I am interested in buying a 450 SEL 6.9 (European car). However the
car has been sitting for quite some time (a few months); in the very
beginning when the car was for sale I already mentioned that the front
of the car went down first meaning probably that the front valve of
the suspension s leaking?
I would like to go for a test drive but how do I check the suspension?
The car is sitting now in his lowest position but when it will be
started it will raise completely I suppose. How do I check now if the
suspension is working well without having to wait for a few days to
let the car sink down?
firstname.lastname@example.org (kris) haute in die Tasten:
IIRC the 450 SEL 6.9 uses a hydro-pneumatic suspension (HP) under license by
Unfortunately Mercedes only took the HP license, but decided to make the HP
his own, while other manufacturers like Rolls Royce/Bentley used straight
parts. So spare parts for the Mercedes HP as well as mechanics skilled to do
maintenance and repair to the Mercedes HP will be quite hard to find.
Unfortunately I have never driven a 6.9 with HP, but I own a Citroen Xantia with
perhaps my remarks may be useful for you.
The main sources of trouble are these:
1. damaged suspension spheres: The HP uses spheres, which are about the size of
fist. These spheres are filled with high pressurized nitrogen (pressure depends
type of the car but can easily be as high as 75 bars, which is 75 times the
pressure of the air you breathe or about 25 times the pressure of a car tire).
spheres also contain a membrane, which separates the nitrogen from the HP fluid
The system works in that way, that a pump presses as much HP into the system
car stands at a good level. The suspension work is made by the nitrogen
the spheres. These spheres tend to lose pressure over the years, besides that
membrane will wear out and lose flexibilty. If the membrane is not damaged, a
sphere may be able to be re-pressurized, but this requires special tools. On
Citroens the spheres are not supposed to be refilled, instead of that you simply
them out and replace them with new ones (like you would chage shocks). One usual
Citroen sphere costs about 60-80 USD, the change is quite easy. The Mercedes
may be different, hard to get and more expensive. Citroen spheres have a life
60.000 to 100.000 miles. I suppose that the Mecedes HP system does only the
job. On some Citroens the HP does also support the braking and the steering, so
cars have up to nine spheres. A 6.9 probably will have only 4 of them.
2. not enough/dirty HPF: Use only the fluid which is cleared by Mercedes for
Do not use motor oil or ATF for that job. This may damage the seals in the
Maybe you can find out how the colour of the Mercedes HPF is. Citroen HPF is
green, and if the fluid in the car looks dark brown, you know that it is due for
change. If the car has been sitting very long and there is a lot of dirt in the
Citroen offers the opportunity to replace the usual HP with a rinsing liquid.
the car for a few hundred miles with that fluid and put in fresh HPF, and the
clean again. There are also several filters in the HP system, which may be
check and clean.
3. worn out or stuck height level correctors: Most Citroens have mechanical
detect the level of the body over the front and the rear axle in order to
defined ground clearance and an even body level, no matter how many people are
in the car and how much luggage is put into the trunk. These mechanical devices
dirty and rusty. A worn or stuck level corrector will result in a car which is
or too low and does not ride softly.
4. leaks in the HP tube system, leaking seals. After you have looked after the
of the HPF in the car, you should carefully inspect the car for leaks.
5. damaged HP pump. If this pump is worn out, the car will not hold its level.
When inspecting the car, I would suggest the following:
1. Have the owner start the car and leave the engine in idle. Gear lever to
brake loose. Stay outsides and carefully watch the car. It should rise within 20
seconds to normal level, and it should do that softly. If the car sits on the
and suddenly the back pops up, something may be stuck there.
2. If the car is up, leave the engine running and press down the body by hand on
corner of the car. You should be able to push down the body by 5 or 6 inches
easily, and the body should come up again softly after that. If you cannot push
the body, something is stuck.
3. If the car has a lever for adjusting the ground clearance, test it (all HP
have such a lever). The car should change its level smoothly.
4. Have a test ride. The car should run smoothly as silk, you should feel like
cloud. If the car runs hard instad, then one or more of the spheres are damaged.
5. after the test drive, leave the car in park, switch off engine. After some 20
seconds, the ar should sink down by two or three inches. This is usual and
worry about. If the car does not sink at all, or if it only sinks on one axle,
the other axle stays up, something may be stuck.
6. carefully inspect the car for leaks
7. ask the owner for proofs of services made. Ask him how old the spheres are
wether he has changed them or not.
When Mercedes developed the W140 S-class, they took a Citroen XM for suspension
benchmarking. His successor got air suspension. So you may expect an
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1) stiff ride-spheres are shot, replace
2) leaking struts, replace or have rebuilt
3) valve, may leak internally, car will usually ride fine but go down over
time 4) pump-pressure test, it can be rebuilt
Its a very simple system.
at 03:10 AM, email@example.com (kris) said:
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