A fuel gauge might sound trivial, but it’s a tested and proven way to avoid getting stranded on the road after running out of gas. While a stuck fuel engine might not affect your engine performance, it will be an inconvenience at one point or another.
How Does a Car Fuel Gauge Work?
Your fuel level monitoring system consists of two parts. A sensing system (the sender) and an indicator (fondly called the gauge)
The sender unit has a sensor and sensing unit that does the majority of the fuel level monitoring.
A typical sender unit consists of:
- A float that floats on the fuel in the tank and has a metal rod that contacts a variable resistor via a flexible wiper
- A variable resistor whose resistance changes depending on the position of the wiper
The difference in resistance determines the voltage dip across the circuit. This dip/voltage difference is monitored by the gauge or a fuel level control module.
The fuel gauge could be anything from a simple dial with a voltmeter tuned to translate the voltage dip into corresponding changes in fuel level or an elaborate digital readout driven by a control module.
Suppose your car has a trip computer that estimates how long you can drive before filling up next or has a digital fuel level display. In that case, chances are you have a control module interpreting the analog signal from your sender unit into information your ECU can use to calculate range and fuel levels.
What Could Go Wrong and Make the Fuel Gauge Stuck?
A couple of things could go wrong along with the fuel level sensing system. Some are cause for alarm, while others are normal, depending on how you fill up your tank.
A Blown Fuse: Gauge Stuck at Zero
Fuel gauges are electrical units meaning a fuse controls them. The first and simplest place to start troubleshooting is to confirm that a fuse is not blown.
Get your vehicle’s service manual and identify the fuse controlling the dashboard gauge and the sender unit.
ProTip: Sender unit often shares a fuse with the fuel pump. The gauge could share a fuse with other gauge cluster displays.
If the fuse is blown:
- Replace and monitor it
- If it blows again, the wiring of the components ( sender or display gauge) are faulty and shorted out.
ProTip: Most dials in a vehicle do a sweep to full before dropping back to the right level. This is a self-diagnostic check to help you tell whether a display gauge is working or not. Not all vehicles do this, though.
A Gauge Stuck on Full for a While After Overfilling
If you recently filled your tank to the brim, expect your fuel gauge to be stuck at full (or past full if it’s an analog dial) for a while.
This happens because the fuel level sensing float is a physical unit that occupies space. It can only go so high.
When it is floating on the fuel at the full level, there is still some space around the float for more fuel to go.
More fuel that your system can’t account for will go into the tank if you continue filling the tank. The amount of extra varies from one vehicle model to another.
You will drive for a couple of miles without noticing a drop in the fuel gauge. Some cars will resume reading after a couple, while others might seem stuck on full for over 50 miles.
However, the gauge will start moving by the time you hit 100 miles. If you drive a distance that normally takes half a fuel tank without noting any differences in fuel gauge levels, chances are something is stuck or damaged.
A Fuel Gauge Stuck on Empty Even If the Tank is Full
A fuel gauge stuck on empty is more common than one stuck on full. This is because most manufacturers design their tanks such that no (or minimum) resistance means empty while maximum resistance from the potentiometer in the sender means full tank.
Pro Tip: Though insightful, this is not a market standard. Some manufacturers have their resistors set the other way round, such that high resistance means empty and low resistance means full.
This fail-safe design means that any fault in the system will most likely force the circuit open, leading to minimum resistance that interprets as an Empty fuel level.
This could happen if:
- The wires connecting to the sending unit are damaged
- An old sending unit has a corroded and broken resister
- A damaged contactor wiper that can’t maintain contact with the potentiometer
- The float in the fuel tank disconnects from the wiper that traces levels on the resistor
- The float is damaged to the extent that it can’t float in your fuel tank
- Rust or gunk gets the wiper stuck at a position where the system detects empty
- A bad ground connection at the sender unit
A Damaged Sending Unit Causes Erratic Readings or Random Sticking
If your sender unit is faulty, expect one of the following.
- Stuck on empty – most common.
- Erratic reading – common just before it fails
- Stuck on full- very rare
Erratic readings happen if the float is in the process of disintegrating and can no longer float accurately. The constant bouncing affects the wiper’s level on the variable resistor, making fuel levels on the gauge jump around.
Fluctuations could also happen if:
- The potentiometer is deteriorating and no longer has a smooth resistance curve
- The wiper contacting the potentiometer is deteriorating and can’t keep firm contact
- Some loose connection between the signal wires to the sender terminals or the fuel gauge/reading module
- Erratic ground connection
Sometimes, the sender gets stuck at one position every time you turn on the vehicle instead of fluctuating. This is common in senders that get steady power as long as the vehicle is on. It could be stuck at full one time, at half the next, and 3/4 full after that.
ProTip: Most modern vehicles have a pulse-controlled sender. A control module pulses power through the sender every now and then to get ‘reports’ on fuel level. This is more power-efficient and reduces the possibility of heating the resistor in the sender unit.
