How to Check a Fuel Sending Unitby Dan Ferrell
There was a time, back in the earliest days of the automobile, when a fuel gauge was something of an academic consideration. First, because given the technology of the time, you were doing well if your car ran long enough between breakdowns to warrant checking the fuel level. And secondly, because fuel tanks and the attached gauges were most often located under seats -- because, safety. Now, most vehicles rely on a float in the tank which, attached to a kind of variable-resistance potentiometer, tells the computer how close you are to walking home.
Loosen the fuel filler cap. Relieve the fuel system pressure on your vehicle. The simplest way is to find the fuel pump relay in the box under your hood, and then pull it up and out. Start the vehicle, and allow the engine to run until it dies. Shut the ignition off, and reinstall the relay.
Check for proper ground on the fuel tank. Clip a 12-volt test light to a power source. If you cannot access one near the fuel tank, use a long jumper wire to the positive terminal of the battery. Then touch the fuel tank with the test light pick. If the light glows with a bright light, you have a good ground; otherwise, your broken ground connection to the tank might be the cause for the fuel sending unit malfunction. Repair as necessary.
Check for incoming voltage to the sending unit. Clip the test light to a good ground on the vehicle. Any unpainted, metal part of the car's under-body will do. Then back-probe the sending-unit power wire. If the light glows with a bright light, the unit is receiving power; otherwise, the power source is disconnected. Check the circuit and make the necessary repairs.
Disconnect the ground battery cable using a wrench. Gain access to the fuel-sending unit on the fuel tank and disconnect hoses and electrical connectors from the sending unit assembly. Depending on your particular vehicle model, you may need to lower the fuel tank or access the unit through an access door on the floor under the rear seat of the car. Check your owner's or service manual.
Detach the fuel-sending unit from the fuel tank. On some models, you may unscrew the fuel sending assembly from the tank using a Phillips screwdriver; others require a tank-unit cam tool or spanner wrench to unscrew the locking ring. Lift the fuel-sending unit off the tank and out of the vehicle.
Set your ohmmeter to the appropriate range to read the lowest and highest resistance on the sending unit. Check your service manual to obtain the resistance range on your particular unit. Hook the ohmmeter leads to the sending unit positive and negative wires. You do not have to worry about polarity for this test.
Turn on the ohmmeter and slide the sending unit's float arm from the lowest to the highest position as you check your meter reading. The resistance reading on the meter should go down smoothly as the float travels up to its highest point. Move the float down, and the reading should go up smoothly on the meter. If you receive erratic readings or readings not within the manufacturer specifications, replace the sending unit.
Place the sending unit float next to your ear and shake it. If you can hear liquid inside the float, it's been punctured and you need to replace either the float or the entire sending unit if the float isn't replaceable.
- If you need help locating or identifying components on your particular vehicle model, refer to your car service manual. You can buy one at most auto parts stores or check one at your local public library.