The mysterious inards of a SLS accumulator
 

I changed out the accumulators on my wagon today. The rearend bounced like crazy going over small bumps. It would almost make a person seasick in the back seat.
I thought I would cut one open to see what was inside for my own interest and all the forum folks as well. There is a ring inside that holds a rubber diaphragm. On one side of the diaphragm is suspension fluid and the other is pressurized gas(not sure what kind of gas). When the diaphragm ruptures it fills the whole accumulator with fluid causing the rearend to bounce.

Rubber diaphragm down:
http://i35.photobucket.com/albums/d1...umulator01.jpg
Rubber diaphragm up:
http://i35.photobucket.com/albums/d1...umulator02.jpg

Cool. Gas is nitrogen I think.

What did you cut that open with? Thanks for the view inside. I've replaced 4, and at least now I can visualize the guts....

My sharp whits!:P

I used an angle grinder. It's really thick steel I was quite surprised.

Wow, so there's no way to test these spheres off of the car?

Fill it with oil and measure the volume it takes vs the volume the body displaces. If the diaphragm is intact the volume of oil will be relatively small.

Cool, I did the same thing with the engine mounts when I replaced them...

http://i3.photobucket.com/albums/y71...s/423502e0.jpg

Yes- take a rod with a blunt end and attempt to push the rod into the sphere. If it pushes down easily the sphere is no good. If it's difficult or you cannot the sphere is good. The volume of oil means nothing- a bad sphere may displace a small amount of oil but show little resistance and be worthless. You'd have to wait for some time for the oil to seep. Pushing a blunt rod takes only seconds anyways. A phillips screwdriver will work in a pinch- just be sure to push in the center.
http://i176.photobucket.com/albums/w...s/PJat6038.jpg

With a little bit of adaptation, you could replace these with Citroen spheres. The later ones use the same hydraulic fluid as the accumulators and they last 20 years. I've got some early ones that have lasted longer. And the older ones are rebuildable with parts still around. They come apart and can be re-gassed and reused for about 20 bucks.

I have several spares in my garage- no need to change nuttin...:D

If you have a dead one, can you send it to me so I can so some experiements? I re-gas the Cit ones and have built a set-up to do it.

I can probably regas the accumulators as well, although they look like some of the newer ones that are not made to be regassed.

I think the failure mode for these items is the diaphragm, which lets the nitrogen leak out. You can't disassemble the sphere (as with a Citroen sphere) and replace it - it's a welded unit. Adapting Citroen spheres might be interesting, as it would provide you with rebuildable parts in the suspension. But you still have the expensive struts to worry about...

I don't have any dead ones- I test them in the yard so i don't waste $ buying a bad one.

Have you ever found any good ones in the junkyard? I've only been able to find blown ones.

I wonder how the gas gets out. Isn't this mounted upright under the car? Even if it was sideways, you'd think some of the gas above the hole would remain in there...maybe its pressurized and when the diaphragm tears the pressure drops to near atmospheric and there's not enough "spring" left.

Nitrogen is probably soluble in the fluid at high pressure. As is gets recycled through the system it's solubility will drop on the low pressure side thus losing the nitrogen to the atmosphere. Pretty soon, no nitrogen left.

Since the gas compresses it "mixes" with the hydraulic fluid and creates a foam with each bounce. The foam recirculates into the reservoir and the gas is released.

I scored a good pr at the local scrappers.

I think they charged me like $4/ea.

yeah for me..........

The diaphragm is round and contained inside the frame of the ring?

I'm having trouble wrapping my brain around how the system works...

The accumulator absorbs shock, correct? What controls dampening? It seems like there must be a valve, or something that slows the fluid down as it moves with the suspension to dampen the movement, much like a conventional shock would. If there wasn't, and there were only nitrogen spheres, wouldn't the sphere just act as a "spring" and the car would just constantly bounce over bumps?

I've seen the SLS fail in two different modes...one is excessive bouncing at all times. (When the rear of the car is bounced manually, it keeps bouncing 6-7 times after you stop). The other failure mode is an ultra harsh ride...you can feel every bump, and imperfections in the road slam you into your seat, and/or make your head hit the roof! Can anyone explain why this happens?

