauge swing over to “full.” As the fuel level decreases the float drops, the actuator rod causes the wiper to move across the resistant strip away from the ground, and less current is passed to the gauge. The needle shows a decreasing reading. When the tank is empty, the float is at its lowest and the wiper is at the far end of the resistant strip from the ground, so very little current is sent to the gauge. The needle doesn’t move far, thus reads “empty.”
Often a float will reach the fullest extent of its mechanical travel before the tank is entirely full or entirely empty. This explains why many cars have gauges that stay on “full” for a long time before starting to drop after a fill-up, and why some cars can run for many miles on what appears to be an empty tank.
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