One of the most commonly-referenced gauges in a car is the fuel gauge. Even people who have never touched a wrench before in their lives and know absolutely nothing about the internal mechanics of cars are aware of what the fuel gauge does.
It’s important to know when you are starting to run low on gas, after all, and the fuel gauge is how you know when it’s time to fill up.
But how exactly do these fuel gauges work, and what components allow them to measure the gas levels in your tank? We’ve taken a closer look at fuel gauges to let you know exactly what’s going on in your car.
What is a Fuel Gauge?
In automotive and aerospace engineering, a fuel gauge is an instrument used to indicate the amount of fuel in a fuel tank. In electrical engineering, the term is used for ICs determining the current State of Charge of accumulators.
Fuel gauges are effectively sensing units for a car. They’re effectively made up of two components: the sensing unit and the indicator. The sensing unit itself works with the help of a float switch, which is connected to a potentiometer.
As the tank empties and the float switch drops, it slides a moving contact along the resistor and creates resistance. At a certain point, this will also turn on a “low fuel” light on some vehicles.
In this particular system, there’s also an indicator unit that measures the amount of electric current flowing through the sensing unit.
When the tank level is high and maximum current is flowing, the indicator points to “F or 1” to indicate a full tank. When the tank is empty and the least current is flowing, then the need points to “E or 0” to show that the tank is empty.
How does a fuel Gauge work?
The operation of a fuel gauge depends on its type, which can either be analog, electronic, or magnetic.
Analog Fuel Level Gauge
An indicator needle typically represents the fuel level in an analog setup. A heated bimetal strip moves this needle once current flows through it.
The bimetal strip can control the needle because of a small voltage regulator inside the instrument panel. The fuel tank-sending unit provides a ground circuit for the fuel gauge and controls the amount of current that flows through it.
The movement of the indicator needle represents the activity that occurs in the fuel tank that contains a variable resistor.
As the fuel level changes, so does the resistance of the fuel tank sending unit. As the sending unit delivers more ground to the heating wire in this type gauge, the bi-metal strip will change shape and move the needle.
This kind of gauge began to be replaced with magnetic style gauges in the early 1970s but remained on some vehicles into the 1980s.
Magnetic Fuel Level Gauge
A magnetic fuel level gauge uses small electromagnetic coils that are connected to a sending unit to display information on fuel level. The same mechanism is used to display other data, such as water temperature and oil pressure.
A magnetic fuel level gauge is more accurate than resistance-type gauges.
In terms of its schematic, the needle should point to “E” if the sensor wire is unplugged and grounded. Meanwhile, the needle should point to “F” if the sensor is unplugged or away from ground.
All gauges use similar sensors regardless of their type. An electronic control unit also operates these gauges and sends information to the powertrain control module (PCM) for engine data.
Instrument clusters on newer vehicles, even if they have needle type gauges, are connected to the vehicle network (class 2, CAN bus, PCI bus, etc.) and have a computer or a chip as part of the instrument cluster, so that a small servo drives each needle based on the input received through the CAN bus or directly received by the computer on the instrument cluster circuit board.
Electronic Fuel Level Gauge
An electronic fuel level gauge works the same way as the analog type. The only difference is that an electronic gauge displays fuel level information using numbers or a bar graph instead of an indicator needle.
Common Gas Gauge Issues
The gas gauge is a relatively simple circuit, but its simplicity means each component is essential to its function. Here are four ways that the gas gauge can fail.
- Sending Unit Failure: This is the most common cause of a gas gauge not working. When the vehicle is in motion, the sending unit is in constant motion, constantly rubbing the variable resistor. Over time, the contacts can wear, leading to an open circuit. The gas gauge might interpret voltage feedback from a dead sender as FULL or EMPTY, consequently pegging the gauge no matter the actual fuel level.
- Circuit Problems: These can cause the gas gauge to stop functioning normally. Depending on the location of the fault, the fuel sender may not have a source voltage, the gas gauge may have no fuel sender voltage or the ground for either one may be interrupted. Loose connections and corrosion can also cause problems, particularly in the fuel pump module, which is usually exposed to the elements.
- Gas Gauge Failure: This is less common, but still a possible issue. If the internal circuit is faulty, the gas gauge may only function in one section, such as between half and FULL or between EMPTY and half. If the internal circuits are shorted, they may peg to FULL or EMPTY. If the circuit is open, the gas gauge will likely sit at EMPTY and never move.
