Fuel gauge reading empty when tank is full

Fuel gauge reading empty when tank is full DEFAULT


(Updated on July 29, 2021)

There are two kinds of people in the world: those who fill their vehicles up when they start to get a little low, and those who regularly see that “empty” warning light on their instrument cluster. Which are you?

It’s not good to let the fuel tank run below about a quarter tank too often, since then the fuel pump can overheat and the pump, filters, or injectors can clog because the pump picks up more sediment at the bottom of the tank. Getting stranded after running out of gas isn’t good for you or the car.

Drivers rely on an accurate fuel gauge to get where they need to go and take care of their cars. What do you do if the gas gauge is fluctuating, stuck on full, or always reads empty?

How Does a Fuel Gauge Work?

empty fuel gauge

It’s useful to understand the exact way vehicles tell drivers how much fuel is in the tank. The system consists of three parts: the gauge, sender (also called sending unit), and circuit.


The fuel gauge is the part on the dashboard that indicates how full the gas tank is from full to empty. It’s part of the instrument cluster and is connected to the sending unit either by a physical wire or by a wireless connection.


The sending unit is the physical assembly located in the fuel tank that senses the level of the fuel and sends the information via electrical signals to the gas gauge or car’s computer. It’s a part of the fuel pump module, which also includes the fuel pump, fuel strainer, and fuel filter.

The sender is made up of a buoyant float connected to a variable resistor via a wiper (a thin metal rod). The resistor is connected to an electrical ground at one end (either the top or bottom, depending on the vehicle).

The wiper slides up and down the resistor as the fluid level changes, along with the float level. When the wiper is closer to the electrical ground, the resistance is lower since there’s less resistant material for the current to travel through, and vice versa.

This voltage usually comes from the car battery (which releases a steady voltage) and the resulting voltage (that has been affected by the resistor) goes to the computer or gas gauge itself.


The circuit is the wiring that connects the car battery, sending unit, gauge, and ground (which is either the body or frame of the vehicle or the electrical system).

Causes of a Fuel Gauge Not Working Correctly

Since there are three parts that are needed to show the driver how much fuel remains, an inaccurate fuel gauge is always caused by one of them.

1) Failure of Sender

bad fuel pump

This is the most common problem since there is a lot of motion inside the unit. Parts can get disconnected, worn, or corroded.

The contacts between the wiper and resistor are always rubbing together. If they wear, a correct signal isn’t sent and the level reads as an unchanging full or empty.

If the float physically separates from the wiper, the level always reads as empty.

A faulty resistor can cause erratic readings on the gauge.

An old and sticky wiper gets stuck at certain levels on the resistor and can then get bumped back into place over time or on some rough road. This results in strange fluctuations of the fuel gauge.

2) Issues with Circuit

electrical problems

A problem with the wiring means that the signal can’t get from the battery to the gas gauge because of an issue along the way.

There can be a problem with the wires receiving a signal from the battery, with the ground from either the battery or the sender, or with the wires between the sender and the gauge itself.

The electrical system of vehicles is well-protected, but wires can still corrode or even break if something snags them (like if the vehicle runs over debris).

3) Failure of Gas Gauge or Instrument Cluster

dashboard warning lights

Depending on the vehicle, the gas gauge may be a lone part or it could be a piece of the instrument cluster. Regardless, the internal circuits can be problematic so that the signal is distorted or doesn’t come through at all.

Since electricity is involved, the gas gauge and/or instrument cluster have fuses that can blow. These are easy and cheap to replace, whereas the entire instrument cluster replacement is rather expensive.

How Do I Know if My Fuel Gauge is Accurate?

A functioning fuel gauge should go down at a steady level, depending on the distance and type of driving, after a fill-up. If the fuel gauge indicates the tank is full for more than 75 miles or reads erratically, there may be a problem.

Similarly, if the gauge drops to empty very soon after filling the tank, it should be checked out.

On cars about 20 years old and newer, there may be a self-test procedure for the instrument cluster that will check that the gauge can move from full to empty. This process is different for every vehicle and can be found in the owner’s manual.

