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Self-levelling rear suspension - confused!

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monarchd's picture
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Okay, while the backend was apart on my recently acquired 1982 300TD, I had the rear hydropneumatic shocks replaced (the old ones weren't leaking, but were in bad shape).

Having recently bought the car, I haven't actually experienced the 'joys' of a self-levelling rear end(!).........I did notice today that with the engine running and my wife sitting on the rear tailgate floor, the rear end did drop.........Am I missing something? Yes, the oiil reservoiir is full, and no, there are no leaks.......if the pump isn't working, how do you know......errrr.....I assume the rear end sags?? Can anyone shed some light on this please!

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johnmci's picture
North Saanich, BC
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1993 500SEL

There are two nitrogen spheres in the back that are part of the hydralic suspension system, where those replaced too? The system depends on a control valve in the rear axle area that routes fluid flow depending on the position of a control level that varies in relationship of the body to the suspension parts. If this control valve is not working or if the lever is incorrectly adjusted you might not get any pressure to the shocks to raise the rear of the car.

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Anonymous
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Thanks for the link

I understand the basic functioning of the self-levelling rear end.

The nitrogen spheres were not replaced - my understanding is that their functioning affects ride quality, rather than ride height - correct?

Based on my description of the situation, what is the simplest check I can do to identify why the rear end is apparently not self levelling? I should add that the rear springs were also replaced (one cracked). The ride height looks normal, i.e. no sag when unloaded.

Anonymous
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Marshall,

Do you know of a definitive test to confirm that the pump is the culprit?

Regards,

Quentin

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Maxbumpo's picture
Mount Pleasant, SC
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1995 E300 Diesel
1987 300TDT

Quentin,

Some other wagon owners made measurements of their cars, and the consensus was that the 'correct' ride height (American version wagon), when all is within specifications, is 12.75 inches (measured from the very center of a rear wheel, where the three points of the star meet, directly upward to the outermost bump of the rear fender lip). If your car keeps the ride height close to that, within tolerances (read those references carefully and you can ferret out official MB tolerances, I remember something like +/- 8 mm which is about 0.3 inches), leave it alone.

I also remember reading that a load of ~200Kg in the cargo area should be held within tolerances, when car is in 'ready to drive' condition (full fuel tank, all equipment loaded (spare tire / first aid / tools etc.), two adult passengers in front seats).

If you are unsure of the state of the nitrogen spheres, replace them, as the hydraulic control valve will be destroyed by constant pressure shocks transmitted directly to it when the spheres are not absorbing them. This is a very conservative course of action, but price of new valve will make you cry compared to price of two new spheres, and you will have confidence that you have eliminated the possibility that the only wear items in the system are not worn but new.

Pumps rarely go bad, unless they have been run dry. The definitive test is to hook up pressure gauges at the control valve and measure in accordance with MB published specs (in the service manual). If there is good flow from return line into the reservoir when the car is not loaded, and it reduces when you load it and then returns to normal flow (what it was like before loaded) after a time (maybe 30 seconds, maybe a few minutes depending on how much fluid is needed to restore correct ride height), then your pump is probably fine. The system does NOT react quickly to changes in load; the volume/pressure of the system design don't allow that.

Send me a private message with your email address if you'd like me to send more info (may take me a few days to respond).

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Max Dillon
Charleston SC
'87 300TD, 334k miles (head is off, getting ready to install a different used head...)
'95 E300 Diesel, 348k miles (daily driving duty)
'73 Balboa 20

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