First a quick history lesson on the MB hydraulic suspension.
Accumulators or Spheres as there sometimes called are the "springs" of most of the 140's rear suspension. The ride leveling system was installed on all 500's and 600's, optional on 420's and not installed in 300/320's - at least in the States. I've heard second hand that it was optional on 500's for most other markets. There are two different systems installed on these cars. The basic is a rear ride height control only and this system has no center console control switch- the system is transparent to the driver. This system has two accumulators. The other system has three accumulators and can be identified by a small switch on the enter console with a pictogram of a shock absorber. This system is called ADS or ADSII (Adaptive Damping System) depending on the model year. Basically this allows you to firm the suspension on all four corners. This system also controls the oil flow in and out of the front shock as well. This article deals with the two accumulator system because that's what I own- a 1997 S500 with ride height control. If you have ADS, the procedure is substantially the same- your just replacing three accumulators.
After about 80,000 miles the ride in these cars starts to deteriorate. Bumps that once went unnoticed now start to jar your hind teeth. When you notice that your starting to avoid manhole covers- it's time. What happens is the Nitrogen pre-charge starts to leak out. In a new accumulator, the pre-charge is set to 155bar- that's 2279psi! When the car hits a bump- hyd fluid is forced out of the shock and into the accumulator via a hydraulic line. This compresses the nitrogen gas inside the sphere- as the suspension expands on the back side of the bump the pressure inside the accumulator forces the fluid back into the shock. As the accumulator ages, the pre-charge leaks out and the void is filled with incompressible hydraulic fluid. This is where the ride stiffens up. When all the pre-charge is gone you basically have a hydraulic lock and the suspension stops working. My mechanic says he's seen cars come in where you can't compress the rear suspension even by jumping on it. It would be funny except that if you let your suspension go that far you risk damaging the hydraulic shocks- there about $870.00 a piece to replace. The accumulators are about $100.00 a piece to replace. You do the math.
In the two accumulator system, they're located approximately under the rear seat- tucked up and bolted to the underside of the floorpan. The driver's side (left-hand drive) accumulator is obscured by the exhaust system - not a problem. On cars equipped with xenon headlights, the right accumulator is blocked by the headlight level control. Simply unplug the electrical connector and remove the controller. Here's a few drawings from ALLDATADIY.com located at the bottom of the text.
On to the messy part!
Regarding the accumulators, the job is pretty easy, if not messy. Jack up the rear. If you don't have two floor jacks, try this- chalk the front tires and use the supplied tire changing jack to lift the car. Place a jack stand under the lift point (rubber doughnut) and set car on the stand. Move to the other side and repeat. Not ideal but it works. Please be careful! You might want to even "chalk" the car jack as it contacts the ground at an angle and on a slippery garage floor- well, it could slip. Bleed off as much pressure as you can using the nipple on the level control valve. I stuck some clear plastic tubing on to the nipple and cracked it open to let the hyd fluid and foam escape (brown foam means your accumulators are indeed bad). Once this is done you can remove the doughnuts or hangers that attach the exhaust system to the under side of the car. I think there were two on either side of the muffler and one just aft of the cats. I just unbolted them from the floor pan. This allows the system to hang down just enough to remove the driver's side accumulator. To remove the accumulator, simply remove the two hyd lines that attach to the accumulator and then remove the three mounting bolts. Prepare for an unholy mess at this point. Despite all my best efforts and warnings from other people I still dropped about a quart and a half of oil on the garage floor. Both sides require some finagling to get the accumulators out of their hiding spots but they will come. Some people have had trouble removing the hyd lines from the accumulators. This seems to be more of a problem for cars located in areas that use road salt. A good flare nut wrench is what is required here. This wrench looks like a standard box end except it has a small gap at the end that will allow you to slide the wrench over the hydraulic line. This wrench will help prevent damage to the soft metal nuts that hold the hydraulic lines to the accumulators. It should be noted that I was able to remove the lines with a standard wrench with no problems. Also be very careful not to cross thread the nuts onto the new accumulators. The hydraulic lines are stiff and if you don't line them up perfectly, you can cross thread the nut. This proved to be the most difficult part of the task. I had one on the right that would absolutely refuse to thread correctly. Patients, a beer, and another whack at it did the job.
Once both accumulators have been replaced I followed ALLDATADIY.com's instructions for filling and bleeding the system. Here's a reprint:
FILLING:
Pour oil into the oil reservoir
Only reuse clean oil
Set level controller lever to "Fill" position (F)
(loosen the ride height linkage- this will allow you to move the lever between FILL and EMPTY)
CAUTION
Risk of accident due to the vehicle starting off automatically when engine is running
CAUTION
Risk of injury due to bruising or burns when intervening while starting the engine or when the engine is running
Start engine, allow to run at moderate speed for approx. 60 seconds
The system bleeds itself automatically
Switch off engine
CAUTION
Ensure that there is sufficient oil in the oil reservoir
The pump must not suck in air under any circumstances
Set level controller lever (arrow) to position "Empty (L)"
After approx. 60 seconds, attach connecting rod or connecting linkage (7) to level control lever (arrows) NOTE:
Replace self-locking bolts and nuts
Place vehicle on its wheels and press down firmly several times.
The vehicle level adjusts itself
Check and correct oil level in oil reservoir
Be careful not to over fill the reservoir. I got a little too concerned about running the pump dry that I overfilled the system. Remember that when you move the lever to FILL, your filling the accumulators and raising the suspension. This will lower the reservoir. When I saw the low level ( lever still on FILL) I topped it off. Oops! When I moved the lever to EMPTY (engine off) The accumulators purged their oil and it flowed back into the reservoir. I heard this strange squirting sound. There was oil all over my garage wall. It had squirted out the overflow which thankfully had been pointing toward the front of the car and not into the engine compartment. Nothing bad happens if you overfill the reservoir- it's just messy and embarrassing.
Good luck!
Paul
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