The owner’s manual also states how many gallons/liters of fuel the tank holds in your vehicle, and might state the fuel efficiency. You can calculate approximately how far you should be able to go on a full tank by using those two numbers.

For better accuracy, it’s best to have an idea of what your vehicle’s fuel economy is by regularly tracking the number of miles between fill-ups and dividing that by the number of gallons of gasoline that was pumped in at the next fill-up. Tracking this can help you notice a number of problems in the car before they become a big issue.

Can You Calibrate a Fuel Gauge?

fuel gauge not working

Modern car manufacturers can use a computer in the car to change the movement of the fuel gauge a bit by comparing the position of the float to a calibration curve that compensates for the shape of the tank. Fuel tanks are often an odd shape for a more compact design, squeezing between the other parts of the vehicle.

This calibration is also why driving up and down inclines doesn’t rapidly change the needle on the fuel gauge as quickly as gravity moves the float itself – the car has been programmed to know that the fuel level isn’t actually changing that quickly.

It is possible to calibrate a fuel gauge yourself, though it’s not a good idea unless you have an exact reason and specific instructions for your car. You may be able to learn the procedure from an expert and calibrate the gauge by using a multimeter, resistor, and a power source. If you are unsure, leave this to a professional so that the car isn’t damaged.

How to Fix a Gas Gauge

fuse box

Depending on the problem, a faulty fuel gauge can be either quick and easy or nearly impossible to fix yourself.

First, check all of the electrical fuses. There is usually a box of fuses under the steering column inside the car and another under the hood. If any appear damaged, replace them.

If the self-test procedure for the gas gauge shows a problem or any other instrument cluster lights don’t work properly, the cluster itself may need to be replaced.

Examine all of the wiring between the battery and the gas gauge and the fuel tank. Replace or repair any broken or corroded wiring.

If none of these solve the problem, the sending unit is probably the problem (as is most common). First, try using a bottle of fuel system cleaner in the fuel tank in case some part of the sender is soiled or corroded. Follow the instructions on the bottle.

If that doesn’t work, the sending unit probably needs to be replaced. This is tricky to do at home and is best left to professionals, since it’s difficult to access inside the fuel tank. The tank itself needs to be drained and removed before working on the sender.

Replacement of the sending unit can range between $200 and $800, depending on the vehicle. In some vehicles the fuel tank is more difficult to access so labor costs will be higher. The mechanic may recommend replacing the fuel pump as well since it is usually in the way.


It’s not a great idea to drive around for long with a fuel gauge that isn’t working correctly, since you may be stranded or the fuel pump may pick up too much sediment and overheat.

This may be a problem you can address for relatively little cost. Understanding the system is very helpful, and then some troubleshooting at home can point you in the right direction for repair.

Sours: https://oards.com/fuel-gauge-not-working/


QUESTION: I have a strange problem in my car, which, surprisingly, has improved my dating life. My gas gauge goes straight to empty when the tank reaches half-full. So I appear to run out of gas a lot, which is a useful illusion on dates. I don't really want to fix it. I just want to know what's going on.


Tom: David, you sly little devil! When you do figure out what causes this, I suggest you immediately market it in college newspapers across the country. You'll be a millionaire.

Ray: Actually, you probably just have a faulty gas tank sending unit. There's a float in your gas tank that floats down as the fuel level drops. As the float goes down, the metal contact attached to it slides down a variable resistor. And the contact's point on that resistor tells your gas gauge how much fuel is left. My guess is that the contact on that float/sending unit isn't touching the bottom half of the resistor anymore.

Tom: It won't hurt anything if you leave it alone. But it will keep you from knowing when you're really about to run out of gas - which can be inconvenient.

Ray: Should you ever decide to fix it (you know, on the off chance someone ever goes out with you more than once and catches on to your scheme), you can have the sending unit in the gas tank replaced for $100 to $200.