On the w123, the SLS works as a system. The dampening comes from the accumulators and coil springs working together with the SLS valve. The valve body has 3 valves inside: fill valve, release valve and a base pressure valve.

The actuators only control ride height based on the adjustment of the SLS valve linkage.

If you have an extra bouncy or hard ride*, you need to replace the accumulators.

*I believe this is also an indicator of the condition of your coil springs. If you have an extra bouncy ride with shot accumulators, the springs are doing their job. If you have a hard, jarring ride, the coil springs are worn.

On other SLS systems where they aren't self bleeding like the w123 and w124, I think the extra hard ride can be due to the fact that hydraulic fluid doesn't compress. When the bladders fail, the gas aerates/foams into the fluid, there is no more dampening/cushion and the ride becomes hard.

This is the way I understand the system after rebuilding the one on my wagon.

The nitrogen it under high pressure. If you look at the original picture I posted, you'll see the sticker that say "WARNING, High Pressure". Yeah the one that I cut right through!:P

Ah, I didn't notice that in the picture. Hope you didn't get a big PFFFT in your eye :D
That makes sense then, the pressure would be much lower and offer less compressive resistance. And then the foaming action that Chad described is probably a good explanation as to where the rest of the gas goes.

I've asked this before. I think there are small orifices in the cylinders that the hydraulic oil must pass through (like a traditional shock absorber), and that provides the dampening. As the cylinders compress, forcing the oil through the orifices, the oil moves through the piping to the accumulators, where it compresses the nitrogen, which then springs it back through the system to the cylinders, and has to pass through the orifices, taking energy from the initial input force. Holy run-on sentence.

Anyway, there has to be something that restricts the flow of the oil, the cylinders make the most sense to me, but it could just as well be in the leveling valve. Just my thoughts, could be completely wrong.

I noticed while removing the accumulators from a junkyard car, that when I cracked the line nut, a lot of foam came rushing out, and smelled kinda funky. It foamed for quite a long time, then there was just a constant flow of fluid. I'm assuming this was nitrogen gas?

So it almost seems like, depending on the circumstances, the nitrogen gas sometimes gets trapped in the sphere, even though the diaphragm is bad?

Either that, or it just takes a LONG time for all of the released nitrogen to bleed from the system, while normally operating otherwise. This almost makes sense to me, and seems like it would cause the "bouncyness" failure mode. If there are bubbles of nitrogen in the pressure lines, the rebound can't be dampened...but it would still provide some shock absorption, since the gas is still being compressed in the lines.

Am I in the right *line* of reasoning? ;)

Can anyone recommend a DIY link to replacing accumulators for the W123 and W124. I am currently trying to replace a pair on my W124 and am having difficulty. not sure about fluid leaking concerns, should I plug something? how much will I lose? (how much replacement fluid should I have on hand), how do I get at the accumulators on the W124. I am removing the interior and still cant see them. The carpeted panel between the 2nd and third row doesnt budge and I think that is where they are. I have removed screws, and it (panel) is still rock solid.

thanks in advance.

It took less than 3 qts for me to refill. That carpeted panel is where they are, I think there are 4 screws.

The panel is thin sheet metal, will yeild a bit but after the four screws (two in the 3rd-seat opening, two on the horizontal surface hiding in the carpet) are removed the anti-rattle foam strips will act like 2-sided tape, takes a little work.

They are pretty straight-forward to replace. I found that installing the new ones was a bit tricky, get the small hydraulic lines' threads started before bolting the accumulators down, the passenger-side one is a ***** to line up as it is very short and difficult to move into position.

As far as fluid, I bought 2liters, which was enough but a 3rd liter was necessary to properly purge. I drained the reservoir, poured a liter of fresh fluid in, stuck the return line into the empty container, continued pouring fresh in until the return fluid was clear.

After you run the car a few times, the dirty fluid from the dampers will mix with the new, and a 2nd purge is not a bad idea if it was really nasty (or unknown).

