Fuel level sensor

The fuel level sensor (fuel gauge sensor) on the Discovery died (symptom: fuel guage reads absolutely empty, yellow "low fuel" light on). The sensor works by varying resistance with the level of the float, low resistance at full tank and high resistance (something like 250 ohms) at empty tank. In the picture below, the arm extending to the lower left is the float arm, and attached to it you can see the wiper of the variable resistor. Note the small wire extending down across the front face of the wiper arm, which had rotted and broken so that the circuit was open. The open circuit made the fuel gauge read empty.

I fixed the sender, for now anyway, using a thin piece of copper wire I obtained by sacrificing a pair of headphones. Now, when the tank is absolutely full, the fuel gauge needle is about 1/2 a needle width above the full line.

Note that, when testing the sensor with the pump installed in the tank, the green/black and black wires lead to the sensor. However, since there are two black wires (ground), one for the sensor and one for the pump motor, and because these ground wires are not common, i.e., not tied together, within the pump, you need to check the correct black wire. On this pump, looking at the 4-pin multiplug socket from the top, with the wiring harness disconnected, the correct black wire is the one adjacent side-to-side, not top-to-bottom (where "top" is where the latch on the harness connector is, towards the front of the car).