Case Study: Mercedes-Benz ABS lamp on: False activation

Sept. 29, 2022

This Mercedes came to the shop with the ABS lamp on, and at low speeds, it felt like the ABS was activating.

Related To: ALLDATA

Vehicle: 2009 Mercedes Benz C 230 4MATIC (204.085), V6-2.5L (272.911)

Mileage: 122,368

Problem: This Mercedes came to the shop with the ABS lamp on, and at low speeds, it felt like the ABS was activating.

Case Details: When the technician went on a test drive with his scan tool connected, he noticed that when driving at about 5-7 mph, the ABS system falsely activated. Also, the right wheel speed sensor (WSS) was displaying no speed.

He replaced the right rear wheel speed sensor and went on another test drive. The right WSS was still showing no speed.

An ALLDATA Tech-Assist consultant advised checking the tone wheel on the axle. They often get corroded and sometimes fall off. If the tone wheel was okay, the next thing to check would be the wiring and connectors from the WSS to the ABS module.

The technician inspected the tone wheel and found the right one was completely corroded.

Confirmed Repair: The technician replaced the tone wheel. On this test drive, the ABS light remained off and the brakes worked as designed. Problem solved!

Reprinted with permission from ALLDATA.

About the Author

Richard Diegle | Senior Automotive Editor for ALLDATA

Rich Diegle has been in the automotive industry since 1978. After graduating from a local community college, he began working at a Nissan dealership as a used car technician. Since then, he’s worked as a Nissan and ASE master automotive technician, shop owner, marketing copywriter, automotive editor, public relations manager, instructor, ALLDATA Tech-Assist team manager, and a senior automotive editor.