Check and change ABS and wheel speed sensors

Here you will find useful information and important tips relating to the ABS sensor and wheel speed sensor in vehicles.

Wheel speed sensors record the rotation of the wheels. This information is used by numerous safety and assistance systems as the most important control parameter. Among other things, this page explains the differences between active and passive wheel speed sensors as well as their functional principle. Workshop professionals will also find highly detailed step-by-step instructions along with a wealth of practical tips for troubleshooting and replacing wheel speed sensors.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


Importance of ABS sensors

The increasing complexity of the traffic situation on our roads is placing high demands on car drivers. Driver assistance systems relieve the burden on the driver and optimise road safety. As a result, state-of-the-art driving assistance systems are now included as standard on virtually all new European vehicles. This also means that workshops are being faced with new challenges.

Nowadays, the vehicle electronics play a key role in all comfort and safety equipment. Optimal interaction between complex electronic systems ensures that the vehicle operates without problems, and this, in turn, increases road safety.
Intelligent communication of data between electronic vehicle systems is supported by sensors. When it comes to driving safety, speed sensors play a particularly important role, and this is reflected by their varied use in a number of different
vehicle systems.


They are used by the control units in driving assistance systems such as ABS, TCS, ESP, or ACC in order to detect the wheel speed.


The wheel speed information is also provided to other systems (engine, transmission, navigation, and chassis control systems) via data lines by the ABS control unit.


As a result of their varied use, speed sensors contribute directly to the driving dynamics, driving safety, driving comfort, lower fuel consumption, and lower emissions. Wheel speed sensors are often also called ABS sensors as they were used in vehicles for the first time when ABS was introduced.

Wheel speed sensors can be designed as active or passive sensors, depending on how they operate. A clear and precise way of distinguishing or categorising them has not been defined.


The following strategy has therefore proven useful in day-to-day workshop activities:

  • If a sensor is only "activated" when a supply voltage is applied and then generates an output signal, this is an "active" sensor.
  • If a sensor operates without additional supply voltage applied, this is a "passive" sensor.


Inductive speed sensor, passive sensors

Operating principle
The wheel speed sensors are positioned directly above the impulse wheel, which is connected to the wheel hub or drive shaft. The pole pin, surrounded by a winding, connects to a permanent magnet whose magnetic effect extends to the pole wheel. The rotation of the impulse wheel and the resulting switching from tooth to tooth space brings about a change in the magnetic flow caused by the pole pin and winding. This changing magnetic field induces a quantifiable, or measurable alternating voltage in the winding (Figure 1).


The frequency and amplitudes of this alternating voltage are in relation to the wheel speed. Inductive passive sensors do not require a separate power supply from the control unit. Since the signal range for signal detection is defined by the control unit, the amplitude level must be within a specific voltage range. Gap (A) between sensor and impulse wheel is provided by way of the axle design (Figure 2).

Active wheel speed sensors

Operating principle
The active sensor is a proximity sensor with integrated electronics that is supplied with a defined voltage from the ABS control unit. A multipole ring can be used as an impulse wheel while at the same time being integrated in a sealing ring of a wheel bearing. Inserted in this sealing ring are magnets with alternating pole directions (Figure 3). The magneto-resistive resistors integrated in the electronic circuit of the sensor detect an alternating magnetic field when the multipole ring rotates. This sinusoidal signal is converted by the electronics in the sensor into a digital signal (Figure 4). It is then transferred to the control unit as a current signal using the pulse-width modulation method.


The sensor is connected to the control unit via a two-pole electric connecting cable. The sensor signal is also transmitted at the same time over a power supply line. The other line is used as a sensor ground. In addition to magneto-resistive sensor elements, nowadays Hall sensor elements are also fitted that permit a larger air gap and respond to the smallest of changes in the magnetic field. If a steel impulse wheel is installed in a vehicle in place of a multipole ring, a magnet is also affixed to the sensor element. When the impulse wheel turns, the constant magnetic field in the sensor changes. The signal processing and IC are identical to the magneto-resistive sensor.


