Further guidance
Wheel speed sensors provide wheel and road speed feedback to ABS and derivative active vehicle safety systems (i.e. stability and traction control etc.).
These systems are designed to provide corrective action (e.g. wheel braking or engine torque limitation) when the vehicle chassis or wheel speeds have exceeded their normal operational tolerances, for example, during conditions of wheel slip, oversteer or understeer etc. Wheel speed sensors are critical to the operation of these systems and, therefore, the safe handling of a vehicle.
Inductive wheel speed sensors encase a wire coil around a magnetically polarised core and operate in tandem with a pulse wheel mounted on a component rotating at wheel speed, for example, a driveshaft or hub. As the pulse wheel passes the sensor, the sensor’s magnetic field is disturbed, which induces an oscillating voltage within the wire coil and the circuit connected to it. The quicker the wheel rotates, the greater the magnetic field disturbance, the greater the induced voltage amplitude and the faster the oscillation frequency.
It is not easily possible to distinguish two wire passive (inductive) and active (Hall or magnetoresistive) wheel speed sensors by their external appearance. Diagnostically, this is inconvenient as active wheel speed sensors must never be subjected to resistance tests: this can damage these units, with the only remedy being the acquisition of a new replacement.
Therefore, you must always either consult manufacturer’s data to identify the fitted type prior to diagnostic testing or carry out a speculative PicoScope check to identify the sensor from its output voltage characteristics.
You can check for a positive supply voltage at one of the sensor connector terminals to determine if you have an active sensor type. However, if the supply voltage is missing due to a fault and you then assume the sensor must be passive and perform a resistance check, you can damage a perfectly good active sensor. This will give you two faults.
An ABS control module expects similar (within a given tolerance) oscillation frequencies from all the vehicle’s wheel speed sensors and uses any differences to calculate the timing and scale of its interventions.
If one, or more, wheel speed signals continuously fall outside of normal parameters the control module may turn the ABS function off (along with associated traction and stability systems). A driver warning light will be illuminated but, as with any electrical fault on ABS, normal hydraulic braking is maintained.
Wheel speed sensors and their pulse rings are exposed to the atmosphere and have to operate under conditions of constant vibration and movement. As such, common faults are:
- Sensor signal failures, caused by chafed or fractured circuit wiring, sensor or connector corrosion, or incorrect sensor fitment.
- Pulse wheel related problems arising from corroded or damaged teeth, incorrect pulse ring fitment (misalignment), contamination from a build-up of ferrous material, or an excessive air gap between the sensor and the pulse wheel.
Wheel speed sensor circuits and connectors are also prone to the atmosphere and possible electrical failures, such as open or short circuits or high circuit resistances.
Symptoms of ABS sensor related faults
- ABS warning light (and other associated warning lights) illumination.
- Diagnostic Trouble Codes (DTCs).
- Brake pedal pulsation or vibration.
- ABS inactive.