Q: I recently purchased an '89 Chevy Astro van. My mechanic has done the necessary repairs, and it looks like we got a fairly good deal. The only problem is that the car stinks of cigarettes and other nefarious "weedlike" odors. The upholstery and carpet have been steam cleaned, and it seems like the air-conditioning unit and the upholstery on the ceiling are the main contributors. Do you have any ideas how we could desmoke the air-conditioning ducts, the ceiling or the whole car in general? Every car person I've talked to so far has been pretty useless.


Ray: Well, we fit that category too, Gil. But that never has stopped us from giving advice!

Tom: You're right about it being a difficult problem. Smoke gets into everything: the seats, the carpet, the headliner. It can even permeate the ductwork to some extent.

If it's really intolerable, then your best bet is to call a company that does fire salvage.

Ray: Right. After a house fire, everything that didn't get burned up smells awful. And there are companies that do nothing but get that smoke odor out. You can usually find them in the Yellow Pages under "smoke odor services." They should be able to help you.

Tom: They'll stick 2,000 of those Christmas tree air fresheners up inside your headliner and say, "You're all set, Gil!"


Q: I'm shopping for a new car and am in a quandary over safety issues. Our plan all along had been to get a minivan. We have two young kids, we own a Sable wagon, and we're ready to move on to the minivan stage of life. I've test-driven a few minivans and really like the new Toyota Sienna. But I also want the safest vehicle I can get, so I drove a Volvo wagon, which I liked. The problem is that the Volvo is not a van and doesn't have the space advantage that we want.

My question is, will the Sienna be as safe as the Volvo?


Tom: Is the Sienna as safe as a Volvo wagon? No. But is it a "responsible" choice? I'd have to say "sure."

Ray: Most of the minivans are reasonably safe vehicles these days. But you do trade off a little safety for their convenience and relatively low price (for the amount of space they afford).

Tom: But if safety is absolutely the most important thing on your list, then a Volvo wagon probably would be a better choice for you. That doesn't mean a minivan is unsafe. It's just not as much of a tank.

Ray: On the other hand, if you drive the Sienna responsibly and keep your kids properly belted or "child-seated" in the back seat where they belong, you'll probably be fine - unless you happen to be broadsided by a Ford Expedition.

Tom: Of course, you could get a Ford Expedition. It's safe because it's huge and heavy. It's got almost as much room as a minivan, although not as conveniently laid out. And then you can be taking up two parking spaces, getting nine miles per gallon in town, and sending other families in minivans swerving in fear.

Ray: It's a tough call, Jane. It basically comes down to how you look at life. If a car accident is your worst fear and you'll sleep better knowing that you've done everything you possibly can to protect your kids in a crash, then you should get the Volvo.

Tom: But if you're the kind of person who is comfortable taking reasonable precautions in exchange for things like convenience, reliability and your preferred lifestyle, the Sienna is certainly a responsible choice.

Sours: https://www.orlandosentinel.com/news/os-xpm-1998-03-26-9803250879-story.html
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In cars, the purpose of the fuel gauge is to indicate the amount of fuel in a fuel tank. It has two parts: the indicator on the dashboard and the sending unit in the tank. A bad sending unit is the most common cause of fuel gauge reading incorrectly. Before we further discuss what makes a fuel gauge read incorrectly, it’s better to first understand how it works.

Auto Repairs Are EXPENSIVE

How does the fuel gauge work?


It is common for a sending unit to use a float connected to a potentiometer. As the tank empties, this float drops and slides a moving contact along the resistor which increases its resistance. Also, when the resistance reaches a particular point, it will also make the “low fuel” light to turn on on some cars. Meanwhile, the indicator unit, which is usually placed on the dashboard, measures and displays the amount of electric current flowing through the sending unit. The needle points to “F” meaning a full tank when there’s a high tank level. The needle points to “E” meaning an empty tank when the least current is flowing and the tank is empty. Some cars use other indicators like “1” for full tank and “0” or “R” for an empty tank.


The fuel gauge system can be fail-safe. When an electrical fault opens, the electrical circuit makes the indicator show that the tank is empty (in theory, telling the driver to refill the tank) instead of full (which would make the driver run out of fuel with no prior warning). Wear or corrosion of the potentiometer will give false readings of fuel level. But this system has a risk attached to it. 


