TechASSIST: ABS (Wheel Speed) Sensors

ABS Sensor Technical Advice


Fundamentally, the anti-lock braking system (ABS) prevents skidding and loss of control under harsh braking. The Anti-lock braking system uses ABS Sensors (also known as wheel speed sensors) to sense conditions in the wheel which could result in a skid or loss of traction. But ABS sensors are also used by other vehicle systems aside from the ABS system including:

  • Stability control
  • Traction control
  • Hill descent control
  • Collision avoidance.

The rapid and accurate feedback from the ABS sensor is vital to an effective anti-lock braking and other safety systems.

The ABS sensor works in conjunction an ABS ring (sometimes referred to as a tone or pulse wheel) that is mounted to a component moving at the same rotation as the wheel. This is often the wheel hub but can also be the brake disc, CV joint or the drive shaft. The ABS sensor is mounted next to the ABS ring and communicates the rotational speed back to the ECU. The way in which rotation in measured depends on the type of sensor and ring in use. The most common sensors are either passive or active.

The ABS Sensors and Ring Diagram

Passive ABS sensor wave
A typical passive ABS sensor wave (The waveform size depends on how fast the wheel spins for passive sensors)


A passive ABS Sensor does not require a power source. The sensor extends a magnetic field to the ABS ring using a coil winding surrounding a pole pin that is connected to a permanent magnet. This type of sensor required a toothed ABS ring as when the ring rotates and each tooth passes the sensor a change in magnetic field occurs in the sensor that is measurable as voltage as communicated to the ABS system control unit which coverts this AC signal to digital.

Active ABS Sensor wave
A typical active ABS sensor wave


In contrast to passive sensors an active ABS Sensor requires its own power source. Active sensors are more accurate and overcome some of the limitations of their passive counterparts. They are able to measure lower speeds and some can even determine wheel rotation direction.

Active sensors utilise the Hall effect, named after Edwin Hall who made the discovery in 1879. The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current.

The hall sensor again uses changes in magnetic field to measure rotation and features a semiconductor chip hall sensor to communicate a digital signal to the control system.

This type of active sensor can use a similar toothed ABS ring to passive sensors but often uses a magnetic or encoder ABS ring. The latter requires no permanent magnet in the sensor and as it is much flatter it can be integrated into the wheel bearing resulting in a much more compact system ideal for smaller spaces.

Toothed ABS Ring

ABS Encoder Ring


When an ABS sensor fails the following symptoms are common

  • ABS warning light illuminates (Stability control and/or hand brake light may also illuminate)
  • Fault code is stored
  • Brake judder at low speeds
  • Wheels locking up


Whilst ABS sensors are a simple and generally robust part, their location on the vehicle makes them subject to harsh conditions and so the following causes are common:

  • Physical damage to sensor from road debris
  • Sensor out of position (or increased distance from wheel bearing)
  • Breaks in wiring
  • Internal short circuits
  • Heavy contamination
  • Corroded, cracked or swollen ABS ring
  • ABS ring blocked, damaged, missing teeth or windows

Problems with the ABS ring can be identified using an oscilloscope and analysing the patterns as follows:

diagnosing oscilloscope patterns


When testing ABS sensors it is important to determine if the sensor type is passive or active. A resistance test can be used on passive sensors but should NOT be used on active sensors as it will damage the component.

When replacing ABS sensors we advice that the installation is thoroughly cleaned before fitting the new part.

Ensure that the new sensor is fully seated into position as even the slightest misalignment can cause issues with the signal.

It is important that all wiring to and from the sensor is secured and correctly routed.

Also ensure that fixing are tightened to the required torque as over tightening can damage the new sensor.

As with any sensor a reset of the ECU may be needed after replacement to clear the fault.