CITROEN XM - Spheres

80 Spheres Suspension
Citroen XM Internet Reference Version 1.0 117
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80 Spheres
Well, they don't last for ever. They can lose pressure so that they just need a recharge or they can suffer
actual damage and require replacement. The cost of replacement can be very high if a large number
needs replacing. There are seven spheres on an XM, you can see two of them at the top of the
suspension struts under the bonnet and one hiding behind the radiator.
But how does the suspension actually work? Ah, I thought you might ask that one. The spheres are gas
filled and the vehicle basically bounces along on the gas. Of course, it's not quite that simple. In the
Hydractive type l system, there are three spheres to each axle and an accumulator sphere. Two spheres
on each axle provide suspension for each corner of the car and the one in the middle is used to link the
two at that end of the car allowing fluid to flow from one side to the other as the car corners and from end
to end as the vehicle accelerates and brakes (and hits bumps, etc). When in 'sport' mode, a valve is
closed which isolates the four corner spheres, this results in a harder ride and is done to cut down on
body roll. The computer decides what mode should be used at any time by reading parameters from five
sensors (steering wheel, accelerator pedal, body movement, road speed and brake switch). The
Hydractive 2 system is similar except for the use of two valves instead of one and a different set of
parameters. The new system allows for a wider range of 'circumstances' to be allowed for and a better
ride has been achieved with no increase in body roll. This new system also incorporates an 'anti-sink'
valve which prevents the XM sinking as fast as the older model once the engine has been turned off.
Thanks to Andy Burnett and the Citroenian, the Citroen Car Club Magazine
I thought you said the spheres were gas filled!
They are. There's a diaphragm in the sphere keeping the gas and the hydraulic fluid apart. The fluid
allows the balance and height of the vehicle to be changed as the gas is 'compressed' by body
movement. The pressure for the hydraulic system is provided by a pump feeding the accumulator
sphere. Ride height can be varied by changing pressure. The higher the pressure, the higher up in the
world your car will be.
What else should I know about the accumulator sphere? The accumulator sphere acts as a damper for
the output of the pump. Without this damping action, failure of a number of components would be very
likely in short order. Failure of this sphere is usually accompanied by fairly loud and regular ticking
noises from the front of the vehicle (every few seconds). It is important to fix this problem without too
much delay since hydraulic failure will stop your vehicle in its tracks. You can test for this condition by
following this procedure: You should start the engine, wait until the car assumes normal ride height and
then push your foot on the brake pedal. You should be able to apply it between 16 and 25 times before
you hear the clicking of the pump. It's a relatively easy task to recharge or replace the accumulator
sphere. The life expectancy is about 100 000 km. Thanks to Julian Marsh
What are the problems with the Hydractive type 1 suspension?
The computer controlling the suspension relied on some dubious multi-point earths. Some early XM's
still have the old earthing blocks which can cause lots of problems. See the electrical questions sections
for further details. Problems here result in the suspension being switched to 'sport' mode for no apparent
reason, resulting in an annoyingly harsh ride. The parameters used by the computer to determine which
mode the suspension should be in are a bit stringent and result in switching to 'sport' mode when the
driver would not expect. Around late J-reg (RP 5539 and above) the computer (stamped H3) had more
generous parameters. The H3 stamped ECU also came with an improvement to the wiring loom, the
multi-point earths which caused so many problems to earlier XM's were dropped Thanks to Andy
Burnett and the Citroenian, the Citroen Car Club Magazine