An electric current is transmitted via the variable resistor to which a float is connected, so that the value of resistance depends on the level of the fuel. In most fuel gauges in cars these resistors are on the inward part of the gauge, for example, inside the fuel tank. Transmission of current through such a resistor has a risk of explosion and a fire hazard. These resistance sensors also indicate an increased failure rate with the dynamic additions of alcohol in car gasoline fuel. Alcohol speeds up the corrosion rate at the potentiometer since it can carry current like water. 


A pulse-and-hold methodology is being utilized by potentiometer applications for alcohol fuel. It also has a periodic signal that is sent to detect fuel level decreasing the potential of corrosion. This is why there is a demand for a safe, non contact method for fuel level. 


How can you tell if your gas gauge is broken?


When there is an issue on the sending unit, the car will experience issues with fuel gauge like a fuel gauge reading incorrectly. This can put your car at risk of running out of fuel. A bad or failing fuel gauge sender will show a few signs that can alert you of a potential problem. 

  1. Fuel gauge behaves irregularly. One of the first signs that you have a problem with your fuel gauge sender is when it behaves erratically. A bad sending unit may cause the fuel gauge to suddenly shift positions, or give an erroneous reading. For instance, the gauge appears at three quarters, and then only a few minutes after it will change to half full, or vice versa. 
  2. Fuel gauge stuck on empty. Another typical sign of a faulty sending unit is a fuel gauge that is stuck on empty. When the float becomes separated or somehow breaks from the arm, it may lead to a malfunctioning fuel gauge and cause it to be stuck on empty. A broken resistor can also cause the fuel gauge to read empty.
  3. Fuel gauge stuck on full. This condition is a less common sign of an issue with the fuel gauge sending unit. A bad fuel gauge resistor can transmit a bad signal to the instrument cluster, causing the gauge to permanently read full. This poses a problem since the driver must know the correct fuel level of the car so he or she won’t run out of fuel. 

The fuel gauge sender is not a regularly serviced part. Usually, it is only serviced when the fuel pump fails. Nonetheless, it plays an important role in the proper operation of your car. If you start noticing signs of a failing fuel gauge, or you suspect there is a problem with the sending unit, have it inspected by a professional technician to determine if you need to replace your fuel gauge sending unit. 

How do you fix an inaccurate fuel gauge?


A stuck fuel gauge on your car is not only annoying, it is also a hassle as you need to keep track of the number of miles you have driven since your last fill-up to prevent you from running out of fuel. A fuel gauge functions using a series of electrical connections from the sending unit to the gauge cluster, and when one of these connections fails, so does your fuel gauge. The good news is, you can troubleshoot the problem yourself and avoid an expensive repair. Here’s how you can do it:


Step 1

Turn the ignition on and off a few times, and observe if the needle on the fuel gauge moves. If the needle does not move, it most likely means you have a blown fuse and it is due for replacement. The fuse box can either be found on the driver’s side dash or in the engine compartment. Refer to your car owner’s manual to find out where your fuse box is located.

Step 2

Test the grounding wire on the sending unit of the fuel tank. This is connected to your car’s fuel tank. You can do this by attaching the negative jumper cable to the vehicle’s frame, and the positive jumper cable to the sending unit’s grounding terminal. When the fuel gauge is working when the jumper cables are connected, it means you need to replace the grounding wire on the sending unit.


Step 3

Disconnect the wiring connected to the sending unit of the fuel gauge and then look at the fuel gauge. If the fuel gauge is now showing an empty reading, it means the fuel gauge is faulty and needs to be replaced. On the other hand, if your fuel gauge is showing a full reading, it means the sending unit is faulty and due for replacement.


Step 4

Check to confirm that all of the wiring is securely connected to the back of the fuel gauge. To do this, you have to remove the dashboard to have access to the instrument cluster. Consult your car owners manual as the process to do this varies from vehicle to vehicle.  

Also check the fuel gauge’s grounding wire by connecting the negative jumper cable to the car’s frame and the positive cable to the grounding terminal of the fuel gauge. Turn the ignition on and check the fuel gauge if it is working or not. If it is working, then the grounding wire should be replaced. 