- Instrument Cluster Failure: This is the least common, and likely the most expensive problem to fix. Modern instrument clusters are fully integrated circuits and may not even feature replaceable bulbs. If the gas gauge fails as part of the cluster, the whole unit must be replaced.
Symptoms of Bad Fuel Gauge
You might notice that your fuel gauge is not correctly reading the amount of fuel in your fuel tank. If this is the case, then your vehicle’s fuel sending unit might be at fault. It can be the gauge, but the fuel tank sending unit is in the most hostile environment, is doing the most physical work, and is the most likely to fail.
Most gauges have some type of electronic “anti-slosh” function to keep the fuel gauge from moving around as the fuel sloshes. Typically, the fuel gauge won’t correct quickly after filling the tank unless the key is cycled.
Here are the symptoms of a faulty fuel sending unit:
Fuel Gauge is Stuck on Empty
The float may break off or separate from the arm. There might also be an issue with the variable resistor. An empty reading should only occur if the resistor is either creating the maximum amount of resistance or not allowing current to flow.
However, there are times when the fuel sending unit will completely stop sending current at certain positions.
Fuel Gauge is Stuck on Full
A fuel gauge that is stuck on full is not usually a sign of a problem with the fuel sending unit. This issue is usually caused by a malfunctioning variable resistor that gives a bad signal to the fuel gauge, causing it to read full all of the time.
However, there are times when it may indicate a problem with the fuel sending unit itself.
Erratic Fuel Gauge
The fuel gauge should not fluctuate or behave abnormally. When everything is working properly, the fuel gauge will continually move toward empty until you refill fuel.
If your fuel gauge is erratic, it might indicate a problem with the fuel sending unit. If you want to verify if your fuel sending unit is faulty, then you’re going to need a multimeter and access to the fuel tank.
If you don’t have a meter or don’t know how to use one, you can simply disconnect and reconnect the sending unit and watch the gauge, because that’s the first step of what most mechanics do, anyway.
Most sending units have a low resistance when full and a high resistance when empty, so if you disconnect the sending unit and cycle the key, the gauge should go past full. There will usually be three wires in the fuel pump gauge connector, and you need to know which wire feeds the fuel gauge.
Two of the other three wires will be grounds (one for the gauge sender and the other for the fuel pump) and the third wire will give power to the fuel pump. But the fuel pump ground and power feed are usually a heavier gauge wire than the fuel gauge wires (although on some vehicles they’re the same size).
Be careful not to confuse the fuel gauge/pump wiring with the fuel tank pressure sensor, which usually only has 3 wires and is sometimes mounted on the same plate as the fuel sending unit.
On many vehicles, the fuel tank sending unit must be accessed from beneath the vehicle rather than simply raising the seat or removing carpet from the upper side of the trunk.
This can be a laborious task depending on where your fuel tank is located. If your car’s fuel tank is underneath your rear passenger seat, you may have to remove the floor trim and carpeting before you see the top of the fuel tank.
How To Fix the Gas Gauge on My Car?
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 wirings 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 owner’s 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.
How Much Does It Cost to Fix A Fuel Gauge?
The cost is going to vary depending on what the issue is. An issue can be resolved with a new fuse costing $10 or a fuel gauge sender replacement can cost up to $800 after labor. The answer is it depends. Here is a breakdown of the common parts that would need replacing and their cost.
Cost Of Replacing the Fuel Gauge Sender
The majority of the time it is the fuel gauge sender which is the problem. This part can be a bit pricey to repair and most of the expense is in labor costs.
Depending on the make and model of your car and the specific sender that you purchase, costs will run on average between $250 and $800. You can save a lot of money replacing this part yourself if you feel comfortable.
Cost Of Replacing the Fuel Gauge Fuse
Another replacement part that may fix your fuel gauge is the fuse. This is very inexpensive and easy to do yourself. On average the cost of replacing a fuse is between $10 and $20 but some specialty fuses will cost around $100.
Cost Of Replacing the Fuel Gauge
If it is the gauge itself that is your issue, you’re in luck. This part is pretty cheap to replace and depending on the make and model of your car will cost between $50 and $200 for the part.
You will have to consider the labor of replacement if you intend on having this replaced professionally. Total costs will be between $200 and $400.