Subaru XT 1988 Service Manual page 761

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4-1 SUSPENSION
Air (Pneumatic) Suspension
This system maintains constant ground clearance regardless
of vehicle load. For this purpose air volume in each air spring
is adjusted according to a signal from a vehicle height sensor
which is installed in each air spring.
Features
The air spring (ground clearance maintaining) adopted on
the 4WD veh icle provides good riding comfort and stabilized
driving even on rough roads.
1) The air spring is adopted in place of the conventional
metal spring to improve riding comfort.
2) A constant wheel stroke is available irrespective of the
load, and this results in reduced bumping shock on irregular
surfaces of the road.
3) The variable damping force mechanism built into the air
spring keeps the damping force low to assure good riding
comfort while the wheel stroke is small. When the wheel
stroke increases, the damping force is made larger to improve
driveability on rough surfaces. This increase in damping force
also improves stability during high speeds.
4) The vehicle posture can always be maintained constant, so
that the light axis of the headlight will be maintained constant.
4
Operation
Ground clearance "constant"
When the ground clearance becomes smaller than the preset
level due to an increase in load, the vehicle height sensor built
into each rolling diaphragm type air spring issues a "low"
signal. If this condition lasts for a certain period of time, the
control unit judges that the ground clearance is low, and opens
the solenoid valve to send compressed air to the air spring
from the air tank (-arrow mark). As the ground clearance
increases, the "low" signal from the sensor disappears, and the
control unit closes the solenoid valve. The ground clearance is
thus always held at a constant level.
If the pressure in the air tank drops,
t~e
tank pressure switch
operates the compressor until the pressure returns to the
specified level.
If the ground clearance becomes larger than the preset level
due to a load reduction, the sensor issues a "high" signal, and
opens the solenoid valve of the respective air spring to allow
air to be released from the air spring (-----arrow mark). As
the ground clearance lowers, the "high" signal from the sensor
disappears, and the control unit closes the solenoid valve.