Wheel Speed Sensor: Testing and Inspection
WHEEL SPEED SENSOR OUTPUT VOLTAGE MEASUREMENTRequired Special Tool:
MB991970: ABS Check Harness
1. Release the parking brake and lift up the vehicle.
2. Disconnect the connector A-02, and connect special tool ABS Check Harness (MB991970) to the wiring harness-side connector.
NOTE: Do not connect special tool ABS Check Harness (MB991970) to the ABS-ECU.
3. Measure the voltage between the relevant signal and ground terminals in the wheel speed sensor circuit and body ground.
4. Manually turn the wheel to be measured 1/2 to 1 turn/second. Measure the output voltage with a voltmeter or oscilloscope.
NOTE: Check the connection of the sensor harness and connector before using the oscilloscope.
Output voltage:
^ Minimal voltmeter reading: 42 mV
^ Maximum voltmeter reading: 300 mV
^ Minimal oscilloscope reading: 120 mV
^ Maximum oscilloscope reading: 600 mV
Probable causes of low output voltage
^ Wheel speed sensor pole piece to wheel speed rotor clearance too large
^ Faulty wheel speed sensor
5. To observe the waveform with an oscilloscope:
^ Front Wheels: Shift into "D" range and drive the wheels.
^ Rear Wheels: Turn the wheels manually at a constant speed
NOTE: The output waveform is low when the wheel speed is low. Similarly, it will be higher as the wheel speed increases. Waveform may also be observed by driving the vehicle.
POINTS IN WAVEFORM MEASUREMENT
NOTE: The wheel speed sensor cable moves in relation to motion of the front or rear suspension. Therefore, it is likely that it has an open circuit only when driving on rough roads but it functions normally when driving on smooth roads. It is recommended to observe sensor output voltage waveform also under special conditions, such as driving on a rough road.