560SEL Hydraulic Suspension - Odd Behavior
 

Thanks to knowledge supplied over the years by members of this forum, I have a pretty good working understanding of my '91 560SEL's hydraulic rear suspension system. Unfortunately, my knowledge is insufficient to figure out an issue I'm presently experiencing with it.

Whenever I cut the engine, hydraulic oil begins gradually bleeding out of the struts and working its way back into the reservoir. After a few hours, the reservoir is nearly full and the car's rear is clearly sagging. The system is otherwise working normally: upon driving for awhile (or upon adding weight to the trunk and getting the RPMs up), the level control valve does its thing and the system is properly re-pressurized. The ride becomes progressively more comfortable as the rear end lifts, and--of course--the suspension fluid in the reservoir drops to the correct/normal level.

From the service manuals, I can't find a clearly described check valve--though I suppose the level control valve might serve as one, and be the source of this annoying issue. FYI: I last changed the accumulators not too long ago, and I flush and refill the system every year or so. There are no leaks.

Any help in diagnosing and dealing with this undesirable de-pressurizing behavior would really be appreciated.

Yes, the leveling valve does serve to block the return of oil from struts/accumulators to the reservoir when it is in the neutral position. Also, when in the neutral position it routes pumped oil back to the reservoir.

First step is to ensure that the valve operating lever is in the neutral position when the car is at normal ride height. The adjusting link is to be disconnected from the operating lever, and the lever pinned to the body of the valve (hole in lever, and bore in valve) in the neutral position. The length of the adjusting link is then set to provide attachment free of tension. In the event that adjustment does not solve the issue, the valve is rebuildable.

On the wagons with the same system, the check valve is integral to the level control valve. What you are describing are the classic symptoms of a failed level control valve. The seals / o-rings inside the valve might need to be replaced (if you can open that valve up), but I tried twice to rebuild that control valve on my 123 wagon with no success.

What about Forward Flow?
 

Thank you both for your helpful replies! It makes sense that the level-control valve necessarily checks backflow from the struts and accumulators, and this certainly seems the most likely source of symptoms my 560SEL is exhibiting.

:confused: But...

I'm not seeing how excess forward flow is "checked". At what point in the system circuit is desired pressurized hydraulic fluid in the struts and accumulators prevented from simply bleeding out toward the reservoir bottle in the normal forward direction? After the accumulators, does the circuit somehow route back to the control valve again to accomplish this regulation? Or is some other mechanism involved? (Clearly there's some way this is accomplished back there, since the reservoir canister up front is not pressurized.)

Thanks again for clarifying how all of this works.

The question presupposes that the hydraulic fluid is continuously circulated thru the strut/accumulator system; it is not. When the control valve is in the neutral position, pump output is directed back to the reservoir. When in the neutral position the control valve serves to isolate the pressure supply from the struts/accumulators. The struts/accumulators can be viewed as being at the end of a dead end branch off of the circulating loop which consists of the reservoir>pump>valve>reservoir.

To raise the rear suspension the control valve allows flow into the struts/accumulators. In addition to returning the strut to normal ride height length, the fluid inflow also displaces the diaphragm in the accumulator, and in so doing increases the operating pressure within the strut/accumulator system. The contrary occurs to lower the suspension.
The flow into/out of the strut/accumulator system is quite small, less than 500cc full cycle.

:bulb2: OMG! That's so logical!! Now this makes so much more sense to me, and I see just why you've indicated a malfunctioning level control valve best describes my 560SEL's present suspension issues. Thanks for the clear (and enlightening) explanation of the struts' and accumulators' isolated position as branches in the circuit.

Frank, in your earlier post, you indicated it's possible to rebuild the valve--though Max warned of difficulties he'd experienced in trying to do so with his. Is there a prepackaged kit available for accomplishing this? And is the rebuild procedure--diagrams, torque specs, etc.--documented either here, in the service guide, or elsewhere? (If feasible, I'd much prefer performing such a rebuild on my own, rather than shipping the valve somewhere and losing use of the car for too long.)

I see new level control valves selling online, and they're relatively expensive at around $400--though, I now understand how central they are to the functioning of the self-leveling suspension. If simply opening my malfunctioning valve up and replacing a few O-rings and gaskets is all it takes, that would obviously be a job well worth tackling...

Thank you for the education and direction!

Although the MB chassis manual for 2nd Gen W126 does not give any rebuild procedures, these drawings will be of aid. Other than the o-rings for the cover and shaft, the wear items are the springs and balls, and the seats for the balls.

A used valve might well be obtained from someone on this forum, to be explored and reconditioned as you are able. The odds of success with that approach may be equal to doing your present valve.