Is your fuel gauge not working or inaccurate? (3 common causes)
There are two types of people in the world: those who wait until the tank is nearly empty and those who regularly see the empty warning light on their instrument panel. What species are you?
Leaving your car with less than a quarter tank full frequently is not good because it can overheat the fuel pump and cause the pump to pick up more debris from the bottom of the tank, potentially clogging the pump, filter, or injectors. Being stranded because you’re out of gas is not good for you or your car. Drivers rely on an accurate fuel gauge to get them to their destination and protect their vehicle. What do you do if the fuel gauge fluctuates, stays full, or always shows empty?
How does a fuel gauge work?
It’s helpful to understand how the vehicle communicates to the driver exactly how much fuel is in the tank. The system consists of three parts: the display, the transmitter (also called the sending unit), and the circuit. Advertisement
The fuel gauge is the part on your dashboard that indicates how full your gas tank is. It is part of the instrument cluster and is connected to the transmitter unit via a cable or wireless connection.
Channel
The sending unit is the physical assembly in your fuel tank that senses the fuel level and sends the information to your car’s fuel gauge or computer via an electrical signal. It is part of the fuel pump module, which also includes the fuel pump, fuel strainer, and fuel filter.
The transmitter consists of a float connected to a variable resistor via a wiper (a thin metal rod). The resistor has one end (top or bottom, depending on the vehicle) connected to electrical ground.
As the fluid and float levels change, the wiper slides up and down against the resistance. As the wiper gets closer to electrical ground, the resistance gets lower because the material offers less resistance to the current to flow, and vice versa.
This voltage is usually supplied by the car’s battery (which outputs a constant voltage), and the resulting voltage (which is affected by resistance) is sent to the computer or fuel gauge itself.
Circuit
The circuit is the wiring that connects the car’s battery, sending unit, display, and ground (the vehicle’s body or frame, or electrical system). Why the fuel gauge doesn’t work properly
Since three parts are needed to show the driver how much fuel is left, fuel gauge inaccuracies are always caused by one of them.
1) Encoder failure
This is the most common problem because there is a lot of movement within the device. Parts can come loose, wear out or corrode.
The contacts between the wiper and resistor are constantly rubbing against each other. When they wear out, they no longer send the correct signal and the level stays full or empty. If the float physically separates from the wiper, it will always read “empty”. A faulty resistor can cause an inaccurate reading on the display.
Old, sticky wipers can get stuck on the resistor in certain places and get pushed back into place over time or on bumpy roads. This causes abnormal fluctuations in the fuel gauge.
2) Circuit Problems
A problem with the wiring means that the signal is not getting from the battery to the fuel gauge due to a problem along the way. There could be a problem with the cable that receives the signal from the battery, the ground from the battery or transmitter, or the cable between the transmitter and the display itself.
3) Fuel Gauge or Instrument Cluster Fault
Depending on the vehicle, the fuel gauge may be a single component or it may be part of the instrument cluster. Either way, there could be a problem with the internal circuitry that causes the signal to be distorted or not get any signal at all.
Because electricity is involved, fuel gauges and instrument clusters have fuses that can blow. These are easy and cheap to replace, but replacing the entire instrument cluster is very expensive.
How do I know if my fuel gauge is accurate?A functioning fuel gauge should drop to a certain level after refueling, depending on distance and driving style. If the fuel gauge shows the tank is full for more than 110 km, or if the fuel gauge reading is erratic, there may be a problem. Similarly, you should check if the gauge goes empty soon after refueling. For vehicles manufactured more than about 20 years ago, there may be a self-test procedure for the instrument cluster to see if the display can go from full to empty.
Your owner’s manual will also tell you how many gallons/liters of fuel your vehicle’s tank holds, and sometimes your fuel economy. Using these two numbers, you can roughly calculate how far you can travel on a full tank of fuel.
The best way to get a more accurate overview of your vehicle’s fuel economy is to regularly track the number of miles driven between gas stations and divide it by the number of gallons of gasoline you get at the next gas station. By keeping track of these issues, you can identify a lot of car problems before they become major issues.
Can you calibrate my fuel gauge?
Modern car manufacturers can use their on-board computers to slightly alter the movement of the fuel gauge by comparing the float’s position to a calibration curve that compensates for the shape of the tank.
It is possible to calibrate the fuel gauge yourself, but it’s not a good idea unless you have exact reasons and specific instructions for your car. You may be able to learn the steps from an expert and use a multimeter, resistors, and power supply to adjust the display. If in doubt, leave it to a professional to avoid damaging your car.
How to Fix a Fuel Gauge
Depending on the problem, a broken fuel gauge may be quick and easy to fix, or it may be nearly impossible to fix yourself.
First, check all the electrical fuses. Your car’s fuse box is usually located under the steering column and another one under the hood. If any parts appear damaged, replace them.
If the fuel gauge self-test indicates a problem, or if other lights in the instrument cluster are not working properly, you may need to replace the instrument cluster.
Check all wiring between the battery, fuel gauge, and fuel tank. Replace or repair any damaged or corroded cables.
If none of these measures fix the problem, the problem is probably with the sender (this is most common). First, if any parts of the sender are dirty or corroded, try putting a bottle of fuel system cleaner into the fuel tank. Follow the directions on the bottle.
If that doesn’t solve the problem, you may need to replace the donor unit. Replacing the sending unit can cost anywhere from $200 to $800 depending on the vehicle. Labor costs are higher because the fuel tank is difficult to access on some vehicles. A mechanic may also recommend replacing the fuel pump, as it is usually in the way.
Bottom Line
Driving for extended periods of time with a fuel gauge that is not working properly is not a good idea.
This may be a relatively cheap problem to fix. Understanding the system and troubleshooting at home to find the right repair method can be extremely helpful.
