AIRmatic control unit, component description - GF32.22-P-5109GZU

MODEL 164.8 

with CODE (489) AIRmatic (air suspension with level adjustment without ADS adaptive damping system) 

with CODE (494) USA version 

Location 

The AIRmatic control unit is located under the driver footplate.

Task 

The AIRmatic control unit is used to control the vehicle level depending on vehicle speed and driver requirements.

Function 

The functions of the AIRmatic control unit are divided as follows:

• Signal conditioning
• Function logic component
• Safety circuit

Signal conditioning 

The following input signals are processed by the function logic component and used for calculations or to provide information:

• Calculation of vehicle speed
  • Wheel speed signal
  • Speed status signal
• Calculation for valve actuation
  • Left front level sensor (B22/8), right front level sensor (B22/9), left rear level sensor (B22/7) and right rear level sensor (B22/10) signal
• Calculation of compressor control
  • Outside temperature, outside air pressure signal
  • Reservoir pressure (AIRmatic pressure sensor (Y36b1))
  • Vehicle speed signal
  • Vehicle level signal
  • Duty cycle
• Calculation of vehicle level
  • Left front level sensor, right front level sensor, left rear level sensor and right rear level sensor signal
• Information signals (operation)
  • Level adjustment switch (N72/1s2) signal
  • Right front door rotary tumbler microswitch (M14/5s1) signal
  • Left front door rotary tumbler microswitch (M14/6s1) signal
  • Left rear door rotary tumbler microswitch (M14/8s1) signal
  • Right rear door rotary tumbler microswitch (M14/9s1) signal
  • Remote control signal

Function logic component 

The conditioned input signals are processed in the logic component and converted into output signals.

• The signal from the wheel sensors is read in by the ESP control unit (N47-5) and carries the following parameters to the AIRmatic control unit via the engine compartment CAN:
  • vehicle speed
  • longitudinal acceleration
  • vehicle status

  A differentiation is made between a moving and a stationary vehicle. Incorrect wheel speed signals are also detected.

• Steering angle sensor signal (N49):

  The vehicle lateral acceleration is calculated with the steering angle sensor signal and the signals from the wheel speed sensors.

• Left front level sensor, level sensor signal level sensor, left rear level sensor and right rear level sensor signal:

  The level sensors provide 2 signals each, which detect the distance between the wheel center and the body on each wheel separately and then transmit it.

• Level adjustment switch signal:

  The control unit uses the signals from the level adjustment switch to determine the driver's preferences for the vehicle level:

  • Raised level
  • Highway

  The driver's requirement remains stored until it is changed again by the driver (also with ignition OFF) or previous speed thresholds are exceeded.

• Level adjustment

  The AIRmatic control unit uses the level adjustment switch setting selected by the driver and the vehicle speed to control the respective level valves thus setting the correct vehicle level according to the vehicle load.

  The AIRmatic is equipped with a central reservoir so that the level control time remains as short as possible (e.g. after loading). If, while loading, a critical level is dropped below, pressure is taken from the central reservoir for noise reasons. During normal control mode when the vehicle is in operation, the suspension struts are supplied directly by the AIRmatic compressor unit (A9/1).

Function requirements 

• Vehicle at rest
• Engine OFF

  If a door is still open after the locking process and if it closed during the run-on time (t = 60 s), as of the condition in which all the doors are locked, the vehicle is nevertheless lowered.

• AIRmatic central reservoir pressure control

  The main control parameters are

  • Pressure in central reservoir
  • External air pressure and
  • vehicle speed.
• Compressor actuation:

  The compressor is controlled via a relay. Since the AIRmatic compressor unit is not designed for continuous operation, the absolute runtime and cooling phase are monitored to prevent overloading of the AIRmatic compressor unit.

• Wake-up function 

  With the aid of the microswitches of the rotary tumblers (via the central locking motors), the rear door locking unit (A72) and the activities of the infrared remote control, the AIRmatic control unit is "woken" with the ignition OFF (circuit 87 = low) by the front SAM control unit (N10) via a hardware line. The AIRmatic control unit then checks and, if necessary, corrects the current vehicle level. The rotary tumbler microswitches and the rear door contact are analyzed by the rear SAM control unit (N10/8) with the result forwarded to the front SAM control unit via CAN-B. When the ignition is switched on the wake-up mode is interrupted by a low/ high transition at circuit 61 and the device returns to the normal mode. Level control is performed in the wake-up mode, e.g. following vehicle loading/unloading operations without switching on the compressor. When the AIRmatic control unit is woken as the result of a door being opened, a time delay occurs which is used for immediate correction of the vehicle level.

  The stationary vehicle is raised by the pressure in the AIRmatic central reservoir. The AIRmatic compressor unit is only used if the AIRmatic central reservoir is empty. It must be ensured that the battery voltage is sufficient to then start the engine

• Lock position 

  To ensure that when intentionally raising the vehicle, e.g. using a vehicle jack, that the suspension struts are not continually deflated, it is necessary to automatically detect any wheel-load reduction and to then initiate a locking position. The locking position serves to prevent actuation of the level valves (deflating processes). If the AIRmatic control unit sets the locking position, nothing is displayed in the instrument cluster (A1) and no errors are saved. The locking position is automatically canceled by the control unit under specific conditions (including wheel speed > 0 km/h), and actuator of the level adjustment switch and then the level adjustment function returns to operation. Output stage actuations via the diagnosis are then performed independently of the locking position status, i.e. they are always possible.

Safety circuit 

The AIRmatic control unit monitors all the important components of the system after they are switched on and during operation. The task of the safety circuit is to detect faulty signals from the sensors, faults in the AIRmatic control unit and faults in the electrical line system. If a fault is detected, the system is switched off and a message is displayed on the multifunction display (A1p13) in the instrument cluster to warn the driver.

The level adjustment is only partially switched off depending on the fault. The system always strives to maintain the selected vehicle level.

The safety circuit also constantly monitors the battery voltage. If the voltage falls below 9 V or exceeds 16.5 V, the system is switched off until the voltage returns to the specified range.