Understanding and Diagnosing the ADS System.doc

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 the Sport 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.

If you feel qualified and up to taking a crack at trouble-shooting the system, you will need to build a test break out harness like the one I made.

I'll be posting directions on how to make one of these sometime in the future. Suffice it to say it is not as handy as the MB tool, but it did the job.

Please note that you MUST be EXTRA careful when working around electronic components. Static electricity can destroy sensitive modules like these. Always ground (earth) yourself SAFELY and properly when handling electronics.

The test harness allows you to pull out the ADS module, and keep it hooked up to the car to do functional testing. There are terminals that plug into the sockets that the module plugs into, and these wires connect to the module itself. In addition, there is a connector that allows access to each terminal to take voltage readings. I will add the test procedure to this page sometime in the future.

There are points in the test procedure that require disconnecting the test harness. This was not feasible with my setup, as it was very labor intensive to disconnect and hook it all back up again. Because of this, there may have been tests that did not provide accurate outcomes. If I were to make another test harness, I would design it so that it could be easily disconnected from the car in order to perform the tests to the letter.

After exhaustive testing, I determined that all sensors and solenoids seemed to be working properly. The only item that did not react as it should have was the Comfort/Sport switch on the center console. I removed it (which was quite a chore in itself due to a previous owner bunging up the fasteners in the wood console piece), and tested it with an ohmmeter. It did not give the readings it was supposed to. I was able to take it apart and clean all the contacts. After this the switch worked the way it was supposed to. After reinstalling it into the car though, it did not fix my problem. The car was still defaulted in Firm.

At this point, one of my rear struts started acting up. It was creaking badly over every little bump, and clearly needed to be replaced. The rear struts are expensive, and I was reluctant to replace it with a self leveling ADS strut since the system wasn't working. I thought up some alternative plans.

Option 1:

Continue testing, swearing, and gnashing teeth to figure out what was wrong. 

Option 2:

For the price of one replacement strut, a complete set of Bilstein shocks could be bought. Would the Bilstein shocks fit? Probably, because all W140 chassis' are the same. I would have to somehow fool the ADS module into thinking that everything was OK in the system, otherwise I would get an ADS caution light all the time. However, I began to suspect that the rear springs in cars with self leveling were designed deliberately weaker than in cars without the self level option. When the struts are commanded to lower in my car, the rear sags quite a bit. This told me that replacing the rear struts with normal shocks would not maintain the proper ride height. The self level struts assist the springs in maintaining the correct height. I would also have to change the rear springs. This option was not ideal.

Option 3:

Buy the correct strut, and force the damping valves to the Comfort setting. This would maintain the ride height properly, and I would still have a functioning self level system. I would have to fool the ADS into thinking that everything was OK in the system. This was the best solution for me, as I did not want to take a chance on buying a new ADS module.

I found an online source for struts, and was able to buy one for $400 including shipping. I replaced the strut, and started to think about how to force the solenoids open for Comfort. To recap, the solenoids must both be energized to open them up. This provides a Comfort setting. What we have to do is feed the solenoids 12 volts while the car is on. This will put the suspension in Comfort mode. It is very important that the 12 volts be active only when the car is running, and that it is fused properly. Each solenoid winding has a resistance of 11 - 14 ohms. Using an average value of 12 ohms and Ohm's law, each solenoid will pull ~1 amp of current. If both shocks are forced open, then we have 4 solenoids x 1 amp each for a total of 4 amps. The rear shocks will also pull 4 amps total.

The big moment arrived to see if my concoction would work. I first pushed down on all the corners of the car again to get a feel for how it felt with the car off. I then started the car up, and pushed down again on the corners. Success! The shocks were clearly much softer, and I was able to get the car to bounce. There is definitely a noticeable difference between Sport and Comfort. If you cannot feel the difference in your car selecting between Sport and Comfort, it's defaulted to firm.

I actually tried this on one corner of the car first, before I attempted this on all four corners. I wanted to make sure this concept worked. I am happy to report that it works, and my ride is now the way it was meant to be. No more jarring over road irregularities.

There are disadvantages to this setup. A 95 mph cruise through slower traffic the other night was a little scary due to the fact that the car was in Comfort mode. At these speeds it would be a good thing to stiffen up the shocks. I felt the car wallowing a bit too much for my liking. My next project will be to modify the circuit to make the Comfort / Sport switch useable.

Also, the ADS module now thinks that the solenoids are not working, because the module is no longer connected to the solenoids. I bought eight 12 ohm power resistors capable of handling at least 1 amp. I will connect the 12VDC feed to one side of the resistor, and the return wire to the other side. I will do the same for the second return wire. This will be repeated for each damper assembly. I think this will fool the ADS module into thinking that there are working solenoids at each shock. This should turn off the ADS caution light that is always on now. I will be doing this once it cools off a little here in Chicago. It's too hot now.

Sometime near the end of this project, I received some interesting information from a friend who had learned that the ADS system  would default to Firm if the battery voltage dropped below 10.5 volts during engine startup. I checked to see if this was occurring in my car, and found that the battery dropped to 10.0 volts during startup. Apparently, the ADS system will not tell you that it is in default if this condition exists. It will not illuminate the caution light, even though the system is in default Firm. I will probably buy a new battery in the near future. When I do this, I will hook the ADS system back up the way it should be to see if this might have been my problem all along.

In the meantime, I'm enjoying my nice soft ride. I hope this helps you understand the system better. And if you find yourself struggling getting your ADS system to work, this option is a decent Plan B.

Check back in the future. I plan on putting updates on this page to share any new discoveries.

Update, August 7 2005

I bought a new battery to see if this would cure my ADS problem. To recap, there is a possibility that should the battery voltage drop below 10.5 volts during start, the ADS system goes into default Firm without giving a caution light. I rewired all damper valves back to the original hookup, and tried it out with the new battery in place. The battery voltage dropped to 11.0 during start. It was 10.0 with the old battery. Although the new battery kept the voltage comfortably above 10.5 volts, I quickly saw that the ADS system was still in Firm mode. The Comfort / Sport switch still had to effect. This was very disappointing. I can only conclude that my Comfort / Sport switch is still not functioning properly, or that the ADS module itself is defective. Oh well, back to hard-wiring the solenoids open.

This time though, I wired in my power resistors in an attempt to fool the ADS module into thinking the damper valves were still hooked up. With the wires coming from the ADS module cut, the module recognizes that there is a problem with the system. It can see that all the solenoids are "open", and illuminates the ADS caution light on the dash. The resistors should fool the ADS module into thinking that the solenoids were intact, and prevent the caution light from illuminating.

Once all the resistors were wired and secured, I started the car up to see if I still got an ADS caution lamp. No light! Success! Now I won't have to look at that caution lamp anymore.

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