Damage to a Mechanical Fuel Gauge
Physical dial fuel gauges can get stuck even when the sender unit is working just fine.
A physical gauge is a calibrated voltmeter-like circuit that responds to voltage changes in a predictable way.
A given voltage reading will deflect the needle to a specific extent. To do this, it needs a coil, a permanent magnet, and a freely moving axis.
If any impact affects the magnet’s orientation or bars the gauge from spinning freely on the axis, you will end up with a stuck fuel gauge.
The physical impact that breaks the protective glass and shoves a shad onto the gauges way will definitely get it stuck.
How Do You Fix a Stuck Fuel Gauge
Fiddling with a fuel level sender or a fuel pump is not advisable. These units work close to highly flammable liquids. Any modification that could cause sparks can cause an explosion.
If the fix isn’t as simple as reattaching loose connectors or cleaning dirty terminals, refrain from rebuilding or repairing the sender unit.
Instead, consider buying a complete replacement. Fuel level sender units rarely get damaged. It’s not like you’ll be replacing it every now and then.
Note that some fuel level senders are part of a complete fuel pump assembly. You can’t replace or modify them without touching the fuel pump.
However, if the fuel sender unit is damaged, the entire assembly is either old or received impact damage that will affect the fuel pump soon.
Will a Damaged Fuel Level Sender Cause a Check Engine Light?
Modern vehicles have sensors and modules monitoring more and more parts of the car than before. From tire pressure monitoring systems to sensors to tell you when light is blown.
Some cars have a way of detecting a faulty fuel level monitoring system by comparing fuel level readings to how much fuel the car has burned over time.
The car’s computer combines information from the fuel injection system to estimate how much fuel it uses per combustion cycle.
Apart from using this information to give you a fuel economy estimate, the ECU can use it to cross-check if the fuel level monitoring system is accurate.
If the fuel gauge remains stuck at full or an arbitrary level even after the ECU estimates it has burned through, for instance, a 1/4 tank worth, then something must be wrong.
It will either pop up the MIL, a dedicated error message or simply drop the fuel gauge to a constant Zero to alert you of the problem.
Not all vehicles can do this. Chances are your manufacturer will be proud if they implement the feature and will have it prominently mentioned in the user manual.
Can a Damaged Fuel Level Sender Cause a Fire?
The biggest risk from a damaged fuel level sender is running out of fuel out in the open since you can’t estimate how much is left in the tank.
They’re designed such that the current flowing through the resistor and the entire unit doesn’t cause sparks.
Tampering with the unit could compromise an otherwise safe design that fails safely. Don’t try to repair it – especially if you’re not skilled. Getting a perfect replacement is the best way out.
Does the Fuel Pump Control the Fuel Level Gauge?
Your fuel pump doesn’t control the fuel level gauge. However, most cars have the fuel pump, and fuel gauge sender assembled into a single unit.
This makes the two co-joined into a single unit that can easily be confused as a fuel pump.
Even though they are different units, they are commonly sold as one. Their subtle differences mean the fuel pump section could fail without affecting the fuel level sender and vice versa.
Why is My Fuel Level Gauge Not Displaying Anything?
If you have a car with a digital dashboard, chances are your fuel gauge display is a digital bar or dial.
Such displays can fail if the gauge cluster itself fails. While this is rare, a failed gauge cluster will be blank. It won’t come on after you switch on the vehicle.
Contact a dealer or a proficient mechanic to troubleshoot and fix your vehicle. Modern gauge clusters and dashboards are complex computer hubs. You need skills and schematics to troubleshoot them and fix potential problems.
How to Bench Test a Fuel Sender Unit
Sometimes, you might want to be sure that the sender unit is faulty. This is a good way to troubleshoot and narrow down.
- Identify the terminals to the fuel sender unit
- Turn the vehicle ON but don’t crank it
- Use a multimeter to confirm that the Terminals in the Fuel Sender have voltage.
- One terminal should give you 12V to the ground, while another should give you a varied voltage.
- If the unit passes this check, you can go ahead and check if changes in the float’s level lead to a change in voltage at the signal.
To do this, you will have to empty the fuel tank and remove the fuel level sender and fuel pump assembly (assuming your car uses a single assembly for these)
Check this too: How To Prime A Mechanical Fuel Pump
Once you have it:
- Confirm that the float unit is intact and in good condition. Not perforated or disintegrated
- Confirm that the float is attached to the wiper and the entire assembly moves smoothly
- Use a multimeter and a 12V source (a battery) to apply power to the input terminals
- Move the slider or float arm with a multimeter confirming the voltage between the ground terminal and the signal terminal.
- The voltage should vary smoothly and proportional to the motion you induce on the float arm.
If the sender unit doesn’t pass the above test, then it’s faulty and needs replacement. Otherwise, clean the terminals and reinstall it to see if that fixes the problem.
If it doesn’t, the problem is somewhere else in the system.