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Understanding and Diagnosing the ADS System
ADS is a fantastic system designed by MB to adjust the damping level of the shock absorbers to help stabilize the car during high speed driving, aggressive driving, or if the driver loses control of the vehicle. The 1995 S600 W140 is equipped with ADS II. Several changes were made to the original ADS system to make it even better than it was. I am still in the process of determining when ADS was introduced in Mercedes Benz cars. There are some sources on the internet that state Mercedes Benz ADS (Adaptive Damping System) premiered on the S600 in 1995. For several reasons, I believe ADS came out sooner, because my vehicle has the ADS II system. I will continue to research this, and post any updated info I find.
I was not satisfied with the ride quality of my car. It should have been much softer than it was. It always felt too firm. I also could not feel a difference between theSport and Comfort modes, which pointed to a problem. To test your car, start it up, and select Sport mode. Push down on each corner as if testing the shocks. There should be give, but it should feel pretty stiff. With the car still running, select the Comfort mode. Again, push down on each corner of the car. You should feel much more bounce now. If you don't, you have a problem.
My saga to understand this system began last year. There is surprisingly very little information on the web about the details of this system, and a wealth of downright inaccurate info out there as well. This page attempts to describe the ADS system, how I diagnosed my vehicle's system, and what I ended up doing.
The suspension system consists of two shock absorbers in the front of the car, and two hydraulic struts in the rear of the car. Let's first go over the front shocks. The front absorbers are the traditional type we are all familiar with. They are oil-filled, and self contained. There are no external oil feeds or ports to them. However, there is a major difference between these shocks and a traditional shock absorber. These shocks have a valve assembly built into the top of them. The assembly consists of two separate valves that are controlled by electrical solenoids. These valves limit the amount of oil that can flow within the shock. By restricting the flow, the level of damping is changed. The more the oil is restricted, the firmer the shock becomes. The less the oil is restricted, the softer the ride becomes. Each valve has two positions, and together they provide four different combinations of damping. Both valves open equals softest ride, and both valves closed equals firmest ride. A combination gives a setting somewhere between firmest and softest.
The rear system is fairly complicated. There are a lot of components in the rear system. The rear "shocks" are not really shock absorbers at all. They are hydraulic struts. They are very similar in principle to the hydraulic rams you see on the front of construction equipment. These struts have a supply line feeding them with hydraulic oil pressurized by the power steering pump. There is a reservoir under the hood, under a removable cover near the left headlamp that holds the hydraulic oil. Check the oil level with the car warm, running, and at normal height. The cap is the dipstick.
The power steering pump is used for two systems, one for the steering, and the other for the rear suspension oil. The pressurized oil is sent to a valve near the rear of the car. This valve is attached to the under body of the car. The valve lever is attached to the rear axle. As long as the car is at the correct height, there is no movement of the arm relative to the valve. However, as soon as extra weight is put into the car, the body squats down. This motion causes the arm on the valve to move, and that opens the supply port for the pressurized hydraulic oil. The oil passes through the valve, and eventually makes it's way into the struts. Since oil is being forced into the struts, they begin to fill up and lengthen. This raises the car until the valve arm is back into normal position, which closes the supply of oil. This is how the vehicle maintains the proper ride height in the rear. When the weight is removed, the same thing happens in reverse. The valve arm moves in the opposite direction, which causes the valve to open a drain port. Oil is allowed out of the struts back into the system, and they get shorter again.Motion stops once the proper ride height is reached. There is a delay built into the valve, otherwise the car would attempt to adjust the height over every bump in the road.
We haven't finished yet, because with a system like this, there is no shock absorbing capability. Fluid does not compress, therefore a system like this would be unbearable to ride in. That is why there are nitrogen spheres, or accumulators as they are sometimes called. The red arrows are pointing to them in the diagram below. Note that this diagram does NOT show a system with ADS. The damping valves are not present in this system.
The accumulators are basically metal spheres with a rubber diaphragm dividing the sphere in half. The hydraulic line attaches to one side of the sphere. On the other side of the diaphragm is high pressure nitrogen. The gas in the accumulator will compress, so this is how bumps in the road are absorbed.
If your ride is too firm, the pressure spheres could be faulty. Over time the nitrogen will slowly leak out of them, or the diaphragms may rupture. If this happens, there is no longer any gas in the system to absorb bumps. You may see a large drop in hydraulic oil level in your reservoir. This is because the space previously occupied by the nitrogen gas has been filled with oil. Changing the pressure spheres is always a good place to start, as they usually need to be replaced around 100,000 miles. Unfortunately, there is no sure fire way to check them in the car without some pretty specialized equipment. If replacing your spheres does not fix the harsh ride, there is another problem.
The rear strut damping rates are adjustable as well. In the case of the rear though, the damper valves are not part of the strut. They are separate assemblies mounted to the body of the car. They are mounted in series in the hydraulic lines coming from the pump going to the pressure spheres.
A switch on the center console inside the car allows you to manually select "Sport" or "Comfort" modes of suspension. However, the ADS module will take over if it senses a need for different settings. Note that the solenoids must be energized to open. If they are not energized, they remain in the normally closed position. When the car is off no power is sent to the solenoids, so you would expect to feel very little give in the shocks. When the car defaults to firm, no power is sent to the solenoids. This fact will help us during diagnosis.
A popular misconception is that all four corners of the car each have a pressure sphere. This is not correct. Only the two rear struts use these pressure spheres. There is one for each rear strut. Some cars equipped with ASD (NOT to be confused with ADS) have a third pressure sphere. This is part of the Anti Slip Differential system, and is not part of the suspension system.
Now that we understand the basics of the system, we need to go over how the damper valves are controlled. There are a variety of sensors in the car that feed signals to the ADS computer module. There are wheel speed sensors, a yaw sensor, vertical acceleration sensors, a steering wheel angle sensor, brake pedal sensor and more that all provide input to the computer. The ADS module processes these signals, and energizes or de-energizes each damper valve accordingly. If for example you slam on the brakes, the vertical acceleration sensors and the brake pedal sensor send signals to the ADS module. The module tells the front damping valves to close, and the rear valves to open. This prevents the nose of the car from diving too much. Another situation can close the valves on one side of the car, and open them on the other side. This would be done in a hard turn to keep the car from leaning too much away from the turn.
ADS II is a very sophisticated system, and when it works right, it really is amazing. When it doesn't work right, it's very difficult to diagnose. Unless you have good electrical as well as mechanical knowledge, it would be best to let a good mechanic diagnose the system.
I discovered that pulling codes using a scanner gave questionable or faulty results. The system is analog, and my scanner is not capable of pulling codes from the system. For example, I kept getting a code for a faulty vehicle load sensor. There is no vehicle load sensor on the 1995 model year. So, any codes I retrieved were pretty much useless.
The first step is to see if the solenoids are supplied with ~12 volts with the car running and in Comfort mode. This is fairly simple to do. Each of the damping valve assemblies has two solenoids in it. They share a common 12 volt feed coming from the ADS module, but each solenoid has a dedicated return path back to the module. We need to tap into these wires to probe for voltage. The front solenoid wires are fairly easy to get to. To access the rear solenoid wiring, you'll need to pull all the trunk liner panels out.