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Air Body Control, Function - GF32.22-P-0004FR

Model 257, 290 

up to model year 2021 

with code 489 (AIR BODY CONTROL) 

Block diagram 

G14549251Courtesy of MERCEDES-BENZ USA

AIR BODY CONTROL, in general 

AIR BODY CONTROL is a combination of a multiple-chamber air suspension and continuous damping adjustment. Both damping and suspension at each individual wheel are adapted to the ambient conditions. Level control ensures optimum suspension and constant ground clearance even when the vehicle is loaded. To improve driving safety and reduce fuel consumption, the vehicle is lowered automatically at higher speeds. Furthermore, it is also possible to adjust the vehicle level manually.

To realize various compliance characteristics, the air suspension bellows consist of several air chambers which are connected or disconnected to each other by switching the air suspension valves.

The AIR BODY CONTROL function includes the following subfunctions:

Wake-up mode 

The AIR BODY CONTROL is woken up by the active suspension FlexRay after the vehicle is unlocked in order to measure the current vehicle level. A critical condition e.g. vehicle level at front axle too low, so that the full wheel angle can no longer be realized, is detected by the AIR BODY CONTROL control unit as a fault. In the instrument cluster, the driver is made aware of this situation by the message "Vehicle being raised, please wait".

IMPORTANT If the vehicle has not been used for a long time (> 4 h), it is raised to the normal level when the engine is started. If an increased vehicle level was set during the last journey or the vehicle's off-time is less than 4 h, the vehicle level last set is retained.

IMPORTANT If the pressure in the central reservoir is insufficient, the vehicle level is corrected immediately.

IMPORTANT The vehicle level is not corrected until after the engine is started, because the AIR BODY CONTROL compressor is only actuated when the engine is running.

Additional function requirements for level control 

IMPORTANT The CDI control unit or the ME-SFI [ME] control unit sends the "Engine running" signal over the drive train CAN, powertrain CONTROL unit and the suspension FlexRay to the AIR BODY CONTROL control unit.

Level control 

The level control system controls the air suspension individually at each wheel. Under normal operating conditions, the air suspension operates with different pressures depending on the load. For the compressed air supply system, the AIR BODY CONTROL control unit actuates the AIR BODY CONTROL compressor through the AIR BODY CONTROL relay in the engine's fuse and relay module. A central reservoir which buffers supply bottlenecks is additionally installed.

Depending on the selected transmission mode, the vehicle level is reduced or increased and can also be changed by pressing the vehicle level button in the left lower control panel.

Selection of transmission mode 

By operating the DYNAMIC SELECT switch in the left lower control panel, the driver can select between four preconfigured transmission modes and an individual transmission mode. Depending on the selected transmission program, one of the following settings is activated for the suspension:

The left lower control panel sends the status of the DYNAMIC SELECT switch over the LCP-LIN, Audio/COMAND control panel (except code 446 (Touchpad only)) or touchpad (with code 446 (Touchpad only)), telematics CAN, head unit, user interface CAN, electronic ignition lock control unit and the suspension FlexRay to the Electronic Stability Program control unit. The Electronic Stability Program control unit then sends the request for the selected drive program over the suspension FlexRay to the AIR BODY CONTROL control unit.

The symbol for the selected transmission mode is displayed in the instrument cluster. The current transmission mode settings are also briefly shown in the Audio/COMAND display.

The AIR BODY CONTROL control unit sends information about the selected drive program over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the head unit and to the instrument cluster. The head unit generates the corresponding image data and sends a switch-on request to the Audio/COMAND display via telematics CAN. The image data is transmitted via the high-speed video link to the Audio/COMAND display.

IMPORTANT Selection of the transmission mode also affects adjustment of the damping.

Selection of vehicle level 

At a speed below 80 km/h, the driver can set the vehicle level by pressing the vehicle level button in the left lower control panel. He can choose between the following vehicle levels:

The left lower control panel sends the status of the vehicle level over the LCP-LIN, Audio/COMAND control panel (except code 446 (Touchpad only)) or touchpad (with code 446 (Touchpad only)), telematics CAN, head unit, user interface CAN, electronic ignition lock control unit and the suspension FlexRay to the AIR BODY CONTROL. control unit. The AIR BODY CONTROL control unit sends the switch ON/OFF request for the LED in the vehicle level button on the left lower control panel in the opposite way. The LED is switched on when the vehicle is at the increased level.

An appropriate message additionally appears in the instrument cluster. The AIR BODY CONTROL control unit sends the request to issue a message over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster.

Detection of vehicle level 

The vehicle level is detected for each individual wheel by the following sensors:

The AIR BODY CONTROL control unit reads in the signals from the level sensors directly and evaluates them.

Adjust vehicle level 

By increasing the air pressure in one of the multiple-chamber spring bellows, the vehicle level is increased at the wheel concerned. Conversely, the vehicle level is reduced if air is released from the multiple-chamber spring bellows. Compressed air is supplied to the multi-chamber spring bellows through the AIR BODY CONTROL central reservoir or through the AIR BODY CONTROL compressor (if the pressure in the central reservoir is not sufficient).

The AIR BODY CONTROL control unit actuates the AIR BODY CONTROL compressor directly through the AIR BODY CONTROL relay, the AIR BODY CONTROL central reservoir filling valve and the corresponding level control valves. To protect against overloading, the AIR BODY CONTROL control unit monitors the operating time of the AIR BODY CONTROL compressor. When the maximum operating time is reached, the AIR BODY CONTROL control unit switches off the AIR BODY CONTROL compressor for the duration of the cooling-down phase. The pressure in the central reservoir is detected by the AIR BODY CONTROL pressure sensor. The AIR BODY CONTROL control unit reads in the signals from the AIR BODY CONTROL pressure sensor directly.

