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  #1  
Old 06-22-2002, 02:43 AM
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560SEL rear suspension

1991 560 SEL, ride is quite bouncy in the rear. I have gotten a few suggestions and I am confused to say the least.

I had decided (per suggestions) that I need to replace the acuumulators. But, was also told that if they are bad the car will be sagging in the rear. It doesn't appear to sag to me. You tell me: http://personal.atl.bellsouth.net/lig/d/n/dnt0512/1991.htmL

I was also told that if the car bounced up and down when I pushed on the rear end that I needed to go through a very expensive replacement of the rear shocks. The car does bounce up and down a few times when pushed on.

My hydraulic tank is 1/2 full and it appears that the filter needs replacing. I understand the replacement of the accumulators is not a very hard job but could anyone provide a description of this process? I order the manuals on CD but they do not help at all. Anyone know where I can get a manual similar to a Haynes or Chilton manual for repairs and maintenance?

Can anyone tell me what I should check and in what order if it were yours? This is my first MB and everything else is in wonderful shape and I love this car! I understand from other posts that I will love it even more when I correct this problem.
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1991 560SEL
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http://www.bellsouthpwp.net/d/n/dnt0512/1991.htmL
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  #2  
Old 06-23-2002, 10:31 AM
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Here some things to know on the self leveling suspension. The rear end is supported partially by the springs and partially by the rear struts.

The strut pressure is controlled by the the level-control unit which is physically located near and attached to the rear sway bar. Under no-load and low-load the level control unit maintains the struts at a nominal pressure to (1) partially support the rear end and (2) allow normal dampening within the struts. It's important to realize that each strut is connected to a nitrogen filled accumulater "ball"; the hydraulic fluid is seperated from the nitrogen by a flexible membrane and it's the movement of the membrane that provides the "shock absorption". But, fluid dampening occurs inside the strut similar to any other "shock".

Under heavier loads the level control unit lever is deflected and increases the system pressure whic counteracts the load, raising the rear end. Dampening and shock absorption still function as I just described. Once the load is removed, the level control unit is supposed to bleed the pressure down to the nominal system pressure which lowers the vehicle to normal height.

There are a lot of thing to fail. If the acculumators fail (ruptured membrane) then there is no Bounce to the car (a very rough ride) but the system pressure will be maintained and the ride height should be normal. If the the level controller fails, system pressure can be too high, too low or it won't adjust for a load. System pressure can be checked with a high pressure gauge (power steering type) and level control function can be checked by moving the level control lever to check its operation.

Struts typically give no problems for about 150,000 miles so are normally not a problem. Since they provide the dampening the "push down on the fender" test will usually give you a warm and fuzzy on their operation. AND they and very expensive!!!

I would have it checked but, if the car is too bouncy it's probably the struts. Try comparing to another s-class with the self-leveling rear; maybe it's suppose to be as bouncy as it feels to you

Hope that helps,

Intruder
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  #3  
Old 06-23-2002, 02:25 PM
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"and it's the movement of the membrane that provides the "shock absorption". But, fluid dampening occurs inside the strut similar to any other "shock". "


A rather good description overall except for the above statement. The accumulator does NOT act as a dampener (theoretically - I'm sure some dampening has to take place do to flow friction). Its job is too act as a hydraulic spring.

If the car was straight lifted by struts on hydraulic pressure the thing would have no suspension (fluid not being compressible). The added lifting capability is gained through adjustable hydraulic springs achieved through the accumulators.

I would replace them, they are always bad. My recommendation is to replace them every 100k miles as the system takes a huge beating as it becomes straight hydraulic struts without the effect of accumulators. They don't fail like light switches. They loose capacity to recieve a bump and it slowly becomes evident.

If, as is so common, they are driven well past the stage of no spring left, they become rigid. Sometime after this the rigidity drives the fluid from the system back into the reservoir. I am not sure exactly what happens to maintain this as the car now is not supported and feels as if there are no shock in the rear (bounces like a basketball). I don't understand what is happening there since if one replaces the accumulators most systems restore themselves.
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Continental Imports
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Last edited by stevebfl; 06-23-2002 at 02:43 PM.
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  #4  
Old 06-23-2002, 03:06 PM
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Thanks for all the helpful info! I don't know what I would do as a first time MB owner without this forum.

I also noticed that I do not have the code for self-leveling suspension on my data card? It has bee posited that this may be because it was not an option, any ideas?
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560SEL rear suspension-datacard.jpg  
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1991 560SEL
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http://www.bellsouthpwp.net/d/n/dnt0512/1991.htmL
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  #5  
Old 06-23-2002, 11:43 PM
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stevebfl,

As an engineer, "shock" , as I'm using in the phrase "shock absorption" means exactly what you said. Shock has an exact meaning to me; I just put it in terms I understand; thanks for putting it in more lay terms.

Without the benefit of a manual, I had to piece together how the system works from crawlling under the car, playing with the level control unit and searching this website.

So, my question for you is. Why would you replace the accumulators first if his vehicle feels "bouncy"? ...Wouldn't a "hard" ride lead you to the accumulators first???

Intruder
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  #6  
Old 06-24-2002, 09:10 AM
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Though I have never practiced, I hold a BS in Metallurgical Engineering. You might say at this point that my engineering specialty is MB autos.

