Mercedes-Benz W202 ABS light on: most common causes and measurement
What usually causes the Mercedes-Benz W202 ABS light to come on?
Common causes are a faulty wheel sensor or its wiring (the front sensors break more easily), a fault between the sensor and the ABS ring, the YJ relay (overvoltage protection relay) and the brake light switch. Low battery voltage can also turn the light on. Read the fault codes first, and do not replace parts by guessing.
How is an ABS wheel sensor measured?
Measure the sensor's resistance with a multimeter at the sensor connector. The resistance of a front sensor should be 850–2300 ohms and a rear sensor 600–1600 ohms. A broken sensor has infinite resistance. If the value is within the limits, the fault may be in the wiring or in a dirty sensor tip.
What is the YJ relay and why does it affect the ABS light?
The YJ relay is an overvoltage protection relay that also feeds power to the ABS system. Its solder joints fail, and then the ABS light comes on and other warning lights may light up too. The relay is in the engine bay on the front passenger side under a cover. Check it first; a new relay is surer than repairing the solder joints.
Why does the ABS light come on at high speed after changing tyres?
If the tyre size differs between front and rear, the rolling circumferences differ. The system interprets the difference as wheel slip and turns the light on as speed increases. Use the same tyre size at every corner. Brakes are safety-critical, so have a workshop check the repair.