What would cause the fuel gauge to stop working?

The fuel gauge is a relatively simple circuit, but because of that simplicity each part of it is essential to its operation. Here are different reasons the fuel gauge stop working:


Malfunctioning Sending Unit

As mentioned previously, this is the most common reason a fuel gauge stops working as it should be. When the car is moving, the sending unit is in constant motion, constantly in contact with the variable resistor. Over time, the contacts can wear, which can lead to an open circuit. The fuel gauge might read the voltage feedback from a dead sender as Empty or Full, as a result pegging the fuel gauge regardless of the actual fuel level.


Circuit Issues

Fuel gauge can stop working as it should be when there’s an issue or issues with the circuit. The fuel sending unit may fail to have a source voltage, the fuel gauge may fail to have a fuel sender voltage, or the ground for either may get interruptions. This would depend on the location of the fault. Corrosion and loose connections can also be the reason for a fuel gauge to stop working, particularly at the Fuel Pump Module as it is often exposed to the elements. 


Fuel Gauge Failure

This does not happen a lot, but it is still a possible issue. If the internal circuit fails, the gas gauge may only work in one section, such as between EMPTY and HALF or between HALF and EMPTY. When the internal circuits are shorted, they may falsely give a FULL or EMPTY reading. The fuel gauge will likely sit at EMPTY when the circuit is open, and may never move.


Failing Instrument Cluster

This is the least common cause and can be the most costly problem to repair. Recent instrument clusters are full-integrated circuits and may not even come with replaceable bulbs. If the fuel gauge fails as part of the cluster, the entire unit must be replaced. 


How can I determine the source of my fuel gauge issue?


Before starting to test your fuel gauge, prepare the following tools: a digital multimeter (DMM), an electrical wiring (EWD), and basic hand tools. Do the following tests to identify the source of the problem:


Instrument Cluster Self Test


Many modern vehicles come with an instrument cluster self test feature that can test computer-controlled instrument clusters. You can find the procedure in your owner’s manual or online. The test involves testing the digital lights and readouts, swiping the gauges through their ranges. Pay close attention to see whether or not the fuel gauge sweeps smoothly from Empty to Full. Keep in mind that some steps of self-test may stop the fuel gauge at ¼, ½ and ¾.


Fuel Sending Unit Test


This test should be done when the tank is lower than half level. This is to prevent fuel splashing. Begin by ensuring that the plug is clean, dry and corrosion-free. The pins should be straight and the connector should be fully seated. Take off the pump module so you are able to manipulate the float arm. Back-probe the connector and check for voltage with the key in the ON position (but do not start the engine). One of the pins should always have 5 V or 12 V. One of the other pins will be voltage feedback to the fuel gauge. Output voltage should increase or decrease (which depends on whether you are moving the float up or down) smoothly when you swing the float arm. If you get an incorrect input voltage, inspect the circuit between the ignition or its voltage source and the sending unit. The issue is likely with the sending unit when the output voltage is incorrect. But when both the input and output voltage are correct, it likely means you have a circuit issue between the fuel gauge and sending unit. 


Fuel Gauge Test


When conducting a fuel gauge test, repeat the voltage test from the sending unit. The voltage must be exactly the same as when you conducted a test at the sending unit. You likely have poor wiring between the fuel gauge and the sending unit, or you likely have corrosion, when the voltage is different. 


These tests should allow you to identify the problem, but be careful of modern computer-controlled instrument clusters and fuel gauges, since they can be confusing to diagnose and fix. It is highly recommended for you to have a professional diagnose complicated systems to save yourself from expensive mistakes. 


How much does it cost to fix a fuel level sensor?

The cost to replace a fuel level sensor ranges from $10 to $170. 



The fuel gauge is there for your convenience. You can be stranded when you run out of fuel. If your fuel gauge is reading incorrectly, working intermittently, or failed, it is highly recommended to have it replaced when you can. It typically does not cost much, and it will save you the trouble of running out of gas or constantly tracking how many miles you’ve driven since your last fill up. 


