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HomeTips & GuidesHow To Wire a Fuel Sending Unit: 6 Steps To Follow

How To Wire a Fuel Sending Unit: 6 Steps To Follow

Ever wondered how the fuel gauge works? We’ll look at how to wire a fuel sending unit to rectify the fuel level reading process and send the correct signal to your stock or aftermarket gauge. 

A fuel gauge might seem like a tiny and insignificant car dashboard accessory, but you’ll be flying blind if it fails to work. 

In such a case, the gauge, fuel-sending unit, or wiring between the two might be faulty. If the culprit is the wiring or fuel-sending unit, follow these steps to wire the system.

Table of Contents

What Is a Fuel Sending Unit

A fuel level sender is a device that sends fuel level data to the fuel gauge on the dashboard. 

Old fuel-sending units relied on magnets mounted on a tube float to get the readings.

 But new units use variable resistors to send a varying voltage signal depending on the float level in the gas tank.

How To Wire a Fuel-Sending Unit

Both old (magnet-based) and new (variable-resistor-based) fuel-sending units have the same wiring setup, so the wiring steps below are universal.

A fuel-sending unit with a rubber boot around the top round sanction that attaches to the tank wall or roof

A fuel-sending unit with a rubber boot around the top round sanction that attaches to the tank wall or roof

Step 1: Locate the Fuel Sending Unit

Most vehicle fuel tanks have the fuel-sending unit mounted on the top part to allow the float to stay above the fluid. 

And almost all have a circular top section. Some have fuel pipes connected to them and electrical connections to the fuel gauge.

Locate this unit. You might have to remove some trim inside the boot to access the device.

Step 2: Test the Fuel Gauge

You need another person to check the fuel gauge when conducting this test. Turn on the ignition switch, then disconnect the fuel gauge signal wire from the sender.

Attach or scratch this wire’s terminal to the gas tank’s body or car chassis, then check the gauge.

 If it indicates a full tank, it is not faulty. The issue is most likely in the sender.

But it can also be the fuel sender unit contacts or the tank lacking proper earthing. So clean the contacts and ensure the tank has a solid ground connection.

A fuel tank gauge sensor

A fuel tank gauge sensor

Reconnect the sender unit and test the system again. You’re good to go if the gauge shows the accurate fuel level.

 Otherwise, you’ll have to remove the sending unit for inspection.

However, if the fuel gauge does not move in any of the tests, the signal-sending wire to the gauge might have a discontinuity. 

Also, you might have a faulty sending unit or fuel gauge.

Let’s focus on the scenarios with faulty wiring or a damaged fuel-sending unit.

Step 3: Repairing Faulty Wiring

Look for three wires, with two being long enough to reach the fuel gauge and ignition switch to bypass the existing wiring. 

Strip about an inch from the ends, then connect one wire to the gauge terminal on the sending unit.

Loosen the screw on this terminal, twist the wire, then loop it around the screw. Tighten it to hold the wire. Connect the other end to the fuel gauge.

Next, connect the short wire to the sending unit’s negative or ground terminal, then link it to the nearest ground connection.

This unit also requires power, so connect the other long cable (ignition wire) to the positive or ignition terminal on the fuel-sending unit. The other end should go to the ignition switch.

Once connected, turn the ignition switch and check if the gauge works. If it does not work, proceed to step 4.

A fuel-sending unit (note the wiring)

A fuel-sending unit (note the wiring)

Step 4: Remove the Faulty Fuel Sending Unit

First, disconnect the negative battery terminal to prevent accidental sparks when handling the open fuel tank. 

You’ll need a socket/crescent wrench or vise pliers to loosen the battery terminal lug bolt. You can also disconnect the positive terminal for maximum safety.

After that, disconnect the wires on the fuel-sending unit, then block the fuel pipe using a plug. Remember to mark these pipes if more than one for correct reassembly.

Next, remove the unit from the tank by undoing the screws, studs, or nuts attaching it around the edge.

A fuel-sending unit (note the wiring terminals, round top, and fuel pipes)

A fuel-sending unit (note the wiring terminals, round top, and fuel pipes)

Most units have bayonet fittings held to the seat by an outer locking ring with lugs. Undo this ring by turning it anticlockwise using a C-spanner. 

Remove the sender carefully while watching the arm that holds the float. 

Step 5: Test the Old Fuel Sending Unit

This step is not mandatory, but you can test the fuel-sending unit using a multimeter to confirm if it is faulty.

 These units usually have their resistance ratings marked on the label when the tank is full and empty. 

So the multimeter readings should be within this range when lifting or lowering the float.

But you’ll need a 12V battery to power the device. Do the tests far from the open fuel tank to avoid exposing the fuel vapor to sparks (if any). 

A functional unit should increase the resistance to the upper limit as you lift the arm.

Remember, these units have potentiometers, which increase the output resistance the more you fill the tank (raise the float). 

But since the unit is faulty, the reading won’t change on the multimeter.

Step 6: Install the New Fuel Sending Unit

Remember the steps followed to disassemble the faulty fuel-sending unit? Do them in reverse to install the new device.

 There’s usually a tag in the sender hole to enable you to fit the float sensor the correct way.

A fuel tank sensor

A fuel tank sensor

Turn the sealing ring clockwise using a C-spanner, then tighten the nuts, studs, or screws in sequence to spread the pressure evenly on the surface.

After that, reconnect the fuel pipes, gauge, ignition (positive), and ground wires.

Reattach the battery terminals, then turn the ignition to test the new system. You should get an accurate fuel gauge reading.

A car fuel gauge indicating almost empty on the dashboard

A car fuel gauge indicating almost empty on the dashboard

Safety Precautions

Before removing the current fuel-sending unit, you must undo the battery lugs to cut power to the system. 

Even though the ignition switch cuts power to the device, disconnect at least the negative terminal to prevent shocks or sparks due to the high potential difference.

In some cars, the fuel-sending unit is on the tank wall. So removing it will leave a hole for the fuel to flow out if it is above that level. 

In such a case, check the fuel level from the filler hole using a dipstick

And if this check fails or indicates the fuel level is above the fuel-sending unit hole, drain the tank before removing the device.

Wrap Up

Fuel-sending units work behind the scenes that make gas gauges work. 

Their removal/installation is more complex than electrical wiring, so replacing the devices requires more work than fixing a wiring issue

Therefore, you should test the wiring first. Dismantling the sending unit should be the last resort. 

That’s it for this article. 

Share your thoughts in the comments below to keep the conversation going.

Hi I am Christa, sales manager of Cloom.

I have extensive expertise and experience in wiring harnesses and I believe I can help you.

And we have a very professional technical team who can clearly understand the customer’s needs and give professional suggestions and solutions after receiving the drawings.

If you also have wiring harness needs, please send me the drawing so that we can give you our quote and start our business.

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