Wheel speed sensors are an important component necessary for systems such as ABS and ESP. Sensor data is also important for chassis control systems, hill start assistants and other similar systems. The task of the sensors is to provide information about the current speed of each wheel separately. In its simplest variant, the ABS controller counteracts wheel locking by reducing the braking force when a wheel stops. In more complex ESP systems, the controller additionally detects loss of traction when starting or accelerating and uses data from other sensors (steering wheel turn, acceleration, vehicle rotation) to actively counteract skidding and maintain the desired trajectory.
As we can see, the performance of these inconspicuous elements has an impact on driving safety. There are two basic variants of ABS sensors: active sensors and inductive sensors. They differ both in construction and operation, as well as in diagnostic methods. Therefore, it is useful to be able to initially identify the type of sensor.
Inductive (reluctance, or passive or passive) sensors were the first to appear. This element has a simple structure – it consists of a magnet, which is the core of the sensor, and a coil. The sensor and the rotating gear wheel (pulse ring) form a magnetic circuit. The rotational movement of the pulse ring causes a change in the magnetic field in the coil. The alternating magnetic field induces an alternating voltage in the winding, close to a sine wave, which is the output signal from the sensor. 
Active sensors (also referred to as Hall sensors) are electronic circuits integrated with a magnetic field sensor (which can be a Hall or magnetoresistive sensor ). The sensors themselves are not suitable for use with a controller, so they are connected to an integrated electronic circuit (usually a small integrated circuit in the sensor housing). If the sensor is to work with a steel pulse ring, then a magnet that generates a magnetic field is placed next to the Hall effect sensor. Magnetoresistive (magnetoresistive) sensors, on the other hand, work in cooperation with a multipole magnetic ring. The main advantage of Hall and magnetoresistive sensors is that they do not require a minimum wheel speed to operate. They measure the signal practically from the start of the vehicle. This is because the ABS sensor system generates a rectangular signal with fixed levels depending on the detection of a magnetic field. 
The customer usually arrives at the service center after the ABS or ESP light on the dashboard comes on. But ABS sensor failure manifests itself not only by activating the ABS light, but also by locking the wheels under hard braking. ABS sensors at the same time are among the components that are quite susceptible to damage due to their location. Proximity to the road increases the risk of dirt, moisture or mechanical damage affecting both the sensor itself and the mating impulse wheel. 
The method of proceeding depends on the type of sensor. If we do not know which sensor we are dealing with, we can measure its supply voltage. To do this, we disconnect the sensor from the controller and measure the voltage on the plug side of the controller. If the sensor is not powered – we are dealing with an inductive sensor (of course, remember that the cause can also be a broken circuit or a defective controller). First, of course, we need to answer the question of where the ABS sensor is, and more specifically locate the sensor connector.
Depending on the vehicle, longer or shorter connection cables are used, and sometimes the sensors have a connector in the housing (such as the golf V abs sensor). If we use the QST-5 sensor tester, we do not need to determine the type of sensor before the measurement, the tester will do it for us. It is enough to connect the sensor itself to the tester after unplugging (the way of connection is arbitrary).
We can check the inductive sensor by measuring its resistance with a multimeter. The result should be similar for a set of sensors. We can theoretically measure its inductance (the sensor is a coil), but not every universal instrument offers such a measurement, and the value itself can be large (on the order of 1 H or more). If we have the QST-5 we can do better – not only will we know the resistance and inductance of the sensor coil, but at the same time we can read the parameters of the output signal. If you turn the wheel during the test, you will see the current and maximum voltage amplitude and frequency indications. In this way, we can compare whether the sensors perform similarly. 
In the case of an active sensor, without additional tools we will not do much. At most, we can verify the supply voltage – see above. We won’t check such sensors with a multimeter – first of all, we won’t know if the sensor is working anyway, and if the meter in question takes measurements at higher voltages, we may even damage the sensor. We can check the active sensor with the QST-5 device. We can not only observe the sensor signal, but also count the pulses per full revolution. To do this, simply mark the position of the wheel and turn it exactly a full revolution during the test. We read the number of pulses from the screen of the device. 
It is worthwhile to gain a basic knowledge of ABS sensors, because it has a concrete impact on the work in the workshop. Often these components are not cheap, especially when integrated into the wheel hub. Better knowledge of the subject means more accurate diagnoses and fewer unnecessarily listed items. Remember that ABS and ESP systems have a significant impact on driving safety. Replacement of the ABS sensor and other system components should be done in accordance with the manufacturer’s service manual. Author: Product Engineer Piotr Libuszowski
DeltaTech Electronics to polski producent i ekspert w branży automotive, który na bazie ponad 25 lat doświadczenia wyznacza trendy w innowacyjnej diagnostyce samochodowej.
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