Part 6
Vehicle Dynamic Suspension (Part 6)AIR SUSPENSION CONTROL
Air Suspension Control Switch (Part 1):
Air Suspension Control Switch (Part 2):
The air suspension control module is able to provide three different ride heights:
^ On-road (normal)
^ Off-road
^ Access.
The ride heights are selected via the rotary control switch mounted on the center console. Rotating the switch upwards or downwards selects a different ride height (providing all the criteria outlined are met). The center of the rotary switch contains the 'Hold' switch. The 'Hold' switch allows the air suspension control module to lock the access mode in crawl mode.
The rotary control switch contains one LED for the 'Hold' function. Four further LEDs are located in a display panel adjacent to the switch and display height change, off-road mode, on-road (normal) mode and off-road mode. The symbols illuminate to show the current selected ride height and if a height change is in progress.
If a requested height change is not allowed, i.e. vehicle speed too fast, the height change LED will flash and a chime will be emitted.
A flashing off-road or access LED indicates that the air suspension system is in a waiting state or the system will override the driver's selection because the speed threshold is too high.
The driver can also ignore the system's warning signals and allow the height to change automatically. for example, increasing the vehicle speed to more than 25 mph (40 km/h) will cause the control module to automatically change the ride height from off-road to on-road mode.
On-Road Ride Height
On-road ride (normal) height is used during most driving conditions. This setting is used as a datum for the remaining ride heights and is defined as 0 mm. All other ride height settings are described as above or below this setting. The on-road height can be active at any vehicle speed.
Access or off-road ride height can be selected from on-road ride height using the rotary control or the drivers door access switch.
Off-Road Ride Height
Off-road mode will only be selectable if the vehicle speed is less than 25 mph (40 km/h). The vehicle will be raised 55 mm (2.2 inch) higher than the on-road (normal) mode to provide additional body clearance and improved approach, departure and breakover angles.
If the vehicle speed exceeds 31 mph (50 km/h), the air suspension control module will automatically lower the vehicle to the on-road mode height. At 25 - 28 mph (40 - 45 km/h) a message is displayed in the message center to warn the driver to slow down or the vehicle will lower.
High Speed Height
High speed height lowers the vehicle by 20 mm, and improves the high speed handling of the vehicle. High speed height is only active when the vehicle is traveling at more than 100 mph (160 km/h) for more than 5 seconds, and is not selectable by the driver.
If the vehicle speed falls to below 80 mph (130 km/h) for more than 30 seconds, the control module automatically returns the vehicle to the on-road height setting. If the vehicle stops while the high speed height is active, the timer is paused, stopping the vehicle rising unnecessarily.
When towing, the electrical connection of the trailer socket will be detected and stop automatic lowering to the -20mm position.
Access Ride Height
Access ride height lowers the vehicle by 40 mm (1.6 inch) when the vehicle is stationary or moving at very low speeds in order to improve ease of access to the vehicle. Access ride height can be selected by operating the access switch on the drivers door module when the vehicle is in on-road height or by turning the rotary control switch downwards once when the vehicle is in on-road height or twice when the vehicle is in off-road height.
Access height can be selected at any vehicle speed. When access height is selected, the response of the air suspension system will depend on the vehicle speed.
Access ride height has three different modes of operation:
^ Access Height
^ Access Select from Off-Road Height
^ Crawl Mode.
Access Height
If the vehicle speed is 5 mph (8 km/h) or less, the vehicle will lower to full access height immediately. The access height LED and the height change LED will illuminate. When the access height is reached, the height change LED will extinguish.
If the vehicle is traveling at less than 12.5 mph (20 km/h) 'Access Height' mode can be selected. The control module will lower the suspension to a part lowered height and will remain at this height for up to one minute for the vehicle speed to be further reduced. The on-road LED will be extinguished as the control module lowers the suspension to the part lowered height. The access height LED and the height change LED will both be illuminated. When the part lowered height is reached, the height change LED will flash. If the vehicle speed is not reduced to less than 5 mph (8 km/h) within the one minute period, the access height request will be canceled and the vehicle will return to on-road height.
If the vehicle speed is more than 12 mph (20 km/h), the control module will wait for up to one minute for the vehicle speed to be reduced. The access height LED and the height change LED will flash while the control module waits for the vehicle speed to be reduced, the on-road height LED will remain illuminated. If the vehicle speed is not reduced sufficiently, the access height request will be canceled after one minute.
Access height may be selected up to 40 seconds after the ignition is switched off, provided that the driver's door has not been opened within this time.
The suspension will automatically rise from access height when the vehicle speed exceeds 6.2 mph (10 km/h). If access height was selected directly from off-road mode, then the system will return to off-road height when the vehicle speed exceeds 6.2 mph (10 km/h). Otherwise the system will return the suspension to on-road height.
Access Select from Off-Road Height
When the suspension is in off-road height, rotating the air suspension rotary switch downwards once and then a second time before the height change LED has extinguished, or pressing the access switch on the driver's door module once, will cause the control module to lower the suspension to access height (providing the speed thresholds are met).
The control module will remember to return the suspension to off-road height automatically if the vehicle speed increases to more than 6.2 mph (10 km/h).
