Logo carsalmanac.com

What is FLS: decoding, purpose, types, principle of operation, characteristics and application

Table of contents:

What is FLS: decoding, purpose, types, principle of operation, characteristics and application
What is FLS: decoding, purpose, types, principle of operation, characteristics and application

To regularly monitor the fuel level in the car and avoid getting into an unpleasant situation on the road, all types of transport are equipped with a special sensor. This device determines how much gasoline or diesel is left in the tank and how far it will last. Every driver should know what a FLS is - a fuel level sensor, where it is installed and how it works.


The sensor is located in the fuel tank and is a metal probe with an electronic head, on the screen of which digital indicators are displayed. The part does not move in the tank and does not wear out, it is designed for tanks with a depth of 40 centimeters to one and a half meters. Digital FLS are quite accurate, with an error of no more than one percent. To understand what FLS is, let's find out how it works.

blow in the tank
blow in the tank


The float hanging from the electronic device is placed inside the tank, it always floats on the surface of the fuel and is directly connected to the variable resistance resistor. When fuel is consumed or, conversely, topped up, the indicators also change.sensor, due to internal pressure. There are several types of fuel level sensors that differ in the method of transmitting information to the electronic unit:

  • floating device;
  • sensitive rod that transmits data with a magnet;
  • ultrasonic sensor;
  • electric capacitor.

Modern expensive brands of cars use ultrasonic sensors that work on the principle of radar. The time and impulses of reflections from the liquid and the walls of the tank are recorded. To identify and eliminate the breakdown of such a device, a complex computerized examination of the car's electronics by specialists is necessary.

But what is a FLS operating on the principle of an electric capacitor? The sensor consists of two tubes with holes inserted into each other. Through these holes, fuel seeps in and fills it, changing the capacitance of the capacitor. Automotive fuel and air are electrically conductive, so the sensitive sensor instantly responds to changes in readings. As the liquid level decreases, the capacitance of the capacitor itself increases. The tubular float sensor operates on the same principle: the cavity of the tube is filled with fuel, and the sensitive float rises and falls, depending on the amount of liquid.

tubular sensor
tubular sensor

Fuel sensors also differ in shape, depending on the shape of the tank for which they are intended: a device with a rectangular float for front-wheel drive vehicles, and with a spherical float for all-wheel drive vehicles.

Data reading

If the car is equipped with an on-board computer, information about the fuel level in the tank is displayed on the screen. The fuel level sensor in the tank sends a signal to a digital converter, turning it into a code, and the computer, reading it, displays information on the dashboard. Such devices are very accurate, but depend on the correct setting and operation of the programmable indicator.

If a computer is not available, the data is processed by a special electronic circuit and displayed on the dashboard.

Faulty FLS

There are several reasons why the fuel level sensor does not work:

  • float not sealed;
  • bent wire holder;
  • case depressurization;
  • open resistor;
  • The sensor is poorly attached to the tank body.

When the float has lost its seal, the sensor will always indicate that there is no fuel in the tank, if there is any. In this case, it is necessary to replace the fuel level sensor completely or change the float. If the float wire holder is damaged, data distortion depends on the side of the rod bend. If it is bent up, the indicator will always show that the tank is full, if it is down, it indicates a lack of fuel. You need to straighten the holder or replace the device. This can happen when driving frequently on uneven terrain, when the tank is hit by a mechanical method, or during an accident. The failure of the LLS housing after an accident and the use of poor quality fuel also lead to a failure in the readings.

float sensor
float sensor

Ifthe variable resistor has broken, the indicator will show either an empty tank or filled to the brim. This also happens when the wire connecting the device to the display unit breaks. The smell of gasoline appears in the car interior, so you should check the tightness of the LLS, inspect the place of its installation and the integrity of the fuel pipes.

Fuel quality

Poor-quality gasoline or diesel can cause a breakdown of the LLS. An increased level of sulfur in the fuel causes corrosion of individual elements of the part, which also leads to complete or partial failure.

Mistakes that distort readings

Incorrect installation of the FLS, setting up and operation of the vehicle may affect the information, distorting it. Therefore, the fuel gauge incorrectly indicates the fuel level in the tank.

