I am trying to locate steps to isolate my 1994 S500's ( W140 series chasis ) rear suspension sag problem 145k miles. (I have searched the archives and am trying to locate a shop manual for sale) I would like to go through ALL the steps to determine what is REALLY causing the problem, before I just start replacing parts willy nilly.

For example how do I determine if the hydraulic pump is even working, I would think that if the pump worked and is still full of fluid the shocks must be good otherwise why isn't the fluid leaking out onto the ground near the rear end etc.?

At your mileage, it would be a good idea to replace your nitrogen cells, which are a component of your self-leveling suspension. They are often the cause of a problem like yours, and even if they're not completely shot they're due to be replaced.

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo

You most likely need to get your air cells replaced as Michael mentioned above. . . This should solve the problem

The 'air cells' will not cause the rear end to sag. If they are 'flat', the car WILL ride very hard.

Either the hydraulic oil is gone (low oil=no pressure), the pump is not working, or the height corrector valve is not working.

Kebowers-

Many of the load-leveling equipped cars relied on that system to assist in maintaining static height, even unloaded. A slight sag could very well mean that the nitro cells are no longer holding up their end of the deal (literally). I was not aware of this until recently, and I have 2 cars so equipped.

Replace the nitro cells-simply can't hurt.

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo

Thanks for all the good suggestions so far, however what I really need is step by step instructions on thoroughly checking the system out.

For example, how do I determine if the pump is working at all?

If it is how do I determine if the level valve is working (and of course where is the level valve and what does it look like) etc.

I can only locate manuals for sale on microfiche otherwise I would read the manual.

where is the nitro cells located at?

Well, I have made some progress, I found the hydraulic system reservoir, nothing was showing on the dipstick, I put in two liters of the MB ZH-M fluid to bring it up to the full level.

I'm guessing that that means the system may have been run dry.

Can anyone tell me where the pump on the 140 chasis is located and how I can tell if it is still operating properly?

The pump is part of your Power Steering pump, it's a tandem design. Not sure on how you might test for damage?

I would be looking for possible leaks now, especially since you found the system to be 2 liters low. That fluid has to be going somewhere. Could two bad cells hold 2 liters of fluid????

I don't know how to specifically test the pump, but you could check leveling function by crawling under the car at the rear, look just right of the differential, you should see the valve. Disconnect the linkage (takes a 10mm open and a 9mm {Ithink, might be an 8}open to counter. Loosen the nut and remove one end of the linkage. Now with the engine running, pull the lever down about an inch, if working the rear should lift. If the direction of movement of lever is wrong, I reckon the back will lower.

The nitrogen cells will be in this same area, mounted up on the underside of the body near the shocks. Can't miss them - the size of a softball and black.

__________________
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

Well, here's the update (this is for other DIYers so you can gain from my experiance) on my 1994 S500 rear suspension. I bought two "reservoirs" this is what MB calls the "nitrogen spheres" and installed them.

Using the car jack I jacked up the passenger side to the jacks max lift and placed a jack stand next to it under the rubber lift bumper, set the brake and chocked the front wheels, this gives plenty of room to crawl under the car on your back.

I located the softball sized sphere on the passenger side (I'll call that the right side) next to and just to the right of the differential.
With a number 10 metric open end wrench I cracked open the smaller line and let it bleed the hydraulic fluid out and caught it in a shoebox size plastic bin. If you stay well to the left of the differential you will not get sprayed. As the pressure came down I gradually loosened the fitting untill no more fluid came out and then completely loosened the fitting. [edited to add that per "jcyuhn" there is a better way to relieve pressure: "The nipple on the leveling valve is there to relieve pressure in the system before servicing it. Use a small diameter hose to collect the oil into a container."]
Next with a number 17 metric open end wrench I loosened the larger fitting and unscrewed it completely (no more fluid came out).
Next remove the three nuts holding the sphere to it's bracket with a number 10 metric open end wrench (a better tool would have been a ratcheting flat spanner) these nuts are a pain as there is no room and you have to flip the wrench every 1/8 turn to loosen them. These nuts appear to be some type of self locking nuts and have to be unscrewed all the way with a wrench. The sphere than just comes right out with no problem.

Refit the new sphere exactly opposite of removal, I left the plastic caps on until the sphere was in place to keep dirt out and put the nuts on first to hold the sphere to it's bracket, but ONLY by finger so that it can move around as you attach the hydraulic lines. Be careful not to cross thread the hydraulic lines, you should be able to finger tighten them almost home.

[I did not move the jack to replace the next sphere, there is plenty of room under the car as it is with the passenger side up]

Next I located the softball sized sphere on the driver side, on the 94' S500 it is just to the right (passenger side) and above the last exhaust resonator about 1 1/2 feet forward of the bumper. Same process as before except that no more fluid came out, apparently all of it bleed out when I replaced the other sphere.

To remove the sphere after it is completely loose you have to remove the two bolts holding the rubber exhaust pipe mounts at the very rear of the car and pull the exhaust pipe down 2 inches ONLY with one hand while extracting the sphere with the other hand. Refit the new sphere exactly opposite of removal.

Next I located the level valve to the drivers side and behind the differential, put a clear plastis tube over the bleed nipple on top of the valve place the other end in my plastic bin and opened the valve a couple of turns. At this point I decided to flush out as much of the old fluid as possible and to do this you MUST start the engine. I let the fluid flow from the bleed tube (it flows very slowly) until the level in the reservoir in the engine compartment was ALMOST empty..

{{By the way this is a good time to remove the filter which is in the reservoir by unscrewing the small return pipe and unscrewing the large nut on top of the reservoir where you fill it. When you pull the insert out you will see the quite large filter, which by the way has a magnet in it to catch metal dust.}}

..and then refilled the reservoir with fluid to the top. When the fluid coming out of the bleed tube was clear, I shut off the engine, crawled back under the car and tightened the bleed nipple.

Remove the jack and then fill the reservoir in the engine compartment to the proper level.

By the way, total new fluid used was 3 liters, I had gotten four just in case. Total time was about 2 hours.

Well? How's the car ride now??

__________________
"If God had meant for us to walk, why did he give us feet that fit car pedals?" Sir Sterling Moss

Michael
2014 E63S Estate
2006 SLK55
1995 E500
1986 Porsche 944 turbo

The ride is much better, but I notice a thump on the passenger side rear when that wheel crosses a sharp bump that I do not hear when one of the other wheels goes over a similar sharp bump.

I'm going to search the archives before posting this possible new question on the forum.

FYI. The nipple on the leveling valve is there to relieve pressure in the system before servicing it. Use a small diameter hose to collect the oil into a container. Much neater than the alternative. One caveat - the height of the car may change when doing this, so don't be under there when you open it up!

Occasionally (rarely?) the shock abosrber on these fails internally. There are really 2 pistons linked together inside it, and if they become disconnected, they will 'thump' together going over bumps.

No fix other than replacement. A good part to get from the recycler's like Potomac 'cause they are $700 or so at dealer I think.