Increasing vehicle level 

Reduce vehicle level 

To control the compressed air supply, the AIR BODY CONTROL control unit also evaluates the atmospheric pressure and the intake air temperature. The CDI CONTROL unit or the ME-SFI [ME] control unit sends corresponding information over the drive train CAN, powertrain CONTROL unit and the suspension FlexRay to the AIR BODY CONTROL control unit.

To realize various compliance characteristics, the air suspension bellows consist of several air chambers which are connected or disconnected to each other by switching the air suspension valves. The AIR BODY CONTROL control unit sends a corresponding request over the suspension FlexRay to the AIR BODY CONTROL Plus control unit, that directly actuates the respective air suspension valves.

Vehicle levels      
Level designation Level [mm] Vehicle speed [km/h] Comment
Comfort 0 <125 -
  -15 > 125 Raise to 0 mm at v < 80 km/h
Sport -15 - -
SPORT PLUS -15 - -
Higher level +25 < 120 Lower to 0 mm at v > 80 km/h for longer than t = 3 min

Damping adjustment 

The continuous damping adjustment function adapts the damping at each individual wheel. This improves driving safety and driving comfort.

Adjustment of the damping depends on the following factors:

IMPORTANT Information on selecting the transmission mode is given in the section "Level control".

The damping is adjusted harder or softer depending on the driving situation. If the sensor system detects a sporty driving style for example, the comfortable basic damping becomes harder automatically. The electronics works continuously within a broadly spread damping performance map. The damping force is adapted at each individual wheel. As a result, the vehicle rolls smoothly even on poor road surfaces, without impairing driving stability. The previously set damping stage is active when the ignition is switched on.

Recording of driving condition 

The driving condition is defined through the following variables:

The vehicle body acceleration is recorded by the following sensors:

The AIR BODY CONTROL control unit reads in the signals from the acceleration sensors directly.

The Electronic Stability Program control unit also sends information on the wheel speeds, brake status and the braking torque over the suspension FlexRay to the AIR BODY CONTROL control unit. This information is used for computation of the vehicle speed and the vehicle body acceleration.

The supplemental restraint system control unit sends information about the turn rate and the lateral acceleration and longitudinal acceleration over the vehicle dynamics CAN, Electronic Stability Program control unit and the suspension FlexRay to the AIR BODY CONTROL control unit.

The AIR BODY CONTROL control unit also receives the following information over the suspension FlexRay, which is used for calculating the ideal damping stage or damping force at each of the shock absorbers:

Damping adjustment 

The input signals (driving condition) are used by the AIR BODY CONTROL control unit to determine the ideal damping stage in each case and to then actuate the following component parts directly:

The continuous damping adjustment is implemented using single-tube gas-filled shock absorbers. The damping valve units are located at the bottom side on the single-tube gas-filled shock absorbers.

A check valve is integrated into each of the main pistons and bottom valve of the single-hose gas-filled strut. These check valves ensure that an identical flow direction is given through the damping valve unit, both in the rebound and the compression stages. This design enables the damping valve unit of a shock absorber to consist of a single regulated proportional valve only.

The flow area in the damping valve unit can be continuously adjusted through actuation of this proportional valve. More or less oil can flow through the damping valve unit to match the set cross section. The damping behavior then becomes correspondingly softer or harder.

With activated ESP®, the damping is automatically switched over to the hard damping stage (Sport) in critical driving situations irrespective of the driver's request in order to achieve a more stable driving condition. The hard damping stage remains active for as long as the driving situation requires this.

Additional function requirements for system and warning messages 

IMPORTANT The electronic ignition lock control unit sends the status of circuit 15 over the interior CAN to the AIR BODY CONTROL control unit.

System and warning messages 

System and warning messages with safety and system-relevant instructions for the driver are displayed on the instrument cluster.

The AIR BODY CONTROL control unit sends corresponding requests over the suspension FlexRay, electronic ignition lock control unit and the user interface CAN to the instrument cluster. There are various system and warning messages with different fault priorities, depending on the severity of the fault and the urgency of the request for action to be taken.

If several faults exist at the same time, then several fault messages will be displayed.

Texts displayed in instrument cluster:

Lock position 

To ensure that, when intentionally raising the vehicle (e.g. with a vehicle jack), air is not continuously released from the spring bellows (which would lower the vehicle), it is necessary to detect this reduction in wheel load automatically and then to initiate a locking position. To determine whether the vehicle has been raised, a small amount of compressed air is released. If the level sensors do not detect a change in the vehicle level, the locking position is activated.

The locking position is a pure software function which prevents the actuation of the level control valves (draining process). The locking position is activated by the AIR BODY CONTROL control unit by evaluating the signals from the level sensors. There is no fault entry and no display in the instrument cluster.

At a vehicle speed above 10 km/h, the locking position is canceled automatically. The level control system is then in operation again.

IMPORTANT The output stages in the AIR BODY CONTROL control unit can be actuated through the diagnosis at any given time, irrespective of the locking position.

  Electrical function schematic for AIR BODY CONTROL   PE32.32-P-2053-97XBA 
  Overview of system components of AIR BODY CONTROL   GF32.22-P-9993FR 
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