Mercedes Active Body Control Description and Operation

Andrew Technician Commack, New York Posted  
Tech Tip
Chassis
1999 Mercedes-Benz CL500 5.0L (119.980) 5-spd (722.6)
C1525 (001) Critical Vehicle Level Front Left
C1525 (002) Critical Vehicle Level Front Right
C1525 (004) Critical Vehicle Level Rear Left
C1525 (008) Critical Vehicle Level Rear Right
C1531 (001) Left Front Suspension Strut Moves Although Locking Valve Is Closed
C1531 (002) Right Front Suspension Strut Moves Although Locking Valve Is Closed
C1531 (004) Left Rear Suspension Strut Moves Although Locking Valve Is Closed
C1531 (008) Right Rear Suspension Strut Moves Although Locking Valve Is Closed
C1525 (064) System Pressure Too Low

Active Body Control Description and Operation:

Applicable Vehicles:

230 SL-Class … : SL320, SL500, SL550, SL55 AMG, SL600, SL65 AMG)

215 CL-Class … : CL500, CL600, CL55 AMG, CL63 AMG, CL65 AMG)

220 S-Class …: S280, S320, S350, S430, S500, S600, S55 AMG, S65 AMG)

231 SL-Class (2012 and up : SL350, SL400, SL550, SL63 AMG, SL65 AMG)

216 CL-Class … : CL500, CL600, CL55 AMG, CL63 AMG, CL65 AMG)

221 S-Class … : S280, S300, S350, S400, S450, S500, S550, S600, S63 AMG, S65 AMG )

In 1999 Mercedes introduced an electronically controlled hydraulic suspension system called Active Body Control (ABC) which continues to evolve in the latest models (ABC+). The purposes of Active Body Control are to control the ride level of the vehicle for optimal comfort or performance and to limit excessive body movement from steering, braking, and accelerating. 

Body Control refers to restraining the yaw, pitch, and roll of the vehicle. The term Active is used to describe the system because it reacts to changes in body acceleration and cancels them out as little as 125 milliseconds. The ABC module monitors the vehicle’s vertical acceleration, longitudinal acceleration, lateral acceleration, vehicle speed, ride level, and strut compression. Using that data, the module calculates the ideal amount of damping for each corner of the suspension and controls the solenoid valves in the valve blocks to adjust the struts accordingly. The process continues in a loop as the module senses, adjusts, and controls the system dynamically, similar to the closed loop system of fuel control.

ABC is a hydraulic suspension system; hydraulic chambers are installed on top of struts with conventional coil springs and dampers. The chambers are used to compress the coil springs to change the effective ride height and to change the spring rate. The dampers in the struts and the coil springs with hydraulic control complement each other to cancel out vibrations from the road in the full range of frequencies.

Active Body Control is composed of a Pressure Supply and Circulation system, an Axle and Strut Valving system, an Electronic Sensing system, and an Electronic Controller (module). You can think of it as a Controller (The ABC Module) with a system of Inputs (body motion sensors) and a system of Outputs (Filling and Emptying Valves for the struts).

The Pressure Supply system begins with a reservoir of hydraulic fluid (Pentosin CHF11S) and the ABC pump on the engine. The ABC pump is built together with the power steering pump as one unit, but they are two separate hydraulic systems. The ABC pump draws fluid in from the reservoir and pumps it to a pressure distribution valve which limits the system pressure to about 200 bar. Pressure should generally read between 180 and 210 bar. A valve on the ABC pump throttles the inlet so that a good supply is available at low engine speeds and excessive flow is avoided at high engine speeds. 

Fluid flows from the pressure distribution valve to the front and rear axle valve blocks and circulates back to an oil cooler in front of the condenser and from the oil cooler to the reservoir. The pressurized oil is continuously flowing in the system to keep it cool and to provide fluid volume when the struts need to expand. 

When the vehicle travels over dips in the road the struts extend to maintain the level. If the struts extend sharply the volume of oil circulating in the system decreases and the system pressure could fall. For this reason, two accumulators (also referred to as pressure reservoirs) are installed in the hydraulic system at the Axle Valve Blocks. The accumulators, which look like cannon balls, are spheres with a diaphragm in the middle separating a pre-charge of pressurized nitrogen in the back and hydraulic fluid in the front. They are pressurized to 100 bar when new, but over time the nitrogen slowly leaks out and the accumulator loses its ability to manage rough road conditions. When the engine is started the pump fills them half way, compressing the gas and pressurizing them to about 200 bar. 

The Axle Valving system controls the passage of fluid into and out of each strut, adjusting it rapidly to meet the demands of the road and driver. You can think of the valve system like the valve used to operate older in-ground hydraulic lifts. In those lifts the shop’s compressed air supply acts as the ABC pump, providing pressure and flow. The lever is moved to direct the compressed air to act on the pistons of the lift and raise the vehicle. In the other position it can be used to exhaust the air and lower the vehicle. Similarly, the ABC valve block has a special solenoid valve that can adjust the restriction to oil flow into the strut over a variable range the same way you would adjust the water flowing from a faucet with the handle.

When the vehicle is parked there is no longer a pressurized supply of fluid to support the struts and the accumulators are not capable of supporting the vehicle. The axle valve blocks incorporate shutoff valves whose purpose is to block oil from escaping out of the strut. This way the vehicle can maintain its ride height when parked and left idle for a time. A very small amount of fluid still seeps out of the struts, so the vehicle will lose height over a week or two for example, this is normal. Additionally, the fluid may be very hot from a drive and as it cools down in the struts it can contract enough to slightly lower the suspension, this is a normal condition.

Diagnosis and Repair information will come in a separate post.

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