ABC
ABC was standard on all S-600, S-55, S-63, and S-65 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.
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Active Body Control (ABC) Fitment
Active Body Control (ABC) is fitted on vehicles with SA Code 487a.
Active Body Control (ABC) 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.
Active Body Control (ABC) Pulsation Damper and Filter DIY Repair
http://www.benzworld.org/forums/w220-s-class/1627917-abc-pulsation-damper-filter-diy-repair.html