Wheel speed sensor - How It Works

General points

Wheel speed sensors are located near wheel hubs or differentials and are used to determine the speed of the outer wheel rim. They are used in ABS, ASR and GPS systems. If the systems are combined the antiblocking system provides the wheel rim speeds via data cables to the other systems. There are Hall-type sensors and inductive sensors. Before testing, it is essential to find out which type of sensor is involved (technical data, parts catalogue).

Function

The rotary movement of the sensor ring mounted on the drive shafts causes changes in the magnetic field in the sensor. The resulting signals are sent to the control unit and evaluated. In the case of the ABS system, the control unit determines the speed of the wheel rim which is used to determine the wheel slip, thus achieving an optimum braking effect without the wheels locking. 

Effects of failure

When one of the wheel speed sensors fails, the following system features are noticeable: 

■ Warning light comes on 

■ A fault code is stored 

■ Wheels lock during braking 

■ Failure of further systems

There can be a number of reasons for sensor failure: 

■ Internal short-circuits 

■ Interrupted cables 

■ Cable short-circuit 

■ Mechanical damage to the sensor wheel 

■ Soiling 

■ Increased wheel bearing free play

Troubleshooting

■ Read out the fault code 

■ Check electrical connections of the sensor cables, the connector and the sensor for correct connection, breaks and corrosion. 

■ Watch for soiling and damage

Troubleshooting with wheel speed sensors is difficult with regard to distinguishing between Hall-type and inductive sensors, since these cannot always be distinguished from one another on the basis of what they look like. Three connector pins do not allow exact assumptions about the respective type involved. The specific manufacturer specifications and the details in the spare parts catalogue have to be consulted here. As long as it is not absolutely clear what type of sensor is involved, an ohmmeter must not be used for testing, since this could destroy a Halltype sensor. If the sensors have a 2-pin connector fitted, they will probably be inductive sensors. In this case, intrinsic resistance, a ground connection and the signal can be determined. To do this separate the connector and test the internal resistance of the sensor using an ohmmeter. If the internal resistance value is 800 to 1200 ohm (depending on the reference value) the sensor is OK. If the reading is 0 ohm there is a short-circuit and MOhm indicates a cable interruption. The ground connection test is carried out using the ohmmeter from once connection pin to vehicle ground. The resistance value has to tend towards infinity. The test with an oscilloscope must result in a sinus signal of sufficient amplitude. In the case of a Hall-type sensor only the signal voltage in the form of a rectangular signal and the supply voltage must be checked. The result must be a rectangular signal depending on the wheel speed. The use of an ohmmeter can destroy a Hall-type sensor

Installation note

Make sure of the correct distance to the sensor wheel and sensor seat.