The point in suspension discussions is springs and absorbers. Each theoretically does a little of the other. To understand the concept they must be named as such.

If you reread my post you will note the section where I explain the condition where the suspension bounces like a basketball. After driving on these depleted accumulators for a time the fluid gets driven from the system and the shocks no long support or dampen. I have seen this condition numerous times. I have seen it take out the pump a few times and cause strut leaks quite often.
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  #7  
Old 06-24-2002, 12:32 PM
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Stevebfl,

Thanks for the benefit of your experiences with this system. I'm only asking so much because I've an E320T that doesn't ride the way it used to and I been trying to figure out why.

That piece of information on the long-term effects of failed accumulators really helps.

Intruder
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  #8  
Old 06-26-2002, 12:00 AM
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Also thanks for all the replies, I finaly found the descriptin on my cd's and plan to get after this this weekend.
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http://www.bellsouthpwp.net/d/n/dnt0512/1991.htmL
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  #9  
Old 11-20-2002, 04:48 AM
funfenix69
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rear accumulators

It is now nov. and how did your weekend project of replacing your accmulators go? I need to do the same thing to mine, but had my mechanic diagnose the problem first. Did you have to get special tools?
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  #10  
Old 11-20-2002, 11:45 AM
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My 2c worth

Those are excellent explanations of the working of the system. I replaced the accumulators on both my 300D wagon and my 560SEL and its a simple job - no more difficult than changing shock absorbers, and the hydraulic pump is self-bleeding to make matters easier.

Both cars had close to 100,000 miles at the time and while they sat level, had an extreme 'pogo stick' quality to the ride over bumps.

Just one caution when you replace these - use a flare nut wrench not a regular open wrench to undo the hydraulic connections. They are brass (?) and easily damaged - a flare nut wrench (a few dollars at your car parts place) will do the job right.

Each accumulatror cost $100 or so, if I recall correctly.
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  #11  
Old 11-20-2002, 01:50 PM
moedip
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what size is the flare nut wrench? 10mm?
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  #12  
Old 11-21-2002, 12:20 PM
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I'm too senile to remember

Probably 13mm or 17mm - but Steve Brotherton is the person to ask. Steve probably does more of these in a day than most of us will do in a lifetime.

I bought my flare nut wrenches in a kit - 10mm through 19mm - $15 or so. Pep Boys or Kragen, but easy to find at most auto parts stores. You use these rarely and on hydraulic fixtures which need little torque, so the the expensive, high quality ones are probably overkill, in my opinion.

J C Whitney has a set for $14.99 at http://www.jcwhitney.com/product.jhtml?CATID=5408&BQ=jcw2
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  #13  
Old 11-21-2002, 09:51 PM
funfenix69
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Smile 560 sel suspension

thanks for the support and pointers, guys. I'm about to save myself a lot of money , since my mechanic is charging me $116/hr for this little project. Now, If I can just get someone to steer me how to look up the appropriate part numbers for the belts I'll be replacing this weekend.......that would be great, guys.
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  #14  
Old 11-24-2002, 07:24 PM
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boumcy 560 sel rear cause?

What can happen to cause a real bouncy (no dampening hardly) on the rear is for the diaphragm in one or both 'accumulators' to rupture. This allows gross mixing of the nitrogen with the oil in the shock and leads to 'no dampening'. I have found it beneficial to always 'crack' loose the hose fitting at the top of the shocks and vent the nitrogen accumulated there.
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  #15  
Old 11-25-2002, 09:32 AM
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Let me point out some theory on the accumulators. Knowing how they work is a real benefit to understanding their fault.

First, one must understand that their function is to be a hydraulic "SPRING". In the box the chamber has some high pressure - say 1000psi. The diaphram is plastered against the sides of the sphere. It is now installed and the motor is started. The pressure of the fluid instantly raises to the pressure of the nitrogen. This is because once filled the fluid is not compresible.

The fluid is now at 1000psi (forgive me I don't remember the actual pressure but 1000 works for understanding). The pump is running and fluid is displacing the diaphram while squeezing the nitrogen. Somewhere at say 1500psi the car is rising and leveled. As this car is now driven the shock is forced up and down by the action of the road. As the shock is compressed the fluid goes into the accumulator and further squeezes the nitrogen. The pressure now hits 2000psi and more than supports the car. The suspension rebounds and the pressure in the sphere drops as the shock rebounds. All this activity is dampened by the regular shock activity of the strut.

On cars that I have owned (2 shop wagons - 280TE, 300TE current) we have replaced the accumulators every 50-75k. The reason behind this goes to the rest of the story. The diaphram holds the nitrogen inside like a tire holds air; only the pressure is tremedous and the acitivity continuous and there is no way to add air. Gradually the nitrogen passes through the membrane and the entering fluid pushes the diaphram further with each year. Eventually the diaphram reaches its stop during some form of shock compression. At first only the big bumps hit bottom, eventually with no nitrogen the hydraulics are non-compressable and the only suspension left is the tires and seat springs. The nitrogen that leaks is dissolved at high pressure in the hydraulic fluid and then in the reservoir at low pressure it is given off into the atmosphere.

BTW, Nitrogen is somewhere around 78% of the air we breath and is used because its available and non reactive.
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