Vehicles with automatic level control that ride harshly in the rear (no give in the suspension) may have one or both dampening actuators which have failed. It is commonly recommended the actuators be replaced in pairs.
Car Problem Reports
Mercedes-Benz S500 Bad Dampening Actuators Can Cause Rough Ride in Rear
Mercedes-Benz S500 Problem
Visitor, 1997 Mercedes-Benz S500, 91,000 mi
rear suspension stays down shock are good i think pump not working no light on dash 4 the ride control