The 2-axle air suspension combines an automatic load compensation on the front and rear axles with the possibility for the driver to set various vehicle levels.
The control unit activates all the valves, the compressor and the LEDs. The control unit activates all the valves, the compressor and the LEDs. To achieve this, information from the height sensors, the button, the K bus and the CAN bus is used.
These elements are fitted beneath a sound-damping cover. These elements are fitted beneath a sound-damping cover.
The suspension travel at the individual wheels is determined by measuring the relative movement between the wheel and the body. Angle of rotation sensors are used to achieve this.
The driver has a rocker switch with up/down function, which can be used to change the height level. Three LEDs are located beside the rocker switch to indicate the current height level.
The accumulator/valve unit is located behind the right-hand sill, beneath the underbody panelling. The accumulator/valve unit consists of: the solenoid valves, the pressure accumulator, the pressure accumulator sensor and the pressure accumulator valve.
The 2-axle air suspension automatically regulates load changes on the front and rear axles. In addition, the driver can set three different heights, for example to achieve greater ground clearance in off-road driving.
There are three different heights:
- Standard: normal driving height
- Off-road: increases compared to normal driving height
- Access: lowered in relation to standard, for entry/exit, loading/unloading
In every case, the current height is indicated by the corresponding, permanently lit LED. As soon as the driver specifies a new target height by pressing the button or a change is initiated automatically by a certain driving condition, the LED of the current level remains lit and the LED of the target level flashes. When the new level is reached, the LED of the last level goes out and the LED of the newly reached level lights up permanently.
A special case is advance selection of the Access level while driving: the Access LED flashes and the LED of the original level lights up permanently. However, this does not mean that adjustment has started. This only takes place when the corresponding speed threshold that permits the Access level is reached or the speed falls below this threshold.
As soon as the ignition is switched on (terminal 15), downward level changes requested by the driver are possible. Upward changes of the level - and thus filling of the accumulator - are only possible when the vehicle engine is running.
These functions are enabled by activation of a compressor and various valves in the air supply system and the accumulator unit.
The control unit receives information of the current height of the vehicle from four height sensors. The pressure in the pressure accumulator is also registered by a sensor. The evaluation of various signals on the CAN and K buses enables recognition of driving situations and activates corresponding adjustments.
The interface to the driver consists of a button and corresponding LEDs. The 2-axle air suspension control unit can also activate text messages in the text display of the instrument cluster. The system is activated by the front axle signal and can thus become active before the vehicle is started.
As is familiar from the 1-axle air suspension system, a height calibration must also be performed for the 2-axle air suspension under certain conditions, e.g. following replacement of a height sensor. All air suspensions can be lowered or raised individually. The required activations are performed using the corresponding routines in the diagnostic program for the 2-axle air suspension. When the vehicle has reached the correct height (see TIS for height values), the height offset is performed. In the case of the 2-axle air suspension system, the offset function is performed on an axle-by-axle basis, i.e. first on the front axle and then on the rear axle. Before the offset command is executed, the height should be rechecked.
On the 2-axle air suspension, the function of the components concerned must be checked with the corresponding service function after repair or part replacement.