  • The sensor is not in the center of the container. If the meter is not in the center of the fuel tank, the liquid splashes in different directions while driving, which leads to fluctuations in the final readings. For cars, the design feature of which does not allow the indicator to be in the middle, special FLS with a bending tube are sold.
  • Rare request for information. When setting up the fuel sensor, set the range for requesting information within 15-30 seconds. This will increase the accuracy of liquid control in the tank.
  • Rough terrain. If the equipment is mainly operated on rough terrain with large slopes, then it is unrealistic to obtain truthful data on the amount of gasoline.
  • The presence of two fueltanks. Some car models are equipped with two tanks, and if you install one diesel fuel level sensor on two tanks, the readings will constantly diverge, since fuel can overflow from one tank to another while driving. In this case, two devices are installed and combined using the setup program.
two sensors for removable tanks
two sensors for removable tanks
  • The tube touches the bottom. When the measuring tube touches the bottom, it is deformed, which significantly affects the accuracy of the readings. The device must be mounted so that there is at least five millimeters of space left to the bottom.
  • Oxidation of the electrical connector. Oxidation of contacts leads to periodic switching off of the sensor. It is recommended to additionally lubricate the connector with grease.
  • Electricity at the limit. Exceeding the limits of energy consumption by technology leads to the device turning off and jumps in the issuance of information. The cause of the blown fuse should be eliminated - the voltage of the on-board network.
  • Defective tank vent valve. When the vehicle heats up, poor ventilation of the fuel tank will affect the data.
  • Setting the sensor. It is recommended to perform a high-precision adjustment of the device every six months, especially when changing the type of fuel.
tank sensor
tank sensor

Replace part

To repair the fuel level sensor or replace it with a new one, it must be removed. First you need to remove the negative terminal from the battery and free the area where the sensor is located in the car. Might have toremove the carpet and part of the upholstery from the trunk. We unscrew the fastening of the safety plate on top of the device, if any, and clean everything from dust. We mark the wires so as not to forget how to connect them back, and turn them off. We dismantle the sensor itself from the fuel tank and remove it.


Installation and connection of the fuel level sensor are performed as follows:

  • clean the remnants of the old sealant at the attachment point;
  • apply the rubber gasket to the hole on the tank, aligning them;
  • insert the electronic unit by lowering the float inside;
  • tighten the bolts after lubricating the gasket with sealant.
sensor installation
sensor installation

Connect the wires, the battery and start the car, checking the data on the dashboard. Four wires of the FLS are connected in this way to the on-board controller:

  • black to black - ground;
  • yellow to yellow - peripheral power;
  • blue to blue wire– line B interface;
  • white to orange - line A interface.

After driving at least thirty kilometers, you need to check the part for leakage - look under the trunk mat for traces of fuel. For a more accurate check, fill the full tank, the indicator should report this.

System setup

After replacing the faulty sensor, we proceed to set up the system. This process allows you to configure the device as accurately as possible and indicate the amount of fuel consumed in liters. The most common and convenient way is to spill the tank,however, it takes a long time. You can use the services of specialists in a car service or do it yourself.

Personal computer and Ls Conf utility program are used for configuration. The device is connected to the computer via a USB port with a special adapter. Sensor readings are recorded when the tank is empty. Then gasoline or diesel is added in portions from one to twenty liters until the graph starts to grow upwards, depending on the volume of the container, and each time the information is recorded in a special table, according to which the consumption graph is then built in a certain period. Topping up fuel and calibrating occurs until the entire tank is full. Press the "Save" button so that the calibration table and settings remain in the sensor's memory.

signal transmission resistor
signal transmission resistor

Flow control

With the help of such a sensor, entrepreneurs or car owners control the consumption of working vehicles. In this way, it is possible to establish where the refueling took place, the volume of fuel poured into the tank and its consumption per 100 kilometers in order to prevent the abuse and theft of fuel by the driver or other unauthorized persons who may not know what FLS is. This applies to vehicles transporting fuel, transport of freight companies and so on.

Popular topic