Hydraulic Suspension Questions - PeachParts Mercedes-Benz Forum

jcantor · #1 04-20-2003, 08:00 PM

For quite some time I have been wondering how well the self leveling system on my '87 560SEL was working. It gives a very firm and somewhat rough and doesn't respond as I'd expect when the car is heavily loaded (ie doesn't return the car to the unloaded height)

Today while doing several other projects under the car, I decided to test the system by disconnecting the control arm on the valve and exercise it manually to see if the car responded. Low and behold it moved up and down beautifully just as it should. This means to me that the system is able to function properly and that the nitrogen cells are probably still sealed. However, I'm still puzzled by the rough ride and lack of response when loaded.

Could the ride quality be a shock issue rather than bad nitrogen cells?

Could the lack of height response be somewhere else in the control linkage?

jlc

stevebfl · #2 04-20-2003, 09:31 PM

Read up on the function of the accumulators. They are your problem, probably on both accounts.

The accumulators are the springs of hydraulic suspension. When water logged they are ridgid. The forces generated under such conditions drives all the fluid from the system so that it can no longer ajust the height.

jcantor · #3 04-21-2003, 06:31 PM

Steve,

I certainly would expect that water logged accumulators would cause the rigid ride but then I would not expect them to adjust the ride height during the testing that I did. I gather that I'm wrong on that account but I'm not sure why.

jlc

MikeTangas · #4 04-21-2003, 09:52 PM

Because the accumulators don't adjust the ride height, they only absorb the impact of the shock. The leveling valve allows more/less fluid to adjust the height and you were changing the level as you moved the lever up and down.

jcantor · #5 04-21-2003, 10:40 PM

Mike,

"I see" said the blind carpenter as he picked up his hammer and saw.

A pair of accumulators are now on my shopping list.

Thanks
jlc

dmorrison · #6 04-21-2003, 11:00 PM

Understanding the system will allow you to see why your problem is the Accumulators .
The pump at the engine provides the presurized hydraulic fluid that enters the line that goes to the ride lever control unit. That is the unit mounted in the back of the car. The control valve has a actuating rod that is attached to the rear sway bar. As the sway bar twists due to loads on the car which are transmitted from the rear suspension. This movement of the rear sway bar moves the control lever. This opens the control valve allowing fluid to enter the hydraulic "shock". The piston. The hydraulic shock is not a "shock absorber" it is an hydraulic actuator or piston. The controller tells the actualtor to rise or lower and the presurized hydraulic fluid does that. Once the hydraulic actuator is at the correct height the control valve "shuts off" the fluid. If that is all the system had then the car would sit at that height with out any ability to move. How the system allows for spring type of movement is to have the accumulator.
The Hydraulic fluid actually enters the accumulator first then bleeds over to the hydraulic actuator. In the accumulator thier is a diaphram on one side is the hydraulic fluid and on the other side is a gas. Remember hydraulic fluid will not compress. It is rather solid in this setup. The gas will compress and its the gas in the accumulator that provides the spring effect in your rear suspension. Why do you have coil springs then. The coil springs carry some of the load and the hydraulic suspension the other.
When the control valve is "finished" with the pressurized hydraulic fluid it goes back to the fluid tank in the engine compartment and is there to start the cycle all over again. The return line has a filter attached to it where it enters the tank. This filter should be changed occasionally along with the fluid.

Dave

hanno · #7 04-22-2003, 02:44 PM

JLC, are you still in good health? It takes a brave man to move the suspension valve while under the car. Be safe! And, yes, its the nitrogen spheres. They lose their charge over time and can no longer compress (they are your shockabsorbers). Before changing or opening anything on the hydraulics, relieve pressure as stated in the manual. A stream of fluid from a fitting can penetrate your body.

Thomaspin · #8 04-22-2003, 06:14 PM

Wow!

Do you have a detailed version?

jcantor · #9 04-23-2003, 09:29 AM

Hanno,

Thanks for the concern. I had the car up on some blocks so even if it dropped to the bottom of it's travel range, I still had plenty of room.

jlc

jcantor · #10 04-27-2003, 01:01 PM

Thanks to everyone for the system discriptions and diagnostics. I've just finished replacing the accumulators as suggested - rather easy job.

A quick spin around the block and the car seems to ride much smoother. However, when I disconnect the actuator lever and move it manually, the height no longer adjusts as it did before (and I thought it is supposed to). Does it just need some time and miles to be fully functional, or is there some other problem?

Thanks
jlc

MikeTangas · #11 04-27-2003, 01:46 PM

I found that when the accumlators were shot, the car adjusted almost instantly when moving the level by hand. After replacing the accumulators the car still adjusted but at a slower rate.

What you can do is load the trunk with about 200 lbs of gear, start the car and watch a fixed point in the mirror. Bring the engine up to 1500 rpms or so and watch the car adjust against that fixed point you are watching.

WANT '71 280SEL · #12 04-18-2004, 01:40 AM

just wondering...What if a 560SEL sags in the rear end? Then what would most likely be wrong, the leveling valve? Then, (on the same car), it also has a rough ride as if the accumulators are bad, would these two problems be connected, as in are they the same part causing both symptoms?
Thanks
David

Nikola600 · #13 05-06-2004, 11:52 AM

How does the front shock absorber work on a 1995 S600 coupe. The parts department at the dealer said his books only show the accumulators for the rear suspension. Are the accumulator, actuator and all valves contained in the "shock" as one unit for the front? The front shocks are very expensive.