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  #1  
Old 06-08-2017, 05:11 PM
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If not leaking, do rear shocks in a W123 300TD wear out?

I have an '85 300TD. SLS was rebuilt by PO over 100K ago and is working as it should. Because I know the shocks have not been replaced in the 3 1/2 years and more than 100K I have driven it, I am certainly going to put new bilstiens on the front. I don't know if the rear shocks have ever been replaced.

What I don't know is how much of a "shock absorber" effect the rear shocks have in addition to leveling the rear of the car depending on the load. I imagine the accumulators also smooth things out. So, my question is this: absent leaking, will the performance of rear shocks fall off over time like those in front?

Threads here about wagon rear shocks are scarce, and I have read that if they aren't leaking,leave them alone. But I don't know how much that advice is just based on cost. I intend to keep the car for the long haul, and have ordered a hitch and will probably get a small trailer for dump and Home Depot runs. (Bathroom and reflooring projects on the horizon)

Thanks in advance for your knowledgeable info.

1985 300TD, 362,000 dd

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  #2  
Old 06-08-2017, 06:09 PM
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if they don't leak don't bother. if the front shocks have a 100k on them and don't bounce back more than twice in rebound after a bounce test don't bother. waste of money. everything with mileage and time wears and if you plan on putting a load on it, its going to wear quicker. its up to you if you want to replace/rebuild them
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  #3  
Old 06-08-2017, 07:24 PM
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The spheres are the shock absorbers. Given the price of replacment rams, I'd wait until there was a real problem.
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Old 06-08-2017, 07:41 PM
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Thanks Mxfrank, that answered my question. The spheres must have some valves to stop bouncing, I guess.
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Old 06-08-2017, 08:18 PM
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The spheres have nitrogen and hydraulic fluid in them. A moveable bladder separates the nitrogen and the fluid. When a bump is hit, the fluid cannot compress but gets pushed against the bladder wall which compresses the nitrogen on the other side of the bladder giving a very smooth ride. Picture a basketball filled half way with oil, a sheet of rubber across the half way point sealing the oil and nitrogen on the other side of the sheet of rubber. If the ball is bounced on the oil side the rubber will flex and compress the nitrogen absorbing the "slam" from the Hydraulic/non-compressible liquid.

As the spheres age the nitrogen escapes and when it is gone the hydraulic fluid which will not compress takes up the space where the nitrogen was by pushing the bladder and gives a very stiff ride.

It is opposite of what we expect, when a shock wears out the ride gets very "wallow like." When the spheres leak out the ride becomes very hard because the fluid cannot compress.

The hydraulic struts control the ride height, the spheres soften the ride. If the struts that look like shocks are not leaking I would not change them.

Hope this helps.
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Old 06-08-2017, 08:27 PM
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Thanks again. I know the PO replaced the spheres.
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  #7  
Old 06-09-2017, 12:59 PM
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This is an area many have problems with mentally. I have a non Mercedes car that I suspect the front shocks are not as good as they should be. It passes the bounce test. The only test that to me is totally valid is to drive a similar car with new components. If yours are still decent the car will handle the same. If not you should feel the difference.

That was also an excellent description of how the spheres work. Back In the days I drove the fintail the car was very shock condition sensitive. It to me is reasonable that the internal valving wears slowly with use.

I have little faith in the bounce test. Other than It will show a totally shot shock. At the same time if a shock is that bad it should be noticeable when you drive. The shocks in tandem from side to side contribute to a sway bar effect as well. The nose of the car tends to tip to the lower side easier as you turn the weaker they get.


On certain brands the original makers of the shocks brand should always be used as replacements. Some alternative brands of aftermarket shocks may only last a few weeks in service.
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Old 06-09-2017, 02:14 PM
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Quote:
Originally Posted by tdhawk View Post
I have an '85 300TD. SLS was rebuilt by PO over 100K ago and is working as it should. Because I know the shocks have not been replaced in the 3 1/2 years and more than 100K I have driven it, I am certainly going to put new bilstiens on the front. I don't know if the rear shocks have ever been replaced.

What I don't know is how much of a "shock absorber" effect the rear shocks have in addition to leveling the rear of the car depending on the load. I imagine the accumulators also smooth things out.

So, my question is this: absent leaking, will the performance of rear shocks fall off over time like those in front?

Threads here about wagon rear shocks are scarce, and I have read that if they aren't leaking,leave them alone. But I don't know how much that advice is just based on cost. I intend to keep the car for the long haul, and have ordered a hitch and will probably get a small trailer for dump and Home Depot runs. (Bathroom and reflooring projects on the horizon)

Thanks in advance for your knowledgeable info.

1985 300TD, 362,000 dd
To answer the question first: Yes, the damping performance will fall off with wear of the valves and the piston seal.

In a unitized dampner (aka, "shock absorber"), an accumulator can be either internal or external. In the case of a double tube design, the space between the two tubes becomes the accumulator. In a gas charged mono-tube design, the accumulator can be internal, with the gas separated from the oil by a floating piston, or external, with a canister which contains the floating piston. The external accumulator design is frequently seen on high performance and/or racing motorcycles, and on various forms of racing cars. In all of these cases the valves that control damping oil flow are located in the piston and piston rod.