Categories BlogSours: https://www.cashcarsbuyer.com/fuel-gauge-reading-incorrectly/

Gauge reading full is fuel empty when tank

Why Is My Fuel Gauge Reading Incorrectly?

At its most basic, a fuel gauge is a handy indicator telling you how much gas or diesel you have in your vehicle and how long you can drive until you need more. It’s a car feature we tend to take for granted until we have a broken gas gauge. Have you been there? You filled up a few days ago, have driven each day and the gas gauge is stuck on full? Or what about you just went to the fuel station to fill up and your fuel gauge is reading empty when the tank is full?

Either way, you know your gas gauge is broken. If you are prone to being anxious, not knowing how long you can drive before running out of gas can be stressful. Not only that, repeatedly driving on a very low gas tank can hurt your vehicle’s fuel pump in various ways. A better understanding of the issue can help you decide the best way to solve the issue. Here are four commons reasons why your fuel gauge is not reading correctly.

How to Tell If Your Fuel Gauge Is Broken

1. Fuel Gauge Sender Failure

More commonly known as a fuel sending unit, the component is what sends the fuel level signal to your dashboard indicator. While you are operating your vehicle, the sending unit is constantly in motion. This activity means it is rubbing against the variable resistor. The more you drive your vehicle, the move likely this friction will lead to an open or shorted circuit. When an open circuit occurs, the gas gauge may interpret the change in voltage from the failed sending unit as either full or empty. This is the most common reason why your gas gauge is reading wrong.

2. Circuit Problems

Circuit Problems – The gas gauge circuit is wiring that connects the gas gauge, sending unit, the battery, and a ground. The provides the power from the car to operate the electrical components of the vehicle. Because the gas gauge runs through this circuitry, if there is a problem such as a loose connection or corrosion within the circuit, this can cause an error with the gas gauge reading. If you feel comfortable under the hood, go ahead and check for loose or dirty wiring. Sometimes it’s the smallest thing causing a seemingly big problem. To learn more about circuitry, check out how car electrical systems work.

3. Fuel Gauge Is Faulty

Fuel Gauge is Faulty – This is less likely than the previous two issues. The gas gauge is the visual indicator between the fuel tank and the sending unit. In other words, it’s what you read inside the cabin of your vehicle telling you how much fuel you have. Typically, if there is an issue with the fuel gauge, there is a problem with receiving input from the sender. This could be from a bad ground or once again related to the circuitry. A proper diagnosis can help determine if this is the issue.

4. Instrument Cluster Failure

Instrument Cluster Failure – Instrument cluster is the industry’s way to essentially say dashboard indicators. This includes your fuel gauge, speedometer and so forth. With some luck, this will not be the issue. While it is the least likely cause your fuel gauge is reading incorrectly, it is the most expensive reason. Most newer vehicles have a fully-integrated instrument cluster meaning if one indicator has an issue, the whole unit needs to be replaced to fix the problem. Make sure to check out any warranties associated with your vehicle if this seems to be the problem.


Having a broken gas gauge or anything wrong with your vehicle is no fun. It puts your safety and the integrity of the vehicle in jeopardy. Understanding the root cause of any problem you may be experiencing with your vehicle along with a trusted mechanic or service center can help ease any stress you may experience. If you have any questions about your fuel gauge or any other issue, let us know and we would love to talk about it.

Sours: https://actransmissiongolden.com/why-is-my-fuel-gauge-reading-incorrectly/
How To Adjust a Fuel Sending Unit. (3 Easy Ways)

Symptoms of a Bad or Failing Fuel Gauge Sender

If your fuel gauge behaves erratically or is stuck on full or empty, you may need to replace the fuel gauge sender.

by Eduardo Ruelas on January 08, 2016

The fuel gauge sender is a component that is found in the gas tank of most road going vehicles. The fuel gauge sender, also commonly referred to as the fuel sending unit, is the component responsible for sending the signal that operates the fuel level gauge in the instrument cluster. The fuel sending unit is made up of an arm, float, and a resistor that changes according the position of the float. The sender float is designed to float on the surface of the fuel inside of the tank. As the level drops, the position of the arm and float will shift and move a resistor which controls the display on the gauge. When the fuel sending unit has an issue it can cause the vehicle to experience issues with the fuel gauge, which can put the vehicle at risk of running out of fuel. Usually a bad or failing fuel gauge sender will produce a few symptoms that can alert the driver of a potential issue.

