Overview
This document contains information on:
General
Function requirements
Function
General
Rear axle level control monitors the rear axle level and actuates the air suspension bellows on the rear axle if necessary. The objective of rear axle level control is to maintain the rear axle level at a constant irrespective of the load condition and of any driving dynamic-related pitch motions of the vehicle.
Rear axle level control is implemented using two air suspension bellows.
Function requirements
Drive train operational
Engine in operation
Function
The system is made up of the following subfunctions:
Start system
Level control
Compressed air supply
Lock position
Start system
The electronic rear-axle level control control unit is activated (woken up) by the active chassis FlexRay after the vehicle is unlocked in order to check the current vehicle level.
Compressed air supply
During normal driving operation, rear axle level control operates with different pressures depending on the load condition. The pressure is supplied by an electric compressor. The electric compressor is actuated by the control unit for rear axle electronic level control.
The duty cycle is monitored constantly to protect against overload. The electric compressor is switched off after reaching the maximum operating time. The electric compressor enters the cooling phase. Excess compressed air in the air suspension bellows of the rear axle suspension struts is reduced by actuating the pressure relief valve. The control unit for rear axle electronic level control receives the current atmospheric pressure and intake air temperature for controlling the compressed-air supply system from the combustion engine control unit.
Level control
The vehicle level at the concerned wheel increases when the electric compressor delivers air into air suspension bellows via a corresponding valve of the rear axle electronic level control valve unit.
Conversely, the vehicle level decreases when air is released from the air suspension bellows by a corresponding valve in the rear axle electronic level control valve unit and at the pressure relief valve in the electric compressor.
The following information is provided to the control unit for rear axle electronic level control to assess the current driving condition:
Steering wheel angle, recorded by the steering wheel angle sensor and the steering column tube module control unit
Wheel speeds, from the ESP® control unit
The wheel speeds are the basis for calculating the vehicle speed. Lateral acceleration is calculated from the wheel speeds and the steering wheel angle.
The following signals are read in by the control unit for rear axle electronic level control to calculate the rear axle level:
Left rear level sensor
Right rear level sensor
Lock position
So that air is not continuously released from the suspension struts (to lower the vehicle) when the vehicle is being raised intentionally (e.g. using a jack), it is necessary to detect this reduction in wheel load automatically and then initiate a locking position. The locking position prevents actuation of the level control valves (release process). If the locking position is detected by the electronic rear-axle level control control unit, nothing is displayed in the instrument cluster and no faults are stored.
The locking position is automatically canceled if the control unit for rear axle electronic level control receives front left and front right wheel speed signals from the ESP® control unit of v > 10 km/h. Level control functions are in operation again.
Output stage actuations of the control unit for rear axle electronic level via diagnosis take place irrespective of the locking status and are always possible.