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:
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.