Crawl Mode (Locked at Access Height)
Crawl mode does not change the vehicle height, but modifies automatic height changes and allows the vehicle to be driven at access height but at a higher speed than in access mode.
Crawl mode is selected when the vehicle is at the access ride height and 'Hold' is selected. If 'Access Height' has been selected, but the air suspension control module has not lowered the vehicle because the road speed is too high, when 'hold' is selected the 'Access Height' request is canceled and the vehicle remains at on-road ride height. The 'Hold' button remains active so that all automatic and manual height changes are disabled.
Crawl mode allows the vehicle to be driven at speeds up to 25 mph (40 km/h) without returning to the on-road height. If the vehicle speed reaches 22 mph (35 km/h), a chime will sound from the instrument cluster sounder and the message center will display a 'SLOW DOWN' warning message. If the vehicle speed exceeds 25 mph (40 km/h), crawl mode is canceled and on-road height will become active automatically.
SYSTEM OPERATION
Under normal operating conditions, the air suspension control module keeps the vehicle level at the 'current' ride height. The incoming height signals from the sensors are passed through filters to remove irregular signals produced by road noise or other irregularities. When the vehicle is stationary or a height change is in progress, the signals are passed through a 'fast' filter, which tracks the true rate of change of height. When the vehicle is moving, the signals are passed through a 'slow' filter. The 'slow' filtered signals remove almost all road noise from the signals and output a true long term average for each corner height. The 'slow' filtered signals cannot be used to respond quickly during height changes.
The air suspension control module 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 control module does not implement any height adjustment changes. When the control module detects that a corner has moved outside of the 'dead band', the control module operates the compressor and/or the valves to raise or lower the corresponding corner(s) back into the target height.
SYSTEM INHIBITS
A number of conditions exist where a change in ride height is undesirable. To counter this, the air suspension control module is programmed with a number of system inhibits. If any of the conditions detailed exist, the air suspension control module will suspend height changes and height corrections.
Compressor
Compressor Timeout
Two temperature sensors are located within the compressor to prevent overheating. If the temperature of the motor brush assembly or the compressor cylinder head rise above pre-set limits, the air suspension control module will inhibit the compressor operation. The limits are detailed in tables under Air Supply Unit.
Cornering
If the air suspension control module registers a cornering force greater than 0.2g it will inhibit all height changes and corrections. The system will remain inhibited until the cornering force falls to less than 0.15g. The air suspension control module receives a message from the lateral acceleration sensor (which is an integral part of the ABS yaw rate sensor) on the high speed CAN Bus for the cornering force.
Rapid Acceleration
If the air suspension control module registers a rapid acceleration greater than 0.2g it will inhibit all height changes and corrections. The system will remain inhibited until the rapid acceleration falls to less than 0.15g. Acceleration is calculated by the control module from a vehicle speed signal received via the high speed CAN bus.
Rapid Deceleration
If the air suspension control module registers a rapid deceleration smaller than - 0.2g it will inhibit all height changes and corrections. The system will remain inhibited until the rapid deceleration rises above - 0.15g. Deceleration is calculated by the control module from a vehicle speed signal received via the high speed CAN bus.
Vehicle Jack
The air suspension control module will inhibit all height changes and corrections if it detects a corner lowering too slowly for more than 1.2 seconds. This is interpreted as the corner identified as moving too slowly being supported on a jack. In this situation, the corner height will not change when air is released from the air spring because the jack acts as a mechanical prop.
The system will remain inhibited until any of these conditions exist:
^ The air suspension rotary switch is moved to the up or down position
^ The vehicle speed rises to more than 9.3 mph (15 km/h) for more than 15 seconds.
Door Open
The air suspension control module will stop all height change requests while any of the doors are open. Vehicle levelling continues with a door open by keeping the vehicle at the height when the door was opened if the vehicle load changes.
DIAGNOSTICS
The air suspension control module can store fault codes which can be retrieved using T4. The diagnostics information is obtained via the diagnostic socket which is located in the instrument panel, in the drivers stowage tray. The socket is secured in the instrument panel and protected by a hinged cover.
The diagnostic socket allows the exchange of information between the various control modules on the bus systems and T4 or a diagnostic tool. This allows the fast retrieval of diagnostic information and programming of certain functions using T4.
Fault Detection
The air suspension control module performs fault detection and plausibility checks. Fault detection is limited to faults that the control module can directly measure, as follows:
^ Sensor electrical hardware faults
^ Valve electrical hardware faults
^ Sensor and actuator supply faults
^ Bus failures
^ Control module hardware errors.
Plausibility checks are checks on signal behavior, as follows:
^ Average height does not change correctly Height changes too slowly
^ Height changes too slowly
^ Gallery pressure Does not increase fast enough when reservoir filling requested Increases when system is inactive Too low when lifting is requested Increases too rapidly when filling reservoir Does not decrease when gallery is vented Pressure varies too much when inactive.
^ Does not increase fast enough when reservoir filling requested
^ Increases when system is inactive
^ Too low when lifting is requested
^ Increases too rapidly when filling reservoir
^ Does not decrease when gallery is vented
^ Pressure varies too much when inactive.
