GF32.33-P-0004GV Electronic Rear Axle Level Control, Function

GF32.33-P-0004GV Electronic rear axle level control, function
- with CODE (488) Steel/air suspension
- as of model year 09

Function requirements, general
^ No overvoltage or undervoltage
^ Circuit 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 rear axle level control system control unit (N97) is the master control unit for the electronic rear axle level control system.
The system is made up of the following component functions:
^ System start-up function sequence
^ Compressed air supply function sequence
^ Level control function sequence
^ Lock position function sequence
^ System and warning messages function sequence

System start-up function sequence
The system is started when bus idle is canceled.

This takes place when the central locking (CL) is unlocked using the transmitter key (A8/1) and a vehicle door is then opened, whereby the rear-end door also counts as a vehicle door. The rear SAM control unit (N10/8) sends the corresponding status for the front doors, rear doors and the rear-end door to the interior CAN. The front SAM control unit (N10) reads-in this information, evaluates it and actuates the rear-axle level control system control unit via a direct line.

Once the rear-axle level control system control unit has been woken, 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 unit (A9/1) being activated. Activation of the 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 performed, and also increases the availability of the vehicle.

Additional function requirements for compressed air supply
^ Engine running (circuit 61 ON) n > 300 rpm

Compressed air supply function sequence
The system operates with different pressures depending on the vehicle load.
The compressed air is supplied by the air compressor motor (A9/1m1) in the AIRmatic compressor unit. This is actuated by the rear-axle level control system control unit via the air suspension compressor relay (F58kG).

The duty cycle is monitored to protect against overload. Once the maximum operating time has been reached, the air compressor motor is switched off and enters a cooling phase. Excess compressed air in the bellows of the rear axle suspension struts is released via actuation of the AIRmatic pressure reduction valve (A9/1y1) located in the AIRmatic compressor unit.

The following information is made available to the rear-axle level control system control unit via the engine compartment CAN to control the compressed air supply:
- Atmospheric pressure and intake air temperature by CDI control unit (N3/9) (with diesel engine) or by ME-SFI control unit (N3/10) (with gasoline engine)
- Ambient air temperature, over the interior CAN from the AAC [KLA] control and operating unit (N22) (without code (581) Comfort automatic air conditioning) or from the comfort AAC control and operating unit (N22/7) (with code (581) Comfort automatic air conditioning) and forwarded by the central gateway control unit to the engine compartment CAN

Additional function requirements for level control
^ Engine running (circuit 61 ON) n > 300 rpm

Level control function sequence
Level control at the rear axle is realized by means of two 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.

The rear-axle level control system control unit calculates the current rear axle level and actuates the left rear level control valve (Y36/8y1) or the right rear level control valve (Y36/8y2) in the electronic rear-axle level control valve unit (Y36/8) accordingly.

To evaluate the vehicle condition at any given time, the rear-axle level control system control unit receives the following information over the engine compartment CAN:
- Rotation angle, recorded by steering angle sensor (N49) and steering column tube module (N80)
- Wheel speeds of front wheels, from ESP control unit (N47-5)

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 directly by the 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 to a bellows due to actuation of the air compressor motor and the level control valve, the rear axle level increases at the wheel concerned. Conversely, the rear axle level decreases if compressed air is released via the corresponding level control valve or via 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 lock position suppresses actuation of the rear axle level control valve unit and the air compressor motor. The rear axle therefore remains at the current level.

If the 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 ESP control unit provides wheel speed information for the front wheels, the rear-axle level control system control unit cancels the lock position and level control starts.

Actuation of the output stages by the rear-axle level control system control unit via the diagnosis function is performed irrespective of the lock 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 multifunction display (A1p13) in the instrument cluster.

To output the messages, the rear-axle level control system control unit sends the corresponding information to the instrument cluster via the engine compartment CAN. The fault codes are also stored in the rear-axle level control system control unit.