Mercedes-Benz W140 ABS and ETS light on: wheel speed sensor
W140 ABS and ETS lights come on - what is the most common cause?
The most common cause is a wheel speed sensor or its wiring. ABS and ETS/ASD use the same sensor data, so one fault can turn on both lights and also drop out traction control and cruise control. Sensors weaken with age.
How do I find a wheel speed sensor fault?
First read the fault codes to see which wheel is at fault. The sensor wire can be broken inside even if the insulation looks intact, and it can break when the steering wheel is turned. Also check that the wire is not rubbing against the brake disc. The sensor resistances can be measured at the control unit connector.
What else should be checked before replacing the sensor?
Check the charging voltage and battery condition, because low voltage can cause the same lights. After brake work the sensor may be damaged or dirty. Brakes are safety-critical: have a workshop check the result of the work.