^ Compressor temperature Sensor voltage too large - head and brush temperature sensor (short circuit to battery) Takes too long to be readable after suitable compressor run time - head and brush temperature sensor Does not increase when compressor is active - head temperature sensor only.
^ Sensor voltage too large - head and brush temperature sensor (short circuit to battery)
^ Takes too long to be readable after suitable compressor run time - head and brush temperature sensor
^ Does not increase when compressor is active - head temperature sensor only.
^ Sensor activity Signal floating Constant articulation when moving.
^ Signal floating
^ Constant articulation when moving.
When a fault is detected, the control module will attempt to maintain a comfortable ride quality and where possible will retain as much functionality as possible.
The system functionality depends on the severity of the fault.
Faults
Faults are categorized into order of severity and effect on the system as follows (with 1 being a minor fault and 5 being a major fault):
^ Height sensor faults (hardware faults) and reservoir valve block failure Retain full functionality with no 'refinements', e.g. cross-link valves inoperative, no compensation for uneven surfaces.
^ Retain full functionality with no 'refinements', e.g. cross-link valves inoperative, no compensation for uneven surfaces.
^ Pressure sensor faults, compressor faults, corner valves stuck shut Road speed signal not available Vehicle returns to on-road height when next requested Levels at 'current' height.
^ Road speed signal not available
^ Vehicle returns to on-road height when next requested
^ Levels at 'current' height.
^ Reservoir valve stuck open, exhaust valve stuck shut if below on-road height, corner valves stuck open if above on-road height Vehicle returns to on-road height when next requested Does not level at 'current' height.
^ Vehicle returns to on-road height when next requested
^ Does not level at 'current' height.
^ Failure of multiple height sensors, cross-articulation when driving, calibration corrupted Vehicle lowers to bump stops.
^ Vehicle lowers to bump stops.
^ ABS module failure, CAN bus failure If the air suspension control module loses communications with the ABS module or the ABS module reports a fault, the air suspension control module immediately returns to the default height, which is below the on-road height. Once at the default height, the control module will continue to level the vehicle at this height. It is unlikely that the fault will be in the air suspension control module. When the fault is repaired, the air suspension control module will resume full functionality but the error will remain in the control module memory.
^ If the air suspension control module loses communications with the ABS module or the ABS module reports a fault, the air suspension control module immediately returns to the default height, which is below the on-road height. Once at the default height, the control module will continue to level the vehicle at this height. It is unlikely that the fault will be in the air suspension control module. When the fault is repaired, the air suspension control module will resume full functionality but the error will remain in the control module memory.
For major faults the control module will not level the vehicle at the 'current' ride height. The control module freezes height changes until it receives a manual or automatic request for height change. The control module will return to on-road height and freezes once on-road height is achieved.
If the suspension is above the on-road height and the air suspension control module cannot lower the suspension, all height changes will be frozen. The control module will issue a message on the high speed CAN bus which is received by the instrument cluster which displays a maximum advisable speed in the message center. An immediate 'freeze' of the vehicle height is caused by one of these:
^ Failure of more than one height sensor - vehicle on bump stops
^ Implausible articulation symptoms detected - vehicle on bump stops
^ Valve or solenoid failure - corner valve stuck open below on-road height or exhaust valve stuck shut above on-road height
^ Stuck corner or whole vehicle (diagnosed using plausibility of the sensor inputs).
If height change is not possible, e.g. exhaust valve failed closed at off-road height or compressor failed at access height, the control module will not level or change height.
If the air suspension control module has a hardware fault, the control module will disable all air suspension functions. Detectable hardware errors include memory error, control module failure, calibrations errors.
Fault Messages
The air suspension has two methods which it can use to inform the driver of a fault in the air suspension system; the air suspension control switch LEDs and the instrument cluster message center.
If the air suspension control module suffers a major failure and there is no air suspension control, all the control switch LEDs will remain unlit.
If a fault occurs and the control module can determine the ride height and the vehicle is not above on-road height, the driver will be notified via a message in the message center. If the control module cannot determine the height of the vehicle, or the vehicle is above on-road height and cannot be lowered, a message is displayed and accompanied with a maximum speed message.
This table shows messages related to the air suspension system.
RESERVOIR
The reservoir supplies pressurized air to the four air springs, via the reservoir valve block, to enable the air suspension system to raise the vehicle more quickly.
The air suspension control module assumes the reservoir has sufficient pressure, which is measured before a vehicle raise is started. The control module then uses a software model to operate the compressor as required.
SYSTEM PNEUMATIC CIRCUIT
This schematic diagram shows the connection relationship between the air supply unit, the reservoir, the reservoir valve block, the cross-link valves and the air springs.
System Schematic Circuit Diagram (Part 1):
System Schematic Circuit Diagram (Part 2):
AIR SUSPENSION CONTROL DIAGRAM
NOTE: A = Hardwired; D = CAN Bus high speed
AIR SUSPENSION CONTROL DIAGRAM (PART 1):
AIR SUSPENSION CONTROL DIAGRAM (PART 2):
