500 SEL Hydraulic Suspension

[Tom Terrell]

My 1983 500 SEL has a real problem with the four point hydraulic suspension. It has always been sluggish to initially raise after a something in the system is changed.

Current Problem: Car lost pressure and will not raise.

1- Cylinders appear to work well with no leaks
2- The four support point Accumulators are new.
3- The Fifth Accumulator is used
4- The Control Manifold ( that connects to the dash cable )
is new.
5- The Hydraulic Pump # 1172360008 is used. I removed
it, cleaned it, and reinstalled. There is no visible wear.
Fluid exits in a steady stream, but with little pressure.
Fluid flow can be stopped by holding your finger over
the output hose end.
6- System filter has been cleaned, and new MB fluid added.
7- No leaks are present in the entire system.

I am thinking that either the pump does not put out
sufficient pressure, OR the control valve on the 5th
accumulator is not working properly.

I do not understand the function of the 5th accumulator or
control valve attached.

Does anybody have a solution to this problem ?

[oxymoron]

spec for pump is 200 bar @ 2000 RPM, so I am gonna say your finger is not capable of holding back 200 bar.

if the rear seal to pump is bad, it leaks suspension fluid back into the engine and will dilute oil, etc. Not a good thing.

take a look here:
http://www.300sel.com/pdf/HydroSuspGuide.pdf

Struts leak internally, making detection difficult. The page above should reference flow rates for testing, but I bet it's the pump, given the new parts you reference.

[stevebfl]

I wish I had the answer, as I have faced the problem scores of times.

The scores of times I'm talking about were times where we got the car up with no other parts. The scores of times don't include the few times we have actually had parts fix the situation.

The first problem I have seen is an airpocket between the reservoir and the pump. I take the line from the pump to the valve off and place it into the reservoir and runn the motor 2000rpms. If this picks the fluid up you won't be able to cap the hose with your finger. If you can't get this state then I have applied airpressure sealled loosely with a shop rag into the reservoir to force fluid into the inlet of the pump.

If the pumps is pushing fluid then the worse case I've seen goes something like this; The pump pumps so slowly that one can place a finger over the outlet hose. One should NOT be able to stop flow. Flow is small but pressure will build. If one places a gauge directly on the line it will peg about instantly and blow up. My gauge goes to 5000psi and it takes a couple seconds, then it better be shut off.

The issue I have always felt was that the flow to the return is too great to get pressure started. I have figured that there is a valve that is not seating unless the flow is huge. I have always solved the problem by a sneeky technique. The problem is that it isn't very easy to explain. The issue is that there is a passage from the height valve block to the regulating valve block that I block (fluid return passage). It is in the lower forward quadrant. I loosen the bolts holding the two valve together and slide a feeler gauge in till I cover the hole. Then I start the car run it till it starts to rise and shut it off and pull the feeler gauge and retighten the valves. I'm not sure I would suggest trying this. If you got a copy of the hydraulic schematic, you will see that the fluid enters the regulating valve from the pump, exits the regulating valve and fills the central reservoir, it then goes back to the height adjusting valve which distributes the flow to the system, the rest returns throw the circuit I'm blocking through the regulator to the reservoir. There is no need to flow through that circuit that is blocked till the car ghas raised.

If none of that works I'm guessing you missed something.

[Tom Terrell]

Steve, I think you are right. I too suspect the height valve and /or the return from it. It makes sense that a little backpressure could free up a sticky valve that is passing too much return fluid.

I won't have time to work on this for a few days, but when I do I'll let you know what happens.

PS. Oxymoron's address to download the hydraulic suspension troubleshooting guide and flow schematic is a must for anyone with these kind of problems. I had tried for a long time to get a complete description of the system, and until yesterday had no luck getting one.