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

MODEL 212.2 as of 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
• Terminal 30

Electronic rear axle level control system, 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 occurs when the vehicle is unlocked and a vehicle door is then opened (the liftgate also counts as 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 (N102) via the interior CAN. The front SAM control unit with fuse and relay module evaluates the signals and sends a wake-up signal via chassis CAN 1 (CAN E1) 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. Level control is performed in wake-up mode, e.g. when the vehicle is being loaded and unloaded, without the AIRmatic compressor (A9/1) being activated.

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 rpm)

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 air suspension bellows of the rear-axle suspension struts is released by actuation of the AIRmatic pressure relief valve (A9/1y1). The electronic rear-axle level control system control unit is sent the following information for controlling the compressed air supply, via chassis CAN 1:

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

Additional function requirements for level control 

• Engine running, signal (n > 300 rpm)

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 current rear axle level and actuates both valves in the electronic level control system level control valve (Y36/8) accordingly. To evaluate the current vehicle condition, the electronic rear-axle level control system control unit receives the following information via chassis CAN 1:

• 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 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 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.

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)

When compressed air is admitted into an air suspension bellows due to actuation of the AIRmatic compressor and of a valve in the electronic level control system level control valve, the rear axle level increases at the relevant wheel. 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.

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 locking position suppresses actuation of the level control valve in the electronic level control system and actuation 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) makes information with regard to wheel speeds at the front wheels available, the electronic rear-axle level control system control unit cancels the locking position and begins level control. Actuation of the output stages of the electronic rear-axle level control system control unit via the diagnosis function is performed irrespective of the locking position status, i.e. actuation is always possible.

System and warning messages function sequence 

System and fault messages with instructions for the driver can be output via the instrument cluster.

To output messages, the electronic rear-axle level control system control unit sends corresponding information to the instrument cluster via chassis CAN 1 and chassis CAN 2 (CAN E2) with the front SAM control unit with fuse and relay module interface. The fault codes are also stored in the electronic rear-axle level control system control unit.