Oh, and you'll need 4 new crush washers for the banjo bolts.

Here's my change on a w123

Jeff,

Any chance you have the part number of the crush washers handy?

Chris

.. suppose for a moment a guy with an SEC with Rear SLS only and new accumulators with all working well.

Then one day he got bored and had a spare set of new accumulators, made a T-fitting and plumbed in the spare accumulators so now each side had 2 good spheres instead of 1 for a total of 4 in just the rear.

I wonder what the change would be. Is the load now spread among the 4 spheres and is more dampened which might be nice with the firmer H&R springs?

Just thinking out loud.
Great pics.

I've had the same question for my wagon.

I've asked a few of my engineer friends, but get cryptic answers: "Frequency and response would be affected by any changes to the fluid system" was the answer with some caveat about discussing further over beer..

I have 2 spare new (Corteco) spheres right now.
I need to see if there is room to mount them next to the existing ones.
Alternatively I could mount 1 extra with some t-fittings.
There is a good hydraulic shop near by that can make me the hoses and cap for the input line on the extra sphere(s).

I suppose if you consider the absurd and installed 1000 spheres, the load spread among them all wouldn't be enough to compress the gas and the ride would be hard (hydraulic lock basically). So what happens in the more reasonable case where the load is spread among 2 spheres, each deflected only half now, and the fluid passing through 2 valve bodies instead of one, but with only half the action.

Might just have to try and see.

These things look similar to "Pulsation dampeners", that we use in the car wash business to even the output flow from our high pressure piston pumps. The type that I use are rebuildable and have a schraeder type valve so as to allow you to charge the unit for appropriate balance based on your pump output pressure.

Most carwash equipment stores will have the liquid nitrogen to fill your dampener. My dampeners have a 1/2" fpt inlet from your source.

Not sure what the SLS system pressure is, but for our car wash pumps we charge the cylinders at 1/2 of pump operating pressure. Ie. pump pressure 1,200 psi., 600 psi., cylinder pressure.

I have found them to be very long lived, without maintenance.

I now have H&R springs front and rear with Bilstien HD up front and SLS in back.
The front end it toyight.
The rear is sloppy loose even with brand new accumulators and rams that don't leak.

Discussions with people smarter than me indicate that just adding more spheres won't help. Its the damping that needs improving.

Would be neat to be able to have adjustable compression and rebound (in and out) damping for the SLS...

Didn't Citroen have that on some of its models? Could the mechanism be adapted to an MB?

I am reading some Citroen technical manuals and I think the answer is yes.
Some even had a switch to choose the firmness.

An addition restrictor orifie, or a variable flow valve that reduced the speed at which the hydraulic fluid could enter and leave the sphere would reduce body movements, no?

I am going to order 4 unidirectional variable valves. 2 for each ram. One for compression, one for rebound.
The are free flowing one direction and can the flow restricted or stopped the other direction. This should give me total control of the in and out. It might be tough to perfectly match the left to the right.

Hoping for less squat, more body control, and if it doesn't work I can just remove it and get back to some other project.

It would be a lot simpler with one single restrictor valve that would control both directions.

I ponder at the necessity of controlling both directions for a normal street car.
Overkill?

** But it seems a lot of the simple valves are uni-directioal anyways. They restrict one way, and free the other.
The tractor guys use 2 valves to control hydraulic items.

Along these lines: http://www.princehyd.com/Portals/0/products/wolverine/WolverineInlineFlowControl.pdf

I finally got the balls together and did my SLS modification.
Used a Parker hydraulic restrictor valve (variable) and a bunch of fitting shenanigans to put it all together.

I have the dial closed down about 4 turns now and the ride is firmer, no harshness, less squat on acceleration and less wagging over big undulating bumps (A sensation that really chapped my hyde). My car has H&R springs with new rams and new accumulators so everything was working as well as it could, just wasn't firm enough. Now I can dial it in how I want.

One if the little bastard fittings is leaking, so I will fix that tomorrow.

Pic
Attachment 105720