Electronic rear axle level control, function - GF32.33-P-0004FL

MODEL 212.2 up to model year 2014 without CODE 488 (Steel/air suspension) without CODE 489 (AIRMATIC (air suspension with continuous damper adjustment)) 

Function requirements, general 

• No overvoltage or undervoltage
• Circuit 30

Electronic rear axle level control system in general 

The electronic rear axle level control system monitors the rear axle level and, if necessary, actuates the air suspension struts on the rear axle to compensate for the effect of loads.

The system is made up of the following subfunctions:

• System startup function sequence 
• Compressed air supply function sequence 
• Level control function sequence 
• Lock position function sequence 
• System and warning messages function sequence 

System startup function sequence 

The system is started by activating the interior CAN (CAN B). This is done by unlocking the vehicle and then opening a vehicle door, whereby the liftgate is also viewed to be a vehicle door. The corresponding status of the front doors, rear doors and liftgate is received by the front SAM control unit with fuse and relay module (N10/1) from the rear SAM control unit with fuse and relay module (N10/2) over the interior CAN. The front SAM control unit with fuse and relay module evaluates the signals and sends a wake-up signal over the chassis CAN (CAN E) to the electronic rear-axle level control system control unit (N97). Once the electronic rear-axle level control system control unit has been activated, the current rear axle level is checked and corrected if necessary. A level control takes place in wake-up mode e.g. by loading and unloading the vehicle without cutting-in the AIRmatic compressor (A9/1) when fill level of the AIRmatic central reservoir is sufficiently high.

Activation of the electronic rear-axle level control system control unit starts an initialization period which is used to correct the rear axle level if necessary.

This reduces the amount of time which passes before level correction is completed, therefore increasing vehicle availability.

Additional function requirements for compressed air supply 

• Engine running, signal (n > 300/min)

Compressed air supply function sequence 

The system operates with different pressures depending on the vehicle load.

The compressed air supply is provided by the AIRmatic compressor. This is actuated by the electronic rear-axle level control system control unit over the AIRmatic relay (K67).

The duty cycle is monitored to protect against overload. Once the maximum operating time has been reached, the AIRmatic compressor is switched off and enters a cooling phase.

Excess compressed air in the rear-axle suspension strut air suspension bellows is released by actuating the AIRmatic pressure reduction valve (A91/y1). The electronic rear-axle level control system control unit is sent the following information for controlling the compressed air supply over the chassis CAN:

• Atmospheric pressure and intake air temperature from CDI control unit (N3/9) (with diesel engine) or from ME-SFI [ME] control unit (N3/10) (with gasoline engine)

Additional function requirements for level control 

• Engine running, signal (n > 300/min)

Level control function sequence 

Level control at the rear axle is realized by means of 2 suspension struts with integral bellows.

The objective of level control is to keep the rear-axle level constant irrespective of the load status and any driving-dynamics related pitch motion of the vehicle. To this end, the electronic rear-axle level control system control unit calculates the rear-axle level at any given time and actuates both valves in the level control valve of the electronic level control system accordingly (Y36/8).

To evaluate the vehicle condition at any given time, the electronic rear-axle level control system control unit receives the following information over the chassis CAN:

• Rotation angle, recorded by steering angle sensor (N49) and provided by steering column tube module control unit (N80)
• Wheel speeds of front wheels, from Electronic Stability Program control unit (N30/4) (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles), except hybrid) or from the Premium Electronic Stability Program control unit (N30/7) (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles), except hybrid) or from regenerative braking system control unit (N30/6) (with hybrid) The signals from the following components are read-in directly by the electronic rear-axle level control system control unit to calculate the rear axle level:
• Left rear level sensor (B22/7)
• Right rear level sensor (B22/10)

If compressed air is now released into a spring bellows through actuation of the AIRmatic compressor and a valve in the electronic rear-axle level control system control unit, the rear axle level on the relevant wheel rises. Inversely, the rear-axle level is lowered when the compressed air is released through a corresponding valve in the electronic level control system level control valve or through the AIRmatic pressure reduction valve. The wheel speeds are used to calculate the vehicle speed and - in combination with the steering wheel angle - to determine the lateral acceleration acting on the vehicle.

Lock position function sequence 

To ensure that compressed air is not continuously drained out of the bellows in order to lower the vehicle when the vehicle is raised intentionally, e.g. with a jack, this reduction in the load on the wheels is automatically detected and a lock position is then activated accordingly. The lock position suppresses actuation of the level control valve in the electronic level control system and that of the compressor. The rear-axle level is then no longer altered by the system. If the electronic rear-axle level control system control unit detects the lock position, nothing is displayed in the instrument cluster (A1) and no errors are saved. As soon as the Electronic Stability Program control unit (except CODE 233 (DISTRONIC PLUS), except CODE 990 (AMG vehicles), except hybrid) or the Premium Electronic Stability Program control unit (with CODE 233 (DISTRONIC PLUS) or with CODE 990 (AMG vehicles), except hybrid) or the regenerative braking system control unit (with hybrid) provides information on the wheel speeds for the front wheels, the rear-axle electronic self-leveling suspension control unit cancels the locking position and starts the level control process. The output stages of the electronic self-leveling suspension control unit are actuated through the diagnosis function, irrespective of the lock position status, i.e. actuation is always possible.

System and warning messages function sequence 

The instrument cluster enables system and fault messages to be displayed with instructions for the driver.

To display the messages the electronic rear-axle level control system control unit sends corresponding information over the chassis CAN to the instrument cluster. The fault codes are also stored in the electronic rear-axle level control system control unit.