HYDROPNEUMATIC STRUTS:
The four-hydropnematic struts that are part of the 450SEL’s 6.9 suspension are an item that has a
life span of approximately 10 years or 100,000 miles. The struts key failure areas are the nylon
seals and the 70 durometer rubber o-rings. These items fail due to heat and contamination. As
the strut moves up and down tremendous heat is generated and the rubber o-rings are literally
baked and unbaked. Overtime these o-rings loose there elasticity and start to collapse, thus
causing the oil leaks. The oil leaks show up in two ways; external and internal oil leaks. The
external oil leaks are visible at the bottom of the strut. Internal oil leaks are distributed back
through the return tube (D2) nd into the reservoir tank. This leak is less apparent but can be
measured by removing the D2 line connection, plastic tube, from the strut. A measurement of
2cc’s in a 4 hour period or 6cc’s in 12 hours. For simplicity, if you get a considerable amount of
fluid in a short time the struts are leaking and should be replaced to prevent further problems.
Contamination in the strut occurs from the rubber boots being torn and exposed to the elements.
The nylon seals (wipers) trap the road debris that enters the strut. Since the gap between the strut
cylinder is infintesimal, very little dirt is exposed to the seal. The main culprit is time and eventually
the seals were out. The rubber boots on the struts should never be comprised. Inspect and wash
them regularly, replace if they are torn. This will improve the longevity of the strut. The struts can
be rebuilt.
ACCUMULATORS:
The accumulators act as the coil spring for the car, in that they contain the nitrogen charge to
supplement the struts. The accumulator life span is the same as the struts. There are 2 types of
accumulators used: Front ones are charged at 75 bars, red color. Rear ones are at 60 bar and
colored blue. The main failure is that the rubber bladder that separates the oil and the nitrogen
collapse and the nitrogen is displaced in the oil. The ride quality becomes hard or bouncy and all
the accumulators should be changed. I do not recommend testing the accumulators since they
can be purchased inexpensively. The accumulators can not be rebuilt.
HYDRAULIC VALVES AND PRESSURE REGULATOR.
The slef leveling valves again have the similar fate of the struts, in that the o rings fail due to age.
Longevity can be up to 10-20 years since the o~rings used are very small. Heat is not a factor in
the rear valve since it is adequately cooled but the front valve is exposed to engine heat and fails
more often than the rear valve. The valves can be checked for external leaks by a visual check of
hydraulic oil leaking from the shaft area. As for internal leaks Mercedes recommends attaching a
plastic line to the (d1) or marked R on the valve and measuring the fluid loss. If the leaks are 8cc’s
in 4 hours or 24 cc’s in 12 hours then the valve should be replaced. This is only true if is leaking
only through the return port. Any other internal leaks can not be diagnosed and if the valve is giving
problem then it should be replaced accordingly. The pressure regulator, which sits on the top of
the supply tank, can only be checked for external leaks. The valves can be rebuilt.
HYDRAULIC PUMP:
The hydraulic pump is a radial piston pump. The maximum delivery pressure for the pump is 200
bars at 2000 rpm. The pump volume can be checked and should yield .2 liters in 30 seconds and
1000 rpm. Very rarely do the pistons fail since the bore and pistons are lubricated with hydraulic
fluid. If the tank runs dry these components can be damaged. The main failure in this pump is the
rear radial seal. When this fails the pump leaks hydraulic oil back into the engine oil. This can
dilute the engine oil and cause damage. Mercedes has added a suspension warning light in the
dash and strict attention must be paid to this.
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