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  #1  
Old 09-28-2004, 09:07 AM
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Help understanding SLS/hydropneumatic

I read lots of the old threads re: sls, hydrpneumatic in the 300td, and have somewhat of an understanding of the system. Could someone simply explain (like to a freshman in H.S.) the major components, their function, and how they work as a system. I know there are variations in model years, but an overview would be very helpful. Thanks.
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  #2  
Old 09-28-2004, 10:10 AM
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Me too. The SLS on my '84 Euro 500SEL doesn't work. I haven't yet even started diagnosing it, but I'd also love to see a detailed explanation on how the sucker works. :p
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1988 Mercedes-Benz 560SEL, 89K miles - daily driver
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  #3  
Old 09-28-2004, 10:30 AM
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I will give this a try and others can fill in where I either get it wrong or leave something out.

In general the system is pretty simple. The "shocks" in the wheel well are really nothing more than hydraulic rams that control ride height. The shock absorbing function is contained in the pressurized spheres, the source of hydraulic pressure is the pump, and the ride height regulator is a three way valve. A reservoir to provide hydraulic fluid to lift the car to the maximum height and then take back the fluid with the car at the lowest ride height is also provided. It has a strainer element in it.

The pump, with the engine running, constantly supplies the control valve mounted on the body of the car above the rear suspension, with oil pressure. The minimum pressure at idle is enough to lift the rear of the car. The valve is set mechanically using a linkage to port pressure to the struts at the wheels when the ride height (the distance from the car frame or body to one of the suspension members) is lowered by load. When the ride height is restored to the set point the valve closes the ports to the struts and goes to a position that allows recirculation leakage so the pump does not overheat. When the load is removed and ride height goes up above the set point the port to the struts is connected to the return line and the weight of the rear squeezes the extra fluid from the struts back to the reservoir until the ride height is lowered enough to reset the valve to the recirculation position.

All the ride functions, absorbing shock and damping responses, comes from the path through the spheres. It is odd when you work on the system and the spheres are bad. Changing the ride height by manipulating the valve results in instantatneous ride height changes, like those bouncing cars with hydraulic suspensions you see, usually in "low rider" mode. With the spheres intact the rate of change is very much delayed especially when the rear end is lowered.

Anyway that is about it. The gas pressurized shocks from Bilstein we are used to are duplicated in terms of shock absorbing (small bumps and the like) using a Nitrogen blanket inside the sphere. The valving and damping functions are provided in the areas of the connections to the hydraulic fluid.

That is about it. Jim
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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #4  
Old 09-28-2004, 10:43 AM
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Getting it

Ok Jim. I think I'm getting it. Thanks for the excellent description of the functions. Are the actuators supplied with hydraulic oil from a line from the valve or a line from the strut?
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  #5  
Old 09-28-2004, 10:45 AM
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Not trying to steal thunder... simultaneous post.

The SLS (hydro pneumatic suspension) on the 123 wagons all function in the same manner with only variation in the 123 wagon chassis SLS being the location of the hydraulic reservoir. Later model 123s (1985) have the reservoir on the left side of the engine, earlier models it is on the right (turbo) side of the engine.

The system is made up of the following components: timing chain driven hydraulic pump (located in the engine head), hydraulic reservoir, position sensing valve, hydraulic struts (2, located inside the rear springs), accumulators (2, located below the cargo area and near the rear springs) and all associated hydraulic tubing.

The engine driven pump supplies hydraulic fluid under pressure (about 130 to 185 bar or 1800 to 2600 psi) to the position sensing valve where, depending on the position of the vehicle body in relationship to the rear wheels, pressurized fluid is admitted to or released from the hydraulic struts and accumulators by the position sensing valve.

Basically, if the car body is lower (ie loaded), fluid is admitted to the struts and raises the body until the valve reaches its shutoff, or bypass point where the fluid is returned to the reservoir. If the car body is too high, fluid is released, lowering the body until the valve's bypass point is reached.

The system's accumulators (two metal spheres approximately the same size as a softball) function as the load supporting devices. Inside each accumulator there is a rubber bladder which is charged with high pressure nitrogen. Hydraulic tubing connects the struts to the accumulators so that when the position sensing valve admits fluid to the struts, it passes through the struts to the accumulators, pressurizing (by compressing the bladders) them until pressure in the accumulators supports the vehicle weight (and load if any). Once this equilibrium is reached, the system's pump continues to pressurize the system and raise the vehicle until the position valve reaches the bypass position. Conversely, if the pressure in the accumulators is greater than what is needed to support the load (ie weight has been removed from the cargo area and the body is too high) the position sensor will release fluid to the reservoir, lowering the body until the bypass position is once again reached.
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  #6  
Old 09-28-2004, 11:20 AM
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Thanks

Ah ha! Very much appreciated. Very well written.
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  #7  
Old 09-28-2004, 11:34 AM
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Quote:
Originally Posted by connerm
Ok Jim. I think I'm getting it. Thanks for the excellent description of the functions. Are the actuators supplied with hydraulic oil from a line from the valve or a line from the strut?
The struts and spheres are in series with the valve. This means there is one path from the valve to each sphere and then from each sphere to its associated strut. Jim
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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #8  
Old 09-28-2004, 11:56 AM
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I totally get it now.

