The function of speed sensors: fault symptoms

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The speed and rotation sensors built into modern automobiles use magnetic effects to take measurements, and there are many different types. The values ​​that these devices determine are necessary for controlling systems in the car, for example, the ABS system. The impeccable functioning of these systems is, in turn, essential for the safe operation of the vehicle. A fault in sensors of this type must be repaired as soon as possible.


1 Measurement principles

2 types of devices

3 Application and function of the speed and rotation sensor

4 Wheel rotation speed

5 Speed ​​or rotation sensor malfunctions

6 How do I know if the speed sensor is defective?


Speed ​​and rotation sensors determine a given angle or path covered in a given time interval. The devices make use of large or small electrical effects, for example inductive, Hall, or magnetoresistive effects. A distinction can be made between electrically active and passive sensors depending on their function. Generally, passive-type devices do not have electronic components at the measurement site and do not require a power supply, while active-type devices, on the other hand, incorporate electronic components for processing the measurement signal and require a power supply.

The rotational speed measurement takes place by means of a sensing device and an incremental-type sensing ring, for example on the car wheels or on the crankshaft. The rings (or sensing wheels) are distinguished according to the pulses they generate during a rotation, allowing a different measurement accuracy. The simplest sensor (or impulse) rings generate only one impulse per rotation and, as such, only allow the measurement of the average speed of a rotation, complex rings generate several impulses per rotation and thus allow the measurement of momentary velocity variations during a rotation.


The bar form is typically preferred for magnetic speed sensors as they are simple to mount. The head of the device is located at a certain distance from the ring and can be arranged perpendicularly or parallel to the axis of this ring. The device can, as such, have a radial, axial or tangential orientation to the ring. The movement of the ring causes a signal in the head of the device, allowing it to measure the rotation.

The principle of electromagnetic induction was used in the first magnetic rotational sensors. These devices are still in use for automotive applications. They are characterized by being passive, cheap, and of robust construction. Even so, their dimension is comparatively larger and they do not allow the measurement of quasi-stationary movements.

More modern devices use the static magnetic field principle to make measurements. They are of the active type and have the advantage that they can be designed in a comparatively compact way, that they are practical, and that they allow measurements of quasi-stationary movements. However, they can be sensitive to interference. Other nearby electromagnetic fields can affect your measurements. This category includes Hall, anisotropic magnetoresistance, and giant magnetoresistance sensors.


Application and function of speed and rotation sensor

Speed ​​and rotation sensors are widely used to carry out measurements for different automotive systems, allowing their management. Devices of this type are used, for example, in:

Wheels. The devices make it possible to determine the speed of rotation of the vehicle’s wheels. The sensor ring is integral to the wheel, presenting the same rotation regime as the latter. Since the beginning of the 21st century, the use of active devices in this function has been common, namely Hall devices with a silicon integrated circuit and a plastic ring with alternately polarized elements. The ring can be integrated into the wheel bearing.

Crankshafts. They determine the revolutions per minute of the crankshaft and its position. The ring is integral to the crankshaft. The most common mounting location is next to the flywheel. Active and passive devices are used for this purpose. Active devices appeal to the static magnetic field principle and use Hall effects, anisotropic magnetoresistance, and giant magnetoresistance.

Gearbox rotations. The devices are used in combination with automatic, automated manual, dual-clutch, and continuously variable transmissions. They serve to optimize the operation of the gearbox. As a general rule, Hall sensors are used in this application.


In modern vehicles, the rotational speed is generally determined for all wheels of the car, which contributes to stability, comfort, and driving dynamics. Electrical signals are sent from the devices via wires to various control modules in the car. The values ​​are of direct relevance to several systems, namely anti-lock brake systems (ABS), electronic stability control (ESP), traction control (ASR), and adaptive cruise control (ACC), as well as to the navigation system, allowing you to determine the speed of the vehicle. Furthermore, the values ​​can be transmitted, for example, to the engine control module and be of relevance to other vehicle control processes.

Driving speed is calculated based on wheel circumference. For this reason, a change in its diameter causes measurement errors. This can occur, for example, when switching to larger or smaller-diameter wheels. In this case, it becomes necessary to recalibrate the speedometer.

The function of speed sensors: fault symptoms


Faulty speed sensor symptoms

The failure of a device of this type can lead to a deterioration in the function of any of the systems that use the value measured by it. Consequently, in certain cases driving safety can be severely compromised. The causes that lead to the failure of these sensors are varied and include mechanical damage, contamination with moisture or dirt, short circuits, ring damage, or loose wheel bearings.

In the presence of a wheel with a faulty speed sensor, the most common symptoms are:

The triggering of a warning light on the instrument panel, usually ABS.

Writing an error code to the vehicle’s OBD2 system.

Wheel lock during braking.

Control system regulation errors.

Failure of other systems.


If you suspect that there is a faulty wheel speed sensor, a diagnosis should be made. For this, it is necessary to have the indicated technical documentation. In addition, it becomes necessary to have a diagnostic tool to read the error memory of the OBD2 system, as well as a voltmeter and an oscilloscope to check the signal and voltage in the device circuit. If you do not have the required tools and skills, we recommend looking for a specialized workshop to carry out the diagnosis and repair. It is a relatively complex procedure.

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