Wheel Speed Sensor: Testing and Inspection
WHEEL SPEED SENSOR OUTPUT VOLTAGE CHECK1. Lift up the vehicle and release the parking brake.
2. Disconnect the ABS-ECU connector, and then use the special tool (MB991219) to measure the output voltage at the harness-side connector.
3. Rotate the wheel to be measured at approximately 1/2 - 1 rotation per second, and check the output voltage using a circuit tester or an oscilloscope.
Output voltage
When measuring with a circuit tester: 50 mV or more
When measuring with an oscilloscope: 120 mV p-p or more
4. If the output voltage is lower than the above values, the reason could be as follow:
- Faulty wheel speed sensor. So replace the wheel speed sensor.
Inspecting Waveforms With An Oscilloscope
Use the following method to observe the output voltage waveform from each wheel sensor with an oscilloscope.
- Start the engine, and rotate the front wheels by engaging 1st gear (vehicles with manual transmission) or D range (vehicles with automatic transmission). Turn the rear wheels manually so that they rotate at a constant speed.
NOTE
1. Check the connection of the sensor harness and connector before using the oscilloscope.
2. The waveform measurements can also be taken while the vehicle is actually moving.
3. The output voltage will be small when the wheel speed is low. Similarly, it will be large when the wheel speed is high.
Points In Waveform Measurement
NOTE The wheel speed sensor cable moves following motion of the front or rear suspension. Therefore, it is likely that it has an open circuit only when driving on rough roads and it functions normally when driving on smooth roads. It is recommended to observe sensor output voltage waveform also under special conditions, such as rough road driving.