Category
TechASSIST BulletinsDate
March 24, 2026
In this ELTA TechASSIST bulletin, we investigate reducing misdiagnosis of Wheel Speed Sensors: their common causes of failure, symptoms, and essential fitting tips to ensure a long-lasting replacement.
Explore the VXPRO wheel speed sensor range, or contact TechASSIST for identification and fitting support.
Before we begin, if you follow Ed from Uckfield Motor Services, don’t miss his latest video explaining all things Wheel Speed Sensors!
Understanding the Role of Wheel Speed Sensors
Wheel Speed Sensors constantly monitor how fast each wheel is rotating and send this information to the vehicle’s electronic control unit (ECU).
If the ECU detects that a wheel is slowing down faster than the others, indicating potential lock-up, it instantly adjusts braking pressure to maintain traction.
Today, the data from Wheel Speed Sensors is used by several vehicle safety systems, including:
- Anti-lock Braking System (ABS)
- Stability control (ESC)
- Traction control (TCS)
- Hill descent control
- Collision avoidance systems
Because of this, accurate and reliable sensor feedback is essential for multiple modern vehicle safety systems.
How They Work
Wheel Speed Sensors operate alongside an ABS ring (also called a tone wheel or pulse wheel). This ring rotates at the same speed as the wheel and is typically mounted on components such as wheel hubs, brake discs, CV joints, and driveshafts.
The sensor is mounted close to this rotating ring. As the ring spins, the sensor detects changes in the magnetic field created by the passing teeth or magnetic poles.
The way this rotation is measured depends on the type of sensor used. The two most common types are passive sensors and active sensors:
PASSIVE SENSORS
Passive sensors are the earliest and simplest type of wheel speed sensor and do not require an external power supply. They consist of a coil winding surrounding a pole pin connected to a permanent magnet. As the teeth of the ABS ring pass the sensor, they disturb the magnetic field.
This change in the magnetic field generates an alternating current (AC) voltage signal in the coil. The signal is sent to the ABS control unit, which converts it into digital data and calculates wheel speed.
ACTIVE SENSORS
These require a power supply and use electronic circuitry to produce a more precise signal. They overcome many of the limitations of passive sensors by providing greater accuracy, especially at low speeds. Some designs can also determine the direction of wheel rotation.
Many active sensors use the Hall effect, a phenomenon discovered by physicist Edwin Hall in 1879. In this system, changes in the magnetic field generate a voltage within a semiconductor sensor chip. This produces a digital signal that is sent directly to the vehicle’s control system.
Active sensors can work with traditional toothed rings, but they are often paired with magnetic or encoder rings. These rings are flatter and can be integrated directly into the wheel bearing, allowing for a more compact and protected design.
Note: When testing, it is important to determine if the sensor type is passive or active. A resistance test can be used on passive sensors but could damage active sensors.
Common Causes of Wheel Speed Sensor Failure
Although Wheel Speed Sensors are generally robust, their position near the wheel exposes them to harsh conditions that can lead to failure.
Common issues include physical damage from road debris, sensor misalignment, damaged wiring, and internal electrical faults. Heavy contamination from dirt or brake dust can also interfere with the sensor signal.
In some cases, the fault may lie with the ABS ring rather than the sensor itself. Rings can become corroded, cracked, swollen, or damaged, and missing or blocked teeth can disrupt the signal sent to the ECU.
These issues can often be identified using an oscilloscope, which allows technicians to analyse the signal pattern during wheel rotation.
What Are the Symptoms?
When these sensors fail or begin to malfunction, several warning signs may appear:
- ABS warning light illuminated
- Stability control or handbrake warning lights illuminated
- Fault codes stored in the ECU
- Brake judder at low speeds
- Wheels locking during braking
Because Wheel Speed Sensors also support other safety systems, a failure can affect multiple vehicle functions.
General Fitting Advice
Before testing or replacing a Wheel Speed Sensor, it is important to identify whether the sensor is passive or active.
A resistance test can be used on passive sensors, but it should never be performed on active sensors, as this may damage the internal electronics.
When replacing a sensor:
- Thoroughly clean the mounting area before installation.
- Ensure the sensor is fully seated in its correct position.
- Maintain the correct gap between the sensor and ABS ring.
- Secure wiring correctly and route it away from moving components.
- Tighten all fixings to the specified torque.
AFTER FITTING
CHECK THE ABS SYSTEM AFTER REPLACEMENT
After replacing a wheel speed sensor, follow the vehicle manufacturer’s repair procedure. Check that the sensor and wiring are correctly installed, then use suitable diagnostic equipment to clear the repaired fault and compare the wheel-speed readings.
Carry out the manufacturer’s specified post-repair checks and any required calibration or learning procedure. When safe to do so, complete a road test and recheck the fault memory. If live data needs monitoring during the test, use a second technician so the driver can concentrate on driving.
IF FAULTS RETURN, RECHECK THE DIAGNOSIS AND FOLLOW THE VEHICLE MANUFACTURER’S TEST PROCEDURE.
By following these steps, technicians can ensure accurate diagnosis, effective repairs, and longer-lasting Wheel Speed Sensor replacements.
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Wheel Speed Sensors (ABS): FAQs
Quick answers to common questions about ABS sensor faults, diagnosis and replacement.
What are the symptoms of a faulty wheel speed sensor?
An ABS or stability-control warning light, stored fault codes or unexpected ABS intervention can indicate a wheel-speed signal problem. These symptoms do not prove the sensor itself has failed: wiring, connectors and the ABS ring can also be responsible.
Does an ABS fault code mean the sensor needs replacing?
Not necessarily. Use the fault code to guide diagnosis, then check the wiring, connections, sensor mounting and reluctor or magnetic encoder ring. Compare wheel-speed signals with suitable diagnostic equipment before replacing parts.
What is the difference between active and passive wheel speed sensors?
Passive sensors generate an alternating voltage as the toothed ring passes the sensor and do not need an external power supply. Active sensors require a supply and use electronic circuitry to produce a signal, including at low wheel speeds. Identify the fitted type before testing or ordering a replacement.
Can I resistance-test a wheel speed sensor?
A resistance test may be suitable for a passive sensor when carried out to the vehicle manufacturer’s specifications. Do not apply this test to an active sensor: it may damage the electronics. Use the correct test procedure for the sensor type and vehicle.
Can a damaged ABS ring cause the same symptoms as a faulty sensor?
Yes. A damaged, corroded or contaminated reluctor ring, or a damaged magnetic encoder ring, can interrupt the wheel-speed signal. Inspect the ring and associated components as part of diagnosis rather than assuming a new sensor will resolve the fault.
What should I check when fitting a replacement sensor?
Confirm the correct application and sensor type. Clean the mounting area, seat the sensor correctly, check the specified air gap and secure the wiring away from moving components. Follow the vehicle manufacturer’s fitting instructions and tightening torques.
Why is the ABS warning light still on after replacing the sensor?
The original fault may still be present, or further checks may be required. Follow the manufacturer’s post-repair procedure, clear the repaired fault with suitable diagnostic equipment and recheck wheel-speed readings. Complete any specified learning or calibration procedure and recheck fault memory after a safe road test. A general engine-ECU reset is not a substitute for diagnosis.
Where can I find the correct VXPRO wheel speed sensor?
Explore the VXPRO wheel speed sensor range and check the vehicle application in the ELTA electronic parts catalogue. For identification or fitting support, contact TechASSIST.
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Tim Brotherton
Marketing & Communications Manager at ELTA Automotive Ltd