How to Test Fuel Sending Units

The fuel-sending unit in your vehicle is mounted inside the fuel tank. These units come equipped with a variable resistor connected to a float through a long wire. The float swings up and down depending on the amount of fuel inside your fuel tank, allowing the variable resistor to increase or decrease electrical current from the sending unit to the fuel gauge on your instrument panel.

Things You'll Need

• Hand-siphon pump
• Approved fuel container
• Wrench
• Floor jack
• 2 jack stands
• 2 wooden blocks
• Ratchet
• Long ratchet extension
• Socket
• Drift punch
• Hammer
• Digital ohmmeter
• 2 alligator clips
• New O-ring seal

Instructions

1. Remove the fuel pump fuse or relay from the vehicle and start the engine. Let it idle until it stalls. Crank the engine for about three seconds to finish relieving the fuel pressure from the system.

2. Siphon the fuel from the fuel tank through the fuel filler tube into an approved container using a hand pump. Bring the level down to 1/4 of a tank or less, if possible.

3. Disconnect the ground (black) battery cable with a wrench.

4. Raise the car rear wheels off the ground using a floor jack and support the vehicle with a couple of jack stands on each side. Block the front wheels with a couple of wooden blocks.

5. Support the fuel tank with the floor jack and unfasten the tank straps using a ratchet, long ratchet extension and socket.

6. Lower the fuel tank a few inches just enough to gain access to the fuel lines and electrical connector on top of the fuel pump and sending unit assembly. Inspect the electrical wires for a loose connection and damage that might prevent the sending unit from operating properly.

7. Unplug the electrical connector and disconnect the fuel hoses form the top of the fuel pump and sending unit assembly.

8. Rotate counterclockwise the lock ring on the fuel pump and sending unit assembly using a drift punch and a hammer. When the lock ring is released, remove the lock ring. Carefully lift the fuel pump and sending unit assembly and O-ring seal underneath out of the fuel tank.

9. Follow the sending unit electrical wires to the connector on top of the assembly. The sending unit is the component with the plastic float hooked to a long wire arm.

10. Connect the probes on a digital ohmmeter to the connectors of the two sending unit electrical wires using alligator clips on the ohmmeter probes.

11. Set your ohmmeter to 20,000 ohms and turn on the meter. Hold the fuel pump and sending unit assembly straight up and make a note of the resistance value on the meter's readout. Carefully swing the float arm up and hold it halfway between the lowest and highest point. Make a note of the resistance value on your meter readout. Finally, swing the arm all the way up and make a note the reading. Your first reading should have the highest resistance value, followed by the second and third. If the sending unit failed this test, replace it.

12. Swing the float arm up and down slowly. Make sure the resistance value changes smoothly as you move the float. If the resistance jumps, fluctuates erratically or goes to infinite, replace the sending unit.

13. Shake the float connected to the wire arm on the sending unit. If you can hear liquid splashing inside the float, replace the float or sending unit.

14. Replace the O-ring seal when reinstalling or replacing the fuel-sending unit.

Tips & Warnings

• Sending unit resistance values vary from one manufacturer to another. If necessary, consult the service manual for your vehicle make and model.

• On some vehicle models, the fuel pump and sending unit assembly is accessible by removing a fastened cover located under the rear seat cushion or the trunk. Check your vehicle service manual if necessary.

• You can find a vehicle service manual for your car make and model in the public library or buy one from your local auto parts store.

References

• "Modern Automotive Technology"; James E. Duffy; 2003
• "Auto Mechanics Fundamentals"; Martin W. Stockel and Martin T. Stockel; 1982