  • Wheel speed detection from standstill. This facilitates speed measurements down to 0.1 km/h, which is relevant to traction control systems (TCS) as soon as the vehicle accelerates from a stop.
  • The Hall sensors detect forward and backward movements.
  • The sensor is smaller and lighter in design.
  • The lack of impulse wheels simplifies the power transfer linkage.
  • Sensitivity to electromagnetic interference is less pronounced.
  • Changes in the air gap between the sensor and magnetic ring have no direct impact on the signal.
  • Virtual insensitivity to vibrations and fluctuations in temperature.


The following system characteristics can be recognised when wheel speed sensors fail:

  • The ABS warning light illuminates
  • Fault code is stored
  • The wheels lock during braking
  • Incorrect or pseudo regulation interventions
  • Failure of further systems


Causes of failure:

  • Breaks in wiring
  • Internal short circuits
  • External damage
  • Heavy contamination
  • Increased wheel bearing clearance
  • Mechanical damage to the encoder wheel


Generally speaking, a malfunction of the ABS/TCS/ESP braking system occurs before the wheel speed sensors are tested.


After the warning light illuminates, the following options are available for troubleshooting and diagnostics:

  • Read out the fault memory
  • Check the supply voltages and signals using a multimeter and oscilloscope
  • Visually inspect the wiring and mechanical assemblies


Diagnostic unit

  • Read out the fault memory
  • Evaluate parameters
  • Compare and evaluate signals from individual wheel speed sensors



  • Voltmeter (check the voltage supply)



  • Check the internal resistance (the internal resistance cannot be measured on active sensors)



  • Signal depiction (evaluation of signal curve)

Troubleshooting wheel speed sensors can be difficult when it comes to differentiating between active and passive sensors as these sensors cannot always be easily distinguished by appearance. Here, specific manufacturers' specifications and the specifications of the respective vehicle manufacturer must be consulted. Due to their advantageous technical characteristics such as accuracy and low structural size, vehicle manufacturers have mainly installed active wheel sensors since 1998.

Requirements for reliable diagnostics

  • Proper documentation in the form of technical data
  • A suitable diagnostic unit, multimeter, or oscilloscope
  • Sufficient technical know-how of the technician and targeted employee training


When performing diagnostics on complex systems, the best technology alone cannot help repair the vehicle. Only well-trained personnel can prevent the random replacement of system components, avoid malfunctions in workshop processes, and strengthen the trusting relationship with the customer.

Observe the following general repair notes:
Repair work to brake systems may only be carried out by qualified specialists.


When carrying out any repairs on the brake system, you must follow the maintenance and safety instructions of the vehicle manufacturer and the product-specific assembly instructions

  • Observe the respective tightening torques without fail.


Fault diagnostics in daily workshop service

Using the following example of "Rear left speed sensor faulty", we will explain the diagnostics work required for an active wheel speed sensor. Your customer has reported a functional problem with the ABS system. The ABS warning light comes on while driving.


In this example, the repair procedure is shown based on a BMW E46. The schematic illustrations, figures, and descriptions are intended solely as explanations of the document text, and cannot be used as the basis for carrying out installation and repair work.


Preparing the diagnostics:

  • In order to allow correct allocation of the vehicle, it is important that the vehicle documents (registration documents) are included with the job sheet.
  • Check the battery voltage. Insufficient voltage supply may cause system failure or result in incorrect measurements or voltage drops.
  • Check the system-related fuses. A quick look in the fuse box might eliminate the first cause of the fault.


Check the service brake

  • Run the vehicle on the brake test stand. We recommend the use of a roller test stand. Potential defects on the brake mechanics can be identified in this way, even under gentle braking. Any imbalance in the brake disc will result in differing wheel speeds when braking, which will change the wheel speed information sent to the control unit.
  • Determine the braking effect.

