Fault interpretation:

When a fault is detected, the ECU will attempt to maintain a comfortable ride quality with restricted functionality of the
air suspension system.

The system functionality depends on the severity of the fault. The faults are defined as minor or major faults.
Minor faults are:
      Most sensor faults (hardware faults and plausibility faults)
      Cross link valve failure
      Reservoir valve failure.

For most minor faults, height changes are inhibited except for a return to standard height. If the suspension is not in
standard height, the ECU will respond to a request for manual or automatic height change to return the vehicle to
standard height. The ECU will continue to level the vehicle at the 'current' ride height.

Major faults are:
     Compressor faults
     Plausibility errors – for example:
     Average height does not increase when lifting and the vehicle is moving. This could be caused by a compressor fault or a fault in the reservoir valve.
     Reservoir pressure decreases when filling requested. This could be caused by a leak in the common gallery in the valve block or connecting pipe.

For major faults the ECU will not level the vehicle at the 'current' ride height. The ECU freezes height changes until it
receives a manual or automatic request for height change. The ECU will return to standard height and freezes once
standard height is achieved.

If the air suspension ECU loses information regarding vehicle speed, the ECU cannot determine if the current ride
height is suitable for the vehicle speed. The ECU immediately returns to the 'default' height, which is 20 mm below
the standard height. Once at the default height, the ECU will continue to level the vehicle at this height. A loss of the
speed signal could be due to a fault in the CAN Bus or a fault in the ABS ECU. It is unlikely to be a fault in the air
suspension ECU. It may, for example, be caused if the battery is disconnected and the steering sensor is not
immediately recalibrated. In this case a CAN Bus fault is recorded in the error memory. If this fault is seen, other ECU's
using the CAN Bus should be also be checked for faults. When the fault is repaired, the air suspension ECU will
resume full functionality but the CAN error remains in the memory.

If the suspension is above the standard height and the air suspension ECU cannot lower the suspension or cannot
determine the vehicle height, all height changes will be frozen. The ECU will issue a message on the CAN Bus which
is received by the instrument pack which displays a maximum advisable speed in the message centre of '35MPH'. an
immediate 'freeze' of the vehicle height is caused by the following:
     Failure of more than one height sensor
     Implausible articulation symptoms detected
     Valve or solenoid failure (does not include reservoir valve)
     Stuck corner or whole vehicle diagnosed used plausibility of sensor inputs.

If the air suspension ECU has a hardware fault, the ECU will disable all air suspension functions. Detectable hardware
errors include memory error, ECU failure, calibrations errors.

 Fault Detection:
The air suspension ECU performs fault detection and plausibility checks. Fault detection is limited to faults that the
 

ECU can directly measure, as follows:
 Sensor hardware faults
 Valve hardware faults
 Sensor and actuator supply faults
 Bus failures
 ECU hardware errors.

Plausibility checks are checks on signal behaviour are as follows:
 Average height does not change correctly
 Height changes too slowly
 Suspension moves in the wrong direction.
 Reservoir pressure
 Does not increase when reservoir filling requested
 Does not decrease when reservoir used to lift vehicle
 Does not decrease when reservoir is vented
 Pressure varies too much when inactive.
 Compressor temperature
 Increases when compressor inactive
 Does not increase when compressor active.
 'Energy' used to change height of corner
 Too much 'energy' used – height change takes too long or long term filtered height does not reach target.
 Sensor activity
 Signal floating
 Inconsistent signal characteristics – signal on one side of axle is varying but other side remains static
 Constant articulation when moving.

The air suspension ECU monitors each corner height signal using the fast filtered signals if the vehicle is stationary
or the slow filtered signals if the vehicle is moving. If the height remains in a 'dead band' which is ±10 mm from the
target height, the ECU does not implement any height adjustment changes. When the ECU detects that a corner has
moved outside of the 'dead band', the ECU operates the compressor and/or the valves to raise or lower the
corresponding corner(s) back into the target height.

When the engine is not running, the 'dead band' target height tolerance is increased to +20 mm and -25 mm. During
'wake-up', the tolerance band is ±20 mm. In all cases, the ECU will bring the corner height as close as possible to the
target height. The ECU monitors the rate of change of height of the corner signals to predict when to close the valve
so that the target height is not overshot.

The following wheel travels are shown for on road and off-road vehicle operation. The difference between the two
operating conditions is a result of operation of the front cross-link valve. When the cross-link valve is opened the
suspension travel is as given for off-road wheel travel. When the cross-link valve is closed the suspension travel given
for on road applies.

The off road mode wheel travel is:
175 mm bump
95 mm rebound
This gives a total of 270 mm off road suspension travel.

The on road standard wheel travel is:
115 mm bump
155 mm rebound
This gives a total of 270 mm on road suspension travel.

General:
The main function of the four corner air suspension system is to maintain the vehicle at the correct ride height,
irrespective of load. Additionally, the system allows the driver to request ride height changes to improve off-road
performance or ease of access or loading. The system automatically adjusts the ride height to improve the vehicle
handling and dynamics when speed increases or decreases.

The system will temporarily inhibit height adjustments when the vehicle is subject to cornering, heavy acceleration or
heavy braking. The inhibit function prevents unsettling of the vehicle by reducing the effective spring rates.

Height changes are also inhibited for safety reasons, when a door is opened and the vehicle is stationary for example.
The air suspension system fitted to Range Rover is controlled by an ECU located behind the passenger side of the
fascia. The ECU monitors the height of each corner of the vehicle via four height sensors, which are mounted in-board
of each road wheel. The ECU also performs an 'on-board diagnostic' function to perform 'health checks' on the
system. If faults are detected, codes are stored in the ECU and can be retrieved using a diagnostic tool.

The independent front and rear suspension offers many design and performance advantages over the conventional
beam axle design.