1. Fuel gauge behaves erratically

One of the first symptoms of a problem with the fuel gauge sender is a fuel gauge that behaves erratically. A faulty fuel gauge sender may cause the gauge to suddenly change positions, or give an inaccurate reading. The gauge may appear to be at three quarters, and then only a few minutes later will change to half full, or vice versa the gauge may appear to be full, only to have the gauge climb higher a short while later.

2. Fuel gauge stuck on empty

Another common symptom of a faulty fuel gauge sender is a gauge that is stuck on empty. If the float somehow breaks or becomes separated from the arm it may cause the fuel gauge to malfunction and become stuck on empty. A faulty resistor can also cause the gauge to read empty.

3. Fuel gauge stuck on full

Another, less common, symptom of an issue with the fuel gauge sender is a fuel gauge that is stuck on full. A faulty fuel gauge resistor can send a bad signal to the instrument cluster which can cause the gauge to permanently read full. This is an issue, as the driver needs to know the accurate fuel level of the vehicle as to not run out of fuel.

The fuel sending unit is not a routinely serviced component, usually only serviced when it, or the fuel pump fails, however it does play an important role to the proper operation of the vehicle. If you fuel gauge is displaying any of the symptoms, or you suspect that there may be an issue with this unit, have the vehicle inspected by a professional technician, such as one from YourMechanic, to determine if the fuel gage sender should be replaced.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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The most common problem that causes the fuel gauge to read incorrectly is a bad fuel sending unit. The fuel sending unit is used to measure the amount of fuel in the fuel tank and communicate the fuel level to the fuel gauge on the vehicle dashboard. This ensures that the driver is aware of the fuel level at all times, as well as warn when the vehicle is low on fuel, and when to visit a gas station before the tank is completely empty. If something happens to the fuel sending unit, the fuel gauge is the first to show it, so let’s take a look at how the two work together.

How a fuel sending unit works

The fuel sending unit located in the gas tank to measure the fuel tank level. Some sending units are attached to the fuel pump assembly, while others are stand-alone units, but both are made up of the same three components: a float, a metal rod/arm, and a variable resistor. These three components work together to measure the vehicle’s fuel level and relay the information to the gas gauge.

Float - The float, which is made of a buoyant composite or foam, floats on top of the fuel in the tank.

Metal rod or arm - The float is connected by a thin metal rod to a contact inside the variable resistor. 

Variable resistor -  A resistor is an electrical device that resists the flow of electricity, and a variable resistor has the ability to adjust the amount of resistance voltage encounters by sliding a contact (wiper) over a resistive element. The wiper is connected to the fuel gauge either directly by wire, or indirectly to an electronic device that communicates with the fuel gauge. In a fuel sending unit, the variable resistor receives power via a small coil from the vehicle’s battery, which is used as the baseline signal.

The fuel sending unit is one of many terms associated with this auto part. Some other terms used include: sending unit, fuel sender unit, fuel tank sending unit, fuel level sender unit, fuel level sending unit, fuel pump sender unit, fuel pump sending unit, fuel gauge sender unit, fuel gauge sending unit, and fuel level sensor. Luckily, these names are interchangeable because they all refer to the same part with the function of measuring a vehicle’s fuel level.

How a fuel sending unit measures fuel tank levels

The float in the fuel tank will move up or down according to the fuel level. The metal rod pivots in relation to the float, which causes the wiper inside the variable resistor to move as well. The wiper moves along a strip of resistive material connected to a ground on one end, and the strength of the electric current from the resistor is determined by the position of the wiper to the ground. The wiper sends the electric current to the fuel gauge, which displays the fuel level accordingly on the vehicle dashboard.

