ABC

Active Body Control (ABC) is fitted on vehicles with option code 487 - ABC SUSPENSION (ACTIVE BODY CONTROL).

ABC (Active Body Control) was standard on all S600, S55, S63, and S65 models. You can tell which suspension you have by opening the hood of your car and examining the top of the shock absorber. If the car is equipped with Airmatic, you will have obvious air lines leading from the top of the shocks. If your car is equipped with ABC, you will have small black electrical plugs leading into the top of both shocks.

Vehicle with ABC.

Vehicle with ABC (shock absorber).

Vehicle with Airmatic.

Vehicle with Airmatic (shock absorber).

Issues

The Active Body Control (ABC) suspension pulsation damper will cause a humming / buzzing noise when not working correctly. If your dampener needs replacing you will hear a howling sound that peaks at 2000 RPM then disappears above that RPM. M-B has a service bulletin about this. See TSB Topic number LI32.50-P-048280 for more detailed information.

Pulsation Damper and Filter DIY Repair

http://www.benzworld.org/forums/w220-s-class/1627917-abc-pulsation-damper-filter-diy-repair.html