![Why is my fuel gauge reading inaccurately and how to fix it? [The Complete Guide]](https://www.autolii.com/cms/storage/uploads/00000000100.png)
- Fuel gauge
- Gas gauge
- Inaccurate reading
- How does it work
- What to do
- How to fix
- Diagnose
- Test
- Vehicle
A fuel gauge is a visual indicator of how much gas or diesel you have in your car.
It helps you determine how much distance you can drive before your next trip to the gas station.
While its function seems so basic, it is one of the most vital pieces of equipment that we cannot drive without.
Sure, you could count the miles with each fill-up, but you cannot always roll the dice.
TIP: It is important to keep your fuel above a quarter of the tank to avoid overheating, high pressure, and increased wear of the fuel pump.
Running low on fuel can cause the pump to pick up more sediments that are resting at the bottom of the tank, which would lead to clogging of fuel filters and injectors.
So if your fuel gauge is no longer working properly, it is important to know if it’s something you can fix or not. Otherwise, you might end up with a more expensive repair.
In this blog, we will answer the following questions:
- How does it work?
- Why is my fuel gauge reading inaccurately?
- How to diagnose and fix the problems with my fuel gauge?
So, let’s jump right into it!
How does it work?
The fuel gauge system is composed of three main parts:
1. The Gauge
The fuel gauge is the visual representation of what’s happening between the fuel tank and the sending unit.
Simply put, it should measure the amount of fuel in the tank.
An analog gauge will move a needle up and down across a dial; and a digital gauge will display a set of bars. Both show how much fuel is left.
When the tank switches to its reserves, it will prompt a low-fuel warning.
You may also learn more about other warning lights from our previous blog.
Source: https://m.made-in-china.com/product/Car-Fuel-Tank-Guage-Sending-Unit-Fits-Hyundai-94460-1m500-921860055.html
2. The Sending Unit
Composed of a fuel pump, fuel strainer, fuel filter, and fuel float, the sending unit works as the “messenger” for the whole fuel tank and gauge assembly.
It senses the level of the fuel in the tank and sends the information via electrical signals to the gas gauge.
The wired system allows a change in resistance, when an input voltage, that is being modified by the fuel sender, is applied from the ignition circuit (Remember Ohm's Law In your physics class?).
This resistance increases when the fuel level drops, and decreases when the fuel level rises.
Consequently, the changes in resistance affect the output voltage that controls the fuel gauge.
The higher the resistance, the lower the output voltage; ergo, lower fuel gauge reading, and vice versa.
3. The Circuit
The circuit connects the fuel gauge, sending unit, battery and ground.
Most modern vehicles today have sending units that are grounded to the electrical system, while some of the older ones are grounded to the body or frame of the car.
Why is my fuel gauge reading inaccurately?
Simply put, an inaccurate fuel gauge reading is just a symptom of an issue/s with its components.
What kind of issues are we talking about here?
Let’s break it down into four main reasons:
1. Sending Unit Failure
This is the most common reason for the inaccurate readings of fuel gauges, which is oftentimes due to the friction between the sending unit and the variable resistor.
Friction happens whenever the car is moving; and the frequent rubbing of the components wears the equipment over time, and leads to an open or shorted circuit.
One of the first signs of a sending unit failure is when it behaves erratically.
An example of this is when the fuel gauge registers at three quarters, but changes into half full within a couple minutes, or vice versa.
Another typical sign of a faulty sending unit is a fuel gauge that is stuck on empty.
It usually happens when the float becomes separated or somehow breaks from the arm – think of it like a toilet tank:
The float, which is a composite foam connected to a thin metal arm, also connects to the variable resistor.
If the float is disconnected, there will be no output voltage that would control the gauge and the reading will be stuck on empty.
On the other hand, a fuel gauge stuck in full could be due to a bad resistor.
A defective resistor will generate a higher output voltage, thus triggering the gauge to permanently read full.
This poses a problem since drivers must know the correct fuel level of the car or they might run out of gas.
2. Circuit issues
The circuit provides the power to operate the electrical components of the vehicle, including the sending unit and the fuel gauge.
The wires are well-protected but can still break if the vehicle runs over debris, humps or potholes.
If there is a problem with the circuit, such as an interrupted ground, a loose connection or corrosion, the fuel sending unit may fail to meet the required input voltage that will send an accurate reading to the fuel gauge.
3. Bad fuse
Aside from the electrical connections, a fuse is also important in providing the necessary input voltage to run the fuel gauge.
In older vehicles, it can be found under the steering column or on the driver’s side. In some cases, it is located under the hood.
