We have created this page to provide you with answers to common ABS Sensor technical questions we are asked. If you cannot find the answer you require, please email the Brake Doctors on brakedoctors@ukpartsdirect.com or use our Brake Doctors LIVE Chat Function.

Q. How does an ABS Wheel Speed Sensor work?

A. This does depend on the type of ABS Wheel Speed Sensor. There are two different types of ABS Sensor – Passive Sensors and Active Sensors. UK Parts Direct stocks a huge range of both Passive and Active Sensors.

Q. How does a Passive ABS Sensor work?

A. Passive ABS Sensors are made up of a permanent magnet around which a copper wire coil is wound. This sensor is mounting in close proximity to the ABS Reluctor Ring. As the ABS Reluctor Ring rotates, the teeth create a small inductive voltage in the copper windings. This voltage signal is monitored by the vehicle ECU (vehicle’s computer) and used as wheel speed information.

Q. How can I tell if my vehicle has a Passive ABS Sensor?

A. Passive ABS Sensors normally have just two wires. One is for Signal and one is for Earth. Also, the vehicle will be equipped with a Metal Toothed type ABS Reluctor Ring.

Q. How does an Active ABS Sensor work?

A. Active ABS Sensors can be likened to the Hall Type Sensor (used in tachometers). The Active sensor pick-up has an amplifier in-built and therefore relies on a supply voltage. This is normally 5v but can be 12v.

The sensor is mounted in close proximity to the Magnetic ABS Reluctor Ring. The Magnetic type ABS Ring consists of a multi-pole (north / south / north / south …) magnetic ring. More and more vehicles are now being produced with integral ABS within the wheel bearing seal and use magnetic powder instead of fixed magnets.

The rotating, alternating, magnetic poles produce a magnetic flux within the sensor element which then amplifies and controls the signal for the ECU (vehicle’s computer) to use as wheel speed information. The output of an Active sensor is digital with a square wave signal, being capable of sending wheel speed information down to 0mph. In contrast, the Passive sensor’s accuracy is usually dubious below 20mph.

Also, some vehicle manufacturers now use active sensors that have small aerials built in, to transmit information to the ECU (vehicle’s computer) via radio waves.

Q. How can I tell if my vehicle has an Active ABS Sensor?

A. Active ABS Sensors normally have three wires. One is for Power, One for Signal and one is for Earth. Also, the vehicle will be equipped with a Magnetic type ABS Reluctor Ring or Integral Magnetic type Hub Unit.

Q. How can I test my ABS Sensor?

A. The first tool for diagnosing an ABS Wheel Speed Sensor failure is the already in-built diagnostic power of the ECU (vehicle’s computer). When an ABS Wheel Speed Sensor fails it generally lights the “MIL” lamp on the dashboard. This points to an ABS fault. We then need to pin-point the wheel that is causing the issue.
Initially, you should look for damaged wires, sensors or a build up of road dirt etc. The next step is to test the sensors individually. Use an oscilloscope connected to the sensor and spin the wheel, check the output pattern for interference and the correct “shape” (passive = sine wave, active = square wave). Passive sensors can be disconnected and tested as a “stand alone” unit. Active sensors, due to their required voltage input, have to be connected with the ignition on when testing.

Q. Why has my original ABS Sensor failed?

A. The main reason an ABS Wheel Speed Sensor fails is the same as for most other automotive sensors, the environment in which it operates. ABS Wheel Speed sensors operate in harsh conditions; they have to cope with extreme vibration, water, temperature fluctuations and road grime and dirt. Also, the connecting wires have to cope with the demands brought by suspension and steering movements.

Q. I have fitted a new pd ABS Sensor but my ABS Light is still on?

A. If the vehicle’s ECU has been reset and there is still an ABS Light / Fault Code showing, please check the ABS Wiring Harness for breaks and/or resistance failures. A resistance check should be carried out from the ABS Module to the end of the ABS Wiring Harness Connector. We are seeing an increasing number of vehicle’s with damaged wiring harnesses / connectors.