Fuel sending units work differently depending on the manufacturer. For example, a Ford fuel sending unit will be further from the ground on a full tank, while a GM fuel sending unit of the same year will be closest to the ground on a full tank. To understand how your vehicle’s fuel sending unit reads resistance, consult the service manual.

When the fuel tank is completely full, the wiper is either closest or furthest from the ground on the resistive material strip. Here is where there is either the least amount of resistance or full resistance respectively. When a vehicle is almost out of fuel, the wiper again is either closest or furthest from the ground as float rests at the bottom of the metal rods travel. Depending on its specifications, the fuel gauge will show full or empty according to what resistance it reads as full or empty.

As the fuel level along with the float lower in the tank, the metal rod connected to the float either moves the wiper further or closer to the ground, which either restricts or increases the amount of electric current being sent to the fuel gauge. The fuel gauge display drops from full according to the resistance change.

What causes a fuel gauge to not work?

While not all faulty fuel sending units act the same when broken, how the fuel gauge is acting can give us some insight on what is broken and needs to be fixed. Here are some signs that the fuel sending unit is bad and some speculations on how and why the fuel sending unit failed.

1. Fuel gauge reading empty when the tank is full

A fuel gauge that only reads empty may be caused by the float separating from the arm, which causes the rest of the components in the fuel sending unit to stop completely. In some cases, a faulty resistor can also cause the gauge to read empty by restricting the signal completely. Corroded wires, especially in the case of a fuel sending unit located on the fuel pump, can stop voltage either from the source or to the fuel gauge. 

2. Fuel gauge is stuck on full

A fuel gauge that only reads full may be caused by a faulty fuel gauge resistor sending the full voltage to the fuel gauge at all times. The fuel sending unit is constantly in motion when a vehicle regularly uses fuel, which causes constant movement of the wiper in the variable resistor. Over time, this can wear down the resistive material strip, creating an open circuit. Another issue may be a defect in the wiring from the fuel sending unit to the fuel gauge causing a shorted signal, or a bad ground wire to the grounding terminal. Although rare, a faulty fuel gauge could be an issue as well.

3. Fuel gauge fluctuates between empty and full

A fuel gauge displays that fluctuates between empty and full may be due to a mechanical failure. The fuel sending unit float arm may ‘stick’ at certain levels, and fall back into place either naturally or with help from vehicle movement. Once the float arm falls back into place, the fuel gauge becomes accurate again. This event can often be replicated, giving more evidence of mechanical failure. In some cases, a faulty fuel gauge could be an issue here as well.

Can you ruin your engine running your gas tank empty?

A fuel gauge not working may not make a vehicle undrivable, but not knowing the fuel level risks the vehicle running out of fuel. While walking to the gas station to get fuel is not ideal, running out of gas is not good for the car either. 

A fuel pump relies on fuel passing through for lubrication and cooling. When the fuel is gone, the fuel pump will overheat and become damaged. If a vehicle constantly runs out of fuel, constant damage to the fuel pump will cause it to fail. 

If a vehicle constantly runs on low fuel, potential debris from the bottom of the fuel tank can clog the fuel pump strainer, or bypass the strainer and clog other components of the fuel system such as fuel lines and fuel injectors. A clogged fuel system can not only cause performance issues but may become a problem for the longevity of other expensive auto parts or the engine itself.

While a faulty fuel gauge may be a low priority on most DIYers, it is beneficial to always fill the tank full and be aware of the fuel level at all times - whether for the sake of the driver, or the vehicle.

How to fix the fuel gauge or fuel sending unit

If your vehicle is experiencing any issues above, there are tests that can be done to confirm the issue is with the fuel sender. We recommended trying all tests before getting into the gas tank and replacing a sending unit that could still be functional. Sometimes the fix can be as simple as changing an instrument cluster fuse. 

For more information on how to fix a gas gauge that is not working, check out our Resource Center article on how to test and replace a fuel gauge and sending unit.

Your fuel sending unit may come separately from your new fuel pump in the box. See below for how to install. 

Sours: https://www.delphiautoparts.com/usa/en-US/resource-center/whats-wrong-my-fuel-gauge-diagnosing-bad-fuel-sending-unit

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