The rear axle air suspension has the task of maintaining the height of the vehicle body as precisely as possible at a pre-defined level under all load conditions. The almost constant level means that the geometric data of the rear axle suspension, such as camber and toe-in, remain unchanged regardless of payload. For the first time at BMW, the entire rear axle load is born by air suspension with the optional extra level control in the E39. This system controls the rear axle level automatically, and without the driver being able to interfere.
The rear axle air suspension is configured so that unnecessary control is largely avoided.
The system offers the following benefits:
- control is independent of vehicle engine
- single-wheel control is possible
- differentiation between load and vehicle condition
- uneven loads can be recognised and compensated for
- compatible with self-diagnosis
- control is interrupted in corners and/of when loading
The entire system consists of the following components:
- Air supply system (LVA)
- Two pneumatic suspension struts
- Two height sensors on the rear axle (left/right)
- Electronic control unit
- Warning lamp (standard instrument cluster) or text display (high instrument cluster)
The system components are described below:
Theair supply system consists of
- Cover with integrated interior noise insulation
- Compressor
- Compressor relay
- Solenoid valve block
- Air pipes, incl. distributor block
The compressor supplies a maximum operating pressure of 13.5 bar. The compressor is mainenance-free. The electric motor is a DC motor.
Important!
To prevent the compressor being thermally overburdened, the time that the electric motor is switched on is monitored: Maximum 8 minutes !
The compressor is supplied with current by the compressor relay.
Important!
the compressor relay cannot be changed! if the compressor relay is damaged, the entire LVA must be changed.
Solenoid valves are used to control the air flow from the comressor to the suspension struts and vice versa. Each pneumatic spring is allocated one valve. The valves can be activated indiviually.
The pneumatic lines (air pipes) create the connection between the solenoid valve block and the distributor block. The pipes are colour coded: blue for right, red for left. A distributor block is used as a pneumatic interface between the LVA and the air pipes to the suspension struts. This is also colour coded on the connections for the air pipes. the distributor block is located in the luggage compartment under the right-hand side member.
The pneumatic suspension strut consists of an air reservoir, a bellows and an additional volume. The bellows forms the air-tight and movable connection between the air reservoir and the suspension strut and, with its effective interior surface over the prevailing air pressure, carries the vehicle load.
The height sensors are located at the rear right and rear left on the respective radius arm bearing block. They are linked by a coupling rod through the rear axle arms. From the respective deflection angle, the control unit receives information of the current height of the vehicle. Both sensors work according to the Hall principle.
If the vehicle is equipped with the optional extra headlight range control, the right-hand sensor configured as a double sensor.
The control unit is installed in its module box in the luggage compartment beneath the right-hand side member.
The air suspension system signal lamp is centrally located in the standard instrument cluster next to the ASC display. On the high instrument cluster, any error messages are given in the text line.
>The system is controlled by a microprocessor, starts working when a door or lid is opened, and continues until approximately 16 minutes after the vehicle is left. The electric drive of the compressor makes the system independent of the vehicle engine.
The following signals are evaluated in the air suspension control unit:
- Consumer cut-off signal
- Height, left
- Height, right
- Terminal 15
- Road speed
- Engine ON/OFF
- Information on the status of the doors and boot lid
The vehicle height is determined by the amount of air in the pneumatic suspension strut. The air quantity in the pneumatic suspension strut is adapted for differing vehicle loads by the compressor and/or the valve in the air supply system being activated.
The LVA supplies air from the luggage compartment to the bellows through the solenoid valves, which are switched by the control electronics, until the specified level is reached.
A pressure check valve protects the system against excessive build up of pressure.
When lowering (vehicle load is reduced), the appropriate solenoid valves and the outlet valve are activated by the electronics. The air flows unhindered into the surrounding area.
Important!
The system can only work properly if the following points are guaranteed: The assembly line mode and the transport mode must be deleted and the height offset properly carried out! Instructions how to delete the assembly line mode and the transport mode, and how to carry out the height offset can be found in the diagnosis programme under the heading "SERVICE FUNCTIONS". When the assembly line mode and/or the transport mode are deleted, the height offset must be carried out!
The safety concept is designed to prevent a system malfunction, especially inwanted control processes, by monitoring the signals and parameters relevant to its operation.. If faults are recognised, the system dependent on the component affected is switched off. The driver is informed of the fault by a signal lamp or a message in the instrument cluster. Faults recognised are stored in the fault memory. Up to three faults can be stored.
Both sides of the vehicle are controlled individually, i.e. the comparison specified actual value is also carried out for both sides of the vehicle individually. When the control system raises the vehicle, the permissible time that the compressor is switched on is monitored.
A non-level surface (vehicle stationary) means when one wheel is resting on an obstacle while the other three wheels are on one level. As such an non-level surface is not compensated for, there is no opposite lean when the vehicle is driven from the obstacle. If the vehicle is heavily laden in this condition, the system will operate to ensure that ground clearance is not impaired. A non-level surface is recognised only in normal or boot mode and so long as no speed signal is received from the control unit.
The system distinguishes between the following types of control: pre/post running mode, normal mode, boot mode, assembly line mode and transport mode.
In the pre/post running mode, the vehicle can only be raised to its specified height if it drops below the specified height in the middle by 40 mm. This reduces the load on the battery before starting the engine, as it only raises the vehicle for heavier loads to increase ground clearance before driving off. Smaller payloads only give short spring deflections; it is thus sufficient to compensate for this inly after the engine has been started.
In normal or boot mode, the specified height is maintained. In normal mode, the load on the rear axle caused by, for example, the fuel tank gradually emptying during a journey, or by the aerodynamic lift on the rear of the vehicle caused by high speeds, are compensated for. In the boot mode, brief changes in load while the vehicle is stationary and the boot lid open, are compensated for.
In its delivery state, a new control unit is in assembly line mode, i.e. the control unit is de-activated. Control processes are not carried out, the safety concept only works with limitations. The fault display is activated in the intrument cluster.
Important!
It is only possible to delete the assembly line mode with the diagnosis programme (SERVICE FUNCTIONS) !
After the assembly line mode has been deleted, the height offset must be carried out. How this is done can be found in the diagnosis programme Menu SERVICE FUNCTIONS, "Height offset".
The transport mode effects a raising of the rear end of the vehicle by about 30 to 40 mm to prevent damage to the vehicle underside during transportation from the factory to the dealer. No control processes are carried out. the fault display is activated in the instrument cluster.
Important!
The transport mode can only be deleted with the diagnosis programme (SERVICE FUNCTIONS) ! For the purposes of renewed transportation, the transport modecan be activated (SERVICE FUNCTIONS). This is independent of whether or not the transport mode was already activated.
After the transport mode has been deleted, the height offset must be carried out. How this is done can be found in the diagnosis programme Menu SERVICE FUNCTIONS, "Height offset".
If the control unit receives no signal from the K-bus, the last signals received (e.g. doors, road speed) remain valid and the control unit remains in the last mode selected until it is de-activated by the consumer cut-off signal. The next time the engine is started, or whenever the engine is started after a communications breakdown, the control unit will remain in pre/post-running mode and the driver is informed of the fault by a fault message in the instrument cluster.