Thanks again. I really like to conceptualize this sort of thing. I'm going to look at a 1985 300td thursday. Guy wants $1500. 200k. Decent shape, but he said it sagged a bit. He also said rear end adjusted up and down a little.

Reading the archives here and with your valuable explanation, I feel a lot better going into the negotiation.
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  #9  
Old 09-28-2004, 12:38 PM
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Stella!
 
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Quote:
Originally Posted by connerm
Thanks again. I really like to conceptualize this sort of thing. I'm going to look at a 1985 300td thursday. Guy wants $1500. 200k. Decent shape, but he said it sagged a bit. He also said rear end adjusted up and down a little.

Reading the archives here and with your valuable explanation, I feel a lot better going into the negotiation.
A wagon with little or no rust which will start and run is a steal at $1500.

FWIW-Tailend sagging could be as simple as a loose/missing link to the position valve or the arm from the sway link is loose. A hard, harsh ride but not sagging, means the accumulators are probably shot (~$90/ea). Leaks anywhere in the system can be expensive to take care of properly but read this for the one I fixed by using tractor fluid.

Be careful of later wagons that appear as if the sunroof was leaking at the front. Some have encountered serious rust in the sunroof pans. See here.
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  #10  
Old 10-10-2004, 09:16 PM
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All well written. I'll save this thread for when I start fixing my 500SEL. Thanks guys
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  #11  
Old 10-11-2004, 09:55 PM
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Bouncing wagons

Jim Smith,

Thanks for the great explanation! My question is about the bouncing, it seems from your description that if the spheres are bad bouncing will result. My recently accquired 280TE with a five speed manual is a great car but anyone riding in the rear will get seasick very quickly. My 240D/300 with heavy duty shocks is a dream to drive and the 280TE as a consequence drives me nuts. So if the bouncing can't be canceled somehow, I'm installing heavy duty shocks and enjoying the results.

Is there a fix?

Thanks,
Ben
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  #12  
Old 10-13-2004, 04:40 AM
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Ridge,

That is correct. When the spheres fail by having the membrane separating the oil from the Nitrogen chamber leak, the Nitrogen goes into solution in the oil and the sphere fills with oil. At this point the rear suspension gets really hard, and the back of the car will bounce around quite a bit. Replacing the spheres will make the car ride as it was intended. Jim
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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #13  
Old 10-13-2004, 01:09 PM
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Dieseldiehard
 
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Quote:
Originally Posted by Ridge
Jim Smith,
So if the bouncing can't be canceled somehow, I'm installing heavy duty shocks and enjoying the results.

Is there a fix?

Thanks,
Ben
Hi, Ben Glad to hear you found a wagon, I figure it would have to be a stick shift for you to own (and a 5-speed to boot!)
I don't believe that HD shocks were ever offered for the rear on the wagons. I had to replace them on one of my TD's when one blew oil out on a cold morning on a trip across West Texas, I left most of my hydro fluid on the ground at a motel that chilly morning
You won't want to replace them until they leak anyhow, they are very co$tly compared to "normal" shocks. The spheres are relatively cheap by comparison and easily changed.
Dieseldiehard
1971 220 (gas) 4-spd manual 106441
1979 300TD w/ 85 turbo engine 295530
1983 300D 243280
1985 300TD 217300
1987 300D 258230
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  #14  
Old 07-28-2008, 08:03 AM
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Hydro pneumatic suspension

All,

I ma new to the site but the articles below in explaining the suspension was a great help.

I have a 2000 S320L and I have having problems with the rear suspension. When you hit a bump the rear suspension will go down and you have to use the Auto Level to bring it back up again.

If anybody knows what might be the cause because the mechanics here do not have a clue.

Michael
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  #15  
Old 07-28-2008, 08:53 AM
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SLS system, nearly the same as the Citroen suspension...

The Hydropnumatic suspension was developed by Citroen, who licences the technology out to the likes of Rolls-Royce and Mercedes Benz...

Here is a link to a Citroen DIY guide that does explain the functioning of the system....

http://www.tramontana.co.hu/citroen/suspension/suspension.php

Its important to know, that very high pressures are used in the SLS system, and that the fluid must be the correct type and changed regularly...
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W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

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