Replacing Accumulators (a.k.a. Pressure Spheres, Spring Actuators)

The rear suspension system in a S600 is fairly complicated. There are a lot of components in the rear system, and in fact, this seems to be one of the least understood systems of the W140.  If your ride is harder than you think it should be, a good place to start is by changing the Pressure Spheres. I suggest that you first understand the system before diving in. I have taken some of my text that describes the system from my ADS Page and copied it here.

The rear "shocks" in W140's equipped with the self-leveling option 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. Read the details on my ADS Page.

Keep in mind that this DIY article is for a W140 with ADS and Self Leveling.

Tools and Materials Needed:

• 1/4" ratchet drive
• 10 mm socket
• 10 mm box wrench
• 11 mm crow's foot line wrench
• 17 mm line wrench
• 19 mm line wrench
• "Custom" 10 and 11 mm box end wrenches referenced in article. 
• Heavy Duty car ramps
• Good Penetrating Oil (not WD40)

Parts Needed:

• Hydraulic Suspension Fluid, P/N 000989910310 (comes in 1 liter bottles)
• Left and Right Nitrogen Chambers, P/N 140 328 05 15
• Possibly steel pressure lines connecting components, if corrosion requires cutting lines to disassemble.

Time Required:

• Lost my records, I estimate anywhere from 2 to 8 hours. Less time for cars without corrosion, more time for cars with corrosion.

Please use caution during this project. The rear suspension oil is under lots of pressure. The oil could spray out under high force. It is important to follow the instructions carefully. You should be wearing safety glasses at all times. If you don't usually wear them, make an exception for this project! This is also one of the messier projects, and the oil smells unpleasant. I would suggest wearing latex gloves to keep the oil off your hands.

I don't have many pictures of this project, as I did not want to handle the camera with oily hands.

Update:

Our friend and fellow W140 owner Gary Siebring had success using the following procedure, but he modified it slightly...he took all the weight off the rear suspension by putting the car on jack stands.  That took all the pressure off the hydraulic system and he didn't get one drop of oil on his head. One other thing he did that should be a part of the following procedure is to soak all line fittings and connections with Penetrating Oil. He soaked his lines for three days, and all of his fittings loosened up. I strongly recommend you do this, as replacing lines is time consuming, difficult, and an expense to be avoided.

Drive the rear wheels up onto the ramps.
Loosen the two ?? mm bolts on the exhaust hangers, and let the exhaust rest on something to keep it from bending. Lowering the exhaust provides a bit more access to the left accumulator.	No photo available
Alldata says to remove the spare tire cover. I did not do this. I suspect it would have saved me a lot of grief, as I had a heck of a time getting to the right accumulator. In fact, I had to remove some of the accumulator mounting bracket to gain access to the pressure lines.	No photo available
The rear system needs to be depressurized. This step requires lots of attention and awareness on your part! Use caution! If you have the car on ramps, the car will lower! Make sure it doesn't lower onto you! There is a ?? mm bleeder screw on the leveling valve. Spray some penetrating oil on it. Place a length of small diameter rubber hose over the bleeder screw nipple. Route the end of the hose into an old coffee can. Slowly crack the bleeder screw open to depressurize the system. Keep your body parts out of any pinch points, because the car may begin to lower. My bleeder screw did not want to break free. I was afraid to force it, as the bracket that the valve mounts to is not very sturdy. I left it alone, and bled the system later by cracking a line open. Don't do this yet.
If your bleeder screw is seized, do the following. Mark the linkage connecting the axle to the body with a permanent marker so you can set it to the same length again. It is important that this setting is repeated, because this affects the rear ride height. Loosen the 10 mm nut on the linkage arm between the axle and the body. Once the nut is loose, you will be able to pull the valve arm to the DOWN position. This opens a port which allows the oil out of the rear struts. The car will lower, so be careful!
Have a catch pan ready. Crack the small pressure line S1 (see figure below) going to the accumulator using an 11 mm line wrench.  Update: Gary modified a cheap 11 mm box end wrench by bending an offset into it, and cutting an opening into the box end as shown making it a line wrench. Heat it good and hot before bending it.       This allowed easier access to the S1 line fitting. Again, use caution, if the car is on ramps, the system is still pressurized! You will see oil leaking out of the attachment point. Don't be surprised if the oil is very dark and foamy (not unlike Guinness beer). In fact, it's a good sign that the accumulators are bad. Once the system finishes hissing and spitting, continue to loosen S1until it can be disconnected from the accumulator. Update: Another MB friend, Steve Urban, was successful using a 11mm crows foot line wrench. You can try this instead of making the above tool.
Loosen the larger line P4 at the accumulator using a 17 mm line wrench. If your car is equipped with the ADS feature, loosen P4 at the damping valve using a 19 mm line wrench. Update: Gary modified a cheap 10 mm box end wrench by bending an offset into it. Heat it good and hot before bending it. He clamped a Vice Grip pliers on the open end. This gave him the extra leverage he needed to break the three 10mm nuts loose.   This is where I ran into trouble. The P4 lines between the accumulators and damping valves were seized. I had to cut the old lines, and reorder new lines. Maybe if I had let the lines soak this wouldn't have been a problem. Left side is part number 1403200753 Right side is part number 1403200853 Loosen the three 10 mm locking nuts 4f holding the accumulator to the mounting bracket. Be careful once you get the last nut off. Your hands will be oily, and the accumulator will slip from your grasp, and your forehead will cushion the accumulators fall! Don't say I didn't warn you.
I was curious what the inside of the accumulators looked like. So I drilled a small hole into the dome to relieve any pressure that might still be in there (Safety Glasses!), and cut it open with a hack saw.

Install everything in reverse order. MB states to replace the locking mounting nuts. I reused mine after I cleaned them thoroughly, and placed a drop of Loctite on the threads.
Use caution when reinstalling the small S1 pressure lines. It is difficult to get them lined up properly. Take the time to make sure they are threaded in straight. If you cross-thread them, you'll be buying new accumulators and lines.
I could not find torque specifications. Obviously, the pressure lines need to be torqued quite a bit to ensure a tight seal with no leaks.
Now the system needs to be cycled to get the air out of the lines. Start the car, and check the oil level in the reservoir. Add oil if necessary. Move the arm on the leveling valve UP towards F. This opens the oil supply to the struts. BE CAREFUL, as the car may slowly rise up. It may take some time to see the car start to raise. Be patient. In addition, once the car does start to raise up, it does so slowly. Remember that all the lines and the struts have to fill up with oil again. Also watch out for hot exhaust pipes. Then move the arm DOWN to L. After a short time, the car should lower. AGAIN! Make sure you don't squash yourself. Do this several times to bleed the struts of any air. Recheck your fluid level often (engine running).
Once the system is completely purged, line up the marks on the linkage again, and tighten the 10 mm nut. This will set the ride height to the proper level.
Double check that there are no leaks anywhere, and verify one more time that the oil level is at the correct level. The old accumulators can be disposed of as scrap metal. Before throwing them away, drill a small hole in the dome to relieve any pressure that might still be inside (Safety Glasses!). This is for the safety of anyone handling them later.
Good Job! Your ride should now be as smooth as glass!

Thanks Gary for sharing your tips and experience!

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