Visual inspection

  • Move the vehicle onto the lifting platform.
  • Check that the wheels are the correct size and have the correct tires.
  • Check the tire pressure and tread depth.
  • Check the wheel bearing play and axle suspension.
  • Check the brake fluid level.
  • Check the wear on the brake pads.
  • Check the connectors and wiring of the sensors to ensure they are correctly positioned and secured, and check for any major damage.

Using the diagnostic unit

  • Connect the diagnostic unit to the 16-pin OBD connector. Depending on the vehicle manufacturer and date of registration, a different diagnostic socket and an additional adapter may be required.
  • Select program.
  • Select vehicle.
  • Select fuel type.
  • Select model.
  • Select required function.
  • Select system.


Depending on the diagnostic unit being used, additional information on the system variants installed in the vehicle may be displayed here. If it is not possible to clearly allocate the system, it is possible – without damaging the respective control unit – to set up diagnostics with the specified control units one after the other. Only the control unit which was correctly identified by the diagnostic unit will establish communication.

  • Start fault diagnostics.


A correct connection and sufficient battery voltage are required in order to establish reliable communication with the control unit. A battery voltage of 12.69 V as evaluated by the control unit can clearly be seen here. Insufficient supply voltage to the control unit could be an indication of a wiring defect or a defect in the vehicle battery.

Read out the fault memory

  • In this example, the fault code "Rear left speed sensor" was stored. In addition to the numerical code, some diagnostic units also provide a definition of the fault code. This makes it easier to decide on the subsequent diagnostics work.

Evaluate the details

  • Here, initial information about a possible reason for the fault is stored. The stated fault code does not necessarily refer to an actual fault in the component. Before you start replacing individual components, read this information carefully in order to determine the next diagnostics steps.

Read out the parameters/measured value block

  • The actual values are displayed here for further analysis. In this case, the faulty signal curve relative to the rear right sensor can clearly be seen. The visible irregularities in the signal curve allow you to narrow down the defect.


If the signal curve does not show any irregularities, first delete the fault. Once this is done, carry out a test drive with the diagnostic unit connected. In the process we recommend evaluating the parameters at the same time so that the cause can be narrowed down if the fault occurs again.

Check the voltage supply

  • Here, we recommend taking the measurement directly at the sensor connector in order to check the entire line between the control unit and sensor.


Practical tip:
Due to the connector design, it is very difficult to take a reliable measurement directly at the plug contacts. It may be helpful to produce an adapter from an old identical sensor.

Check sensor installation point and impulse ring

  • Remove the sensor
  • Check the sensor and impulse ring for damage.


In our example, a defect has been found on the sensor cable. A cable break in the supply cable, caused by mechanical damage, resulted in a loose connection on the connector housing.

Replace the wheel speed sensor

  • Clean the sensor installation point. Clean the contact surface using a wire brush or, if required, with sandpaper.
  • Replace the wheel speed sensor. Please make sure that the sensor cable is properly routed and fixed in place.
  • Observe the tightening torque. Comply with any torque values specified by the vehicle manufacturer.

Read out the fault memory

  • Delete the stored fault.
  • As a result of the diagnostics work on the vehicle, additional faults may be detected by the control unit. These must be deleted before the test drive.

Carry out a test drive

  • In order to check the wheel speed signal after having replaced the sensor, subsequently carry out a test drive with the diagnostic unit connected and evaluate the parameters at the same time.

Final test

  • After carrying out the test drive, read out the fault memory again. As the systems in the vehicle are networked, a defect in the ABS system will also be stored in other control units. Here, we recommend querying all control units and deleting the stored faults.

Please note the vehicle manufacturer's information when carrying out any testing and diagnostic work. Depending on the manufacturer, additional vehicle-specific testing methods may have to be taken into consideration. Ensuring optimal interaction between humans and technology has become more important than ever. Only workshop specialists with the latest expertise and who know how to optimally use their technical equipment will be ready to tackle the challenges of the future.


Troubleshooting tree for wheel speed sensors
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