Fuel tank sending units are tubelike devices that send a signal to your vehicle's fuel gauge telling it what to display. Some older senders use mechanical floats to read the fuel level, either along a track or on a cantilever arm, and some models use a sender that reads electrical resistance of the remaining fuel. Sending units also come with two different wiring configurations, two-wire and three-wire, which must match up to the number of posts on the back of the fuel gauge for it to function correctly.
Disconnect the negative battery cable so that there will be no accidental spark when making electrical connections. This is especially critical when working with a fuel system.
Open the access panel above the sending unit. If your vehicle does not have an access panel, you will need to remove the brackets that secure the fuel tank and disconnect the filler neck, then pull the fuel tank out enough so you can reach the sending unit on top of the tank.
Determine how many posts your sending unit has. The number of posts on the sender should match the number of posts on the back of the fuel gauge, either two or three. Cut the appropriate number of wires, two should be long enough to reach the back of the fuel gauge if it is a three-wire sender, and the third long enough to reach the nearest chassis ground connection. If it is a two-wire sender, only one needs to reach the gauge.
Strip 1/2-inch of insulation from the ends of each wire, and twist the copper strands. Slide the ends of each wire into a ring connector, and connect two wires to the positive and ground posts of the sender, securing each of them with a washer and locking nut.
Slide the ring connector of the wire connected to the positive post of the sender to the post labelled "sndr" on the back of the fuel gauge. Secure it in place with a washer and locking nut.
Slide the ring connector of the negative wire around the connecting post on the nearest chassis ground connection. This will be a part of the metal frame that has not been painted. Secure the connector with a washer and locking nut.
Reconnect the negative battery cable and turn on the ignition to test your sending unit and fuel gauge.
Different manufacturers recommend different coloured wires for the ground and fuel gauge connections. Try to match these recommendations for your application. Use AWG 16-gauge or heavier wiring.