What to do if the fuel gauge does not work
Content
It is always important for the driver to know for what mileage he will have enough remaining fuel in the tank. The calculation of specific values of instantaneous or average mileage, the number of liters of fuel in the tank, and reserve mileage is performed by the on-board computer, but the fuel level sensor (FLS) supplies the initial information to it.
Since the shape of the tank is unchanged, the volume has a known functional dependence on the level.
The purpose of the fuel gauge in a car
Distinguish between a pointer and a sensor. The first is located on the dashboard and is an arrow or digital pointer.
In any case, the numbers are duplicated by an analog scale, it does not matter, in the form of a display section or a separate device with a magnetoelectric drive of the arrow. This is more a tribute to tradition than a necessity, but that's the way it is.
The pointer is connected to the sensor, and the electrical characteristics of both devices are selected in such a way that the error is the minimum allowable at any point on the scale.
It is not necessary to have a linear characteristic of the pointer and FLS. Moreover, they are almost always non-linear. But when two characteristics are superimposed one on the other, and additional non-linearity of the scale is added to them, then the displayed information can be trusted.
In the case of computer processing of the sensor signal, you don’t have to worry about the reliability of the readings. The software controller is able to implement any most intricate function, even if it is not expressed analytically. It is enough to calibrate the readings, which is done during development.
The most complex form of the tank, where, depending on the position of the fuel level, the movement of the sensor's driving element is affected by very different amounts of liquid in units of volume, is set in the device's memory in the form of a table.
What's more, the owner can always enter their own correction factors during the customization procedure for even more accurate readings. This is how universal on-board computers, installed as additional equipment, usually work.
Location of the device
LLS is always placed directly in the fuel tank. Its design is resistant to gasoline or diesel fuel vapors and access is through a flange on the top of the tank, usually integrated with a service port for the fuel pump.
The sensor itself is also often included in a single module with it.
Types of fuel level sensors
There are many principles for converting position into an electrical signal.
Some fix exactly the position of the liquid level, that is, the boundaries between substances of different densities, but it is quite possible to measure the volume directly. There is no special need for this, and the devices will be more complicated and more expensive.
There are several basic principles:
- electromechanical;
- electromagnetic;
- capacitive;
- ultrasonic.
There may also be differences in the way of communication with the pointer:
- analog;
- frequency;
- impulse;
- encoded directly by the data bus algorithm.
The simpler the device, the more it is produced, the price is almost decisive. But there are also special applications, such as commercial or sports, where accuracy and stability are more important.
The device and the principle of operation
Most often, surface control is carried out using a float. It can be connected to the converter in different ways.
Float
The simplest is to connect the float to the measuring potentiometer using a lever. Moving the position of the current collector causes a change in the resistance of the variable resistor.
It can be in the simplest wire version or in the form of a set of resistors with taps and contact pads, along which a slider walks, connected to the float through a lever.
Such devices are the cheapest, but also the most inaccurate. When connecting a computer, they have to be calibrated by control fillings with known volumes of fuel.
Magnetic
You can get rid of the lever by connecting the potentiometer to the float with a magnet. A permanent magnet connected to the float moves along the system of contact pads with taps from calibrated film resistors. Steel flexible plates are located above the platforms.
Depending on the position of the magnet, one of them is attracted to it, closing on the corresponding platform. The total resistance of a set of resistors varies according to a known law.
Electronic
The presence of electronic components in the sensor allows a wide variety of devices to be included in this category. For example, a capacitive sensor, where two capacitor plates are located vertically in the tank.
As it fills with fuel, the capacitance of the capacitor changes due to the difference in dielectric constant between air and fuel. The measuring bridge captures the deviation from the nominal and translates it into a level signal.
The ultrasonic sensor is a miniature emitter of high-frequency acoustic waves and a receiver of the reflected signal. By measuring the delay between emission and reflection, the distance to the level can be calculated.
According to the type of interface, development is proceeding in the direction of separating the sensor into an independent node of a single vehicle bus. Like all other devices, it is able to transmit information on this bus in response to a request from the dashboard.
Common problems
FLS failures are recorded by its noticeably erroneous readings or their complete absence. In the most common case of mechanical connection with a float and an analog potentiometer, the pointer needle begins to twitch, overestimate or underestimate the readings. This is almost always due to the mechanical wear of the contact group of the variable resistor.
The second frequent case is a change in the density of the float due to degradation of the material or filling it with fuel. Up to complete drowning and constant zero readings.
Electronic sensors in the event of a malfunction of the elements simply cease to give readings. Sometimes this is due to wiring that is less protected from external influences. Indicators fail much less frequently.
How to check the operation of the sensor
For each device containing a potentiometer, there is a calibration table for the relationship between resistance and fuel level.
It is enough to take measurements with a multimeter in ohmmeter mode at several points, for example, an empty tank, a reserve stock, an average level and a full tank.
With significant deviations or breaks, the sensor is rejected.
Methods for repairing a fuel gauge
Modern FLS cannot be repaired and are replaced as an assembly. After checking the wiring and testing the resistance at the connector, the sensor is removed from the tank along with the pump and the float on the lever.
This will require access to the top of the tank, usually located under the rear seat cushion or in the trunk. The sensor is removed from the pump module and replaced with a new one.
An exception may be noticed breaks in the wiring. Soldering and isolation of break points is carried out. But usually the cause of failure is the wear of the friction surfaces in the potentiometer.
Its restoration is theoretically possible, but impractical, the repaired device is unreliable, and the new one is inexpensive.