Note that there are two types of wheel speed sensors, which can cause ABS failure if used incorrectly.

A Volkswagen Golf Car was repaired for rear-end collision, but it was found that the ABS warning light was on. At this time, the wheel anti-lock function was invalid. After repeated checks and repairs, it was found that two kinds of wheel speed sensors were in use at the same time in Volkswagen. If they were misfitted, ABS wheel anti-lock and ESP body stability control functions would fail. Here's a reminder to the repairman and fellow drivers.

Fig. 1. Golf instrument ABS and anti-skid control warning lights

There are two kinds of wheel speed sensors in Volkswagen Vehicle System. One is the traditional electromagnetic induction type, which is commonly called the "passive wheel speed sensor" in the industry. There are permanent magnets and coils inside. The signal generating disc is gear shape. When the gear rotates, the electromagnetic coil will feel when the top of the tooth and the root of the tooth rotate close to the sensor. Low pulse voltage should be used to detect the speed of wheel rotation. This kind of sensor has low precision and high quality.

Only one kind of wheel speed sensor is Hall type. It uses the principle that the electron flow is acted by Lorentz force in the permanent magnetic field. Hall voltage will be generated on both sides of Hall wafer to send out signals. In the industry, it is often called "active wheel speed sensor", which can sense signals of extremely high speed. The amplitude of signal voltage is very stable. In modern automobiles, a kind of speed or position sensor is often used.

Fig 2. Electromagnetic induction wheel speed sensor

Originally, it was the wrong wheel speed sensor in maintenance. Hall wheel speed sensor was installed in the repaired rear wheel, but the other three wheel speed sensors of the original car were electromagnetic.

In fact, the circuit of these two sensors can be distinguished. After pulling out the wheel speed sensor, turn on the ignition switch and measure the two ends of the computer side. If there is a 10 volt voltage, the Hall wheel speed sensor should be installed. If it is zero volt, the wheel speed sensor of electromagnetic induction type should be installed, because the electromagnetic sensor itself can send out electric pulse signal, which belongs to "active sensor" and does not need external power supply.

Measuring the resistance of the sensor can also be distinguished. The coil resistance of the electromagnetic sensor is about hundreds to thousands of euros, while the internal resistance of the Hall wheel speed sensor is infinite.

Fig 3. Hall wheel speed sensor

ESP body stability control system is based on ABS wheel anti-lock control system. That is to say, only if ABS system is good, ESP system can work. Because of the wrong wheel speed sensor, ABS system can not work. Of course ESP system can not work properly, so both ABS warning lamp and ESP warning lamp are lit up. The functions of the wheels they control also fail.