For new cars, the fuse is already integrated with the instrument cluster.
4. Instrument cluster malfunction
Instrument cluster is the industry’s term for dashboard indicators, which basically include your fuel gauge.
A non-functioning instrument cluster is the least common reason for inaccurate fuel gauge, but it can be the most expensive to repair.
This is because modern vehicles usually have fully-integrated instrument clusters and may not even come with replaceable bulbs.
This means that if one indicator has an issue, the entire unit must be replaced.
A proper diagnosis can help determine if this is the issue.
How to diagnose and fix the problems with my fuel gauge?
Safety Considerations:
Expect that you will be dealing with gasoline, inspecting underneath the hood, and testing electrical wires.
Make sure to prepare your personal protective equipment and proper tools:
- Mechanic gloves
- Safety glasses
- Digital Multimeter
- Screwdriver set, if necessary
- Socket set, if necessary
- Trim removal tools, if necessary
Once all your tools and equipment are ready, read through the following steps:
- Run an instrument cluster self-test
- Check the fuse
- Inspect the wiring
- Perform a fuel sender test
- Perform a fuel gauge test
1. Run an instrument cluster self-test
Many modern vehicles come with an instrument cluster self test feature that can test computer-controlled instrument clusters.
The procedure involves doing a series of tasks including turning on and off the?headlights?a certain number of times and pressing the odometer button.
You can find the procedure in your owner’s manual or online.
2. Check the fuse
Step 1:
Turn the ignition on and off a few times, and observe if the needle on the fuel gauge moves.
In some cases, the fuse will run multiple gauges, so you might notice something else isn’t reading correctly at the same time.
Result:
If the needle does not move, it most likely means you have a blown fuse.
If you find a blown fuse, replace it and test the gauge again.
However, if the fuse continues to fail, you might have a larger electrical system problem on your hands.
3. Inspect the wiring
Step 1:
The best place to start is at the gas tank itself.
On most vehicles, the gas tank will have a wiring harness that handles the power for the fuel pump and sending unit.
Older vehicles, however, have no electric pump and only have a single wire for the sending unit, and a ground wire attached to one of the screws of the sending unit to ground out the frame nearby.
Step 2:
It is important that you check the ground connections first.
Ground failure is a common issue due to rain, snow and salt that are being splashed at the back of the vehicle.
Step 3:
Thoroughly inspect the wiring, clean, and re-attach. Make sure you reattach it to the appropriate location.
Step 4:
If you find a broken wire, you need to replace it.
4. Perform a fuel sender test
Device:
Digital Multimeter (DMM): set your multimeter to ohms.
Reminder:
This test must be done when the fuel level is below half the tank to prevent fuel splashing.
Step 1:
Make sure that the plug is clean, dry and rust-free. The pins must be straight and the connector should be fully seated.
Step 2:
Take off the pump module so you are able to manipulate the float arm.
Step 3:
Back-probe the connector and check for voltage with the key in the ON position (but do not start the engine).
Step 4:
Touch off one probe to the sending terminal, and the other probe to ground. Slowly move the float up and down, and you should see the resistance change on the multimeter. After you’ve received your resistance results, compare them to the standard for your vehicle.
Result:
You’ll need to know what the standard resistance is for the make and model of your vehicle.
Most older sending units are “0-30” or “0-60” ohms. This means that when the tank is empty, the resistance will read 0 ohms, and it will read 30 or 60 ohms when full.
You can find this in the repair manual for your car.
On newer sending units that incorporate the fuel pump into the assembly, this is more difficult, but oftentimes, test procedures can be found online.
If you see no change in resistance, or if it is just a few ohms off you’ll know that it is your sending unit that is causing you issues.
In some cases, corrosion is keeping the float of the sending unit from doing its job properly, and a thorough scrubbing from a fuel system cleaner will get it back to proper working conditions.
If this doesn’t work you can remove and replace this part yourself or have your car sent out for repairs at the mechanic.
5. Perform a fuel gauge test
Device:
Digital Multimeter (DMM): set your multimeter to ohms.
Steps:
You can also test the fuel gauge by checking the resistance.
On newer vehicles, the gauge cluster and fuel gauge are much more difficult to test.
You have to remove the instrument cluster from the dash and you may have to disassemble the cluster to remove the gauge.
It’s always a good idea to check with a professional when diagnosing complicated systems in order to avoid expensive mistakes.
Results:
The result must be exactly the same as when you conducted a test at the sending unit.
While gauges themselves can fail, generally the problems are found in wiring or the sending unit, rather than the gauge, so it’s best to rule out all the other possibilities before tackling the gauge itself.