The MB/Sachs SLS struts are a variation of the external canister design. To provide load bearing capability a larger difference in piston area from the rod side of the piston to the plain side is required than for a damping application only. Consequently, the struts are constructed with an unusually large diameter piston rod. The area of the piston on the plain side then is much larger than on the rod side, and when pressurized results in an extension force that is much greater than the retraction force. Voila', a load bearing dampner.
The difference in the external accumulators is one of design, rather than one of function; the canister design has a floating piston between gas and oil, while the MB "sphere" has a diaphragm between gas and oil.
The damping function and valves are in the strut, not in the accumulator.
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  #9  
Old 06-09-2017, 02:27 PM
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The spheres are not the "shock absorbers". The spheres are springs, just like the coil springs; they work together. They also "store" SLS fluid so that the valving inside the struts can control the compression and rebound. If you compress a strut with a bump, the piston is forced upwards. But the only way it can move upwards through the SLS fluid is to "wait" for fluid to be forced through the valving. (I have no idea what valving is used, but it doesn't matter for this discussion, although it does for the OP's question) This is what dampens bumps and stops the car from pogoing up and down after a bump. Now, since the piston is going up, and the volume of the strut above the piston is greater than below, (because of the volume of oil the piston rod displaces) that extra fluid has to go somewhere. If the leveling valve is closed, it can only go to the sphere. If the leveling valve is open, it can go to the reservoir (lower mode) or to the sphere (raise mode). This serves to increase resistance to the upward movement, stop it, then force the piston back down through (probably) a different set of valves to get back to a position of equilibrium.
The function of the strut most closely resembles a conventional shock absorber, the difference being that a conventional shock absorber is not completely full of oil. This is to allow for the different volumes of displacement on one side of the piston vs the other. (And in the last 30 years, a gas charge) Of course, the strut also plays a role in ride height, unlike a conventional shock absorber.
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Old 06-09-2017, 04:48 PM
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Thanks for all the information, everyone. I told my son about my question last night and in about 15 minutes he found what I couldn't in many hours of searching. I think he found a scanned service manual. In short, it says the struts do double duty, level and shock absorbing. Like Frank and resto said. I tried to cut and paste pertinent parts of a link he emailed me here, but it is beyond my limited computer skills. I'll see if he can help with that, so I can share what he found.

I'll contact PO. if he didn't change the struts they're probably the originals.

'85 300TD. 362,000 dd.
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  #11  
Old 06-09-2017, 05:43 PM
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Here goes. I do not own a model with this type rear suspension. So an in depth examination of the system is not merited by me.

Perhaps poor shocks it the rear might stress the spheres more than intended.That is both a guess and something that may be worth considering. My gut feeling is they would be dealing with quicker applied loads if the shocks were bad. Quicker to me would indicate more pressure as well.

Since the spheres are far from cheap and shocks probably are by comparison. It might be a no brainer to change them if uncertain.

If you have driven the car for many thousands of miles. If there is any edge cupping on the tread. To me it would be a certainty that they are pretty bad. Even a small amount would be a concern. This is at the pretty extreme end of the shocks life though.

Others that have obviously had personal experience with this system might post in the next few days. The above are just a few quick thoughts that crossed my mind if I were in your situation.


One problem on older cars when we buy them. Most times we never know what has or has not been done. If the front shocks where bad when you purchased it . Yet the last owner replaced the spheres. Is interesting.


There also might be some literature from Mercedes existing somewhere on this issue.

Last edited by barry12345; 06-09-2017 at 05:54 PM.
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  #12  
Old 06-12-2017, 12:47 PM
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A quick search from our sponsor shows the struts to be about $300 vs $100 for a sphere. Each.

Tire cupping has nothing to do with shocks or struts. Tire cupping is a product of alignment or ride height. SLS does control ride height, but the pump and level control valve are responsible for ride height, not the struts or spheres. Even on a non SLS car, the shocks have NOTHING to do with ride height, unless they are air shocks or have their own springs (we used to call them over riders, not sure they exist anymore).
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Old 06-13-2017, 12:45 PM
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That is pricey. I always assumed knowing assumptions are bad at the same time. That a tire might skip off the road with bad shocks and when connecting again. The cupping was new wear from rotational difference. Or the return contact force was higher than if the shocks were working.


I know where there is a junk wagon still intact. I think I can locate the owner. It has the naturally aspired engine. I wll ask him if he put any new parts on it in the last year or so it was on the road. He did not drive it much.


This car will be crushed with no local demand for parts. So if anything usable is available it will be cheap. On the other hand he may have just pushed it till it died.
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  #14  
Old 06-13-2017, 01:46 PM
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SLS struts aren't shocks, nor are they like standard front struts used in many cars. Although they have pistons which move up and down and pump fluid, they have no valving, and the shock absorber effect is a function of the spheres. A bad sphere can blow out the seals on a strut, but bad struts don't cause bad spheres.
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  #15  
Old 06-13-2017, 02:53 PM
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Has anyone used put an SLS system into the front suspension?

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