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86 Suspension Explanation
Suspension
normal driving the microcomputer-based ECU actuates be blenoid valves to
bring the 'third spring' into the hydraulic circuit of each axle. This
increases the volume of compressible gas by 50% and so reduces the
suspension stiffness, improving ride comfort. Damping force is also reduced,
since each solenoid valve additionally opens an orifice to permit fluid to
flow between all three springs on each axle. Conversely, when the ECU
decides that driving conditions demand a firmer suspension setting, it
actuates the solenoid valves to isolate the 'third spring' assemblies from
the system and stop the transverse flow of fluid between the main suspension
struts. This results in an increased spring stiffness, firmer damping and an
anti-roll characteristic.
Sensors
The ECU responds to eight signals; position of the control switch (SPORT or
COMFORT), steering wheel position and rate of turning, vehicle speed,
accelerator pedal movement, braking force, body displacement and door and
boot light switches. The steering wheel and body displacement sensors are of
the twin photo-interrupter type and operate in a similar way to those
described for the Mitsubishi ECS system.
The vehicle speed sensor, which is shared with the instrument pack, is a
simple Hall-effect detector installed in the final drive housing. A take-off
shaft, which is driven by the gearbox output shaft, is fitted with a polar
ring (a magnetic ring composed of several alternate north and south poles).
The Hall-effect detector faces the polar ring and produces a small Hall
voltage pulse (about 1 mV) each time a north pole moves past it. The gearing
is arranged such that one pulse is obtained for each 20 cm of vehicle
travel, giving a highly accurate method of speed measurement. Accelerator
pedal position is derived from the throttle position sensor; a conventional
rotary potentiometer is used. Braking force is measured indirectly, using a
pressure switch. When the fluid pressure in the braking circuit exceeds 35
Citroen XM Internet Reference Version 1.0
127
3/3/99
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