Level Control On Rear Axle, Basic Function - GF32.32-P-9900A

Vehicles with level control system on rear axle only 

Overview 

This document contains information on:

• General 
• Function requirements 
• Function 
  • Start system 
  • Compressed air supply 
  • Level control 
  • Lock position 

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 

Start system 

The control unit for rear axle electronic level control (or the suspension and damping system control unit) is activated (woken up) by the active suspension 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. Once the maximum operating time has been reached, the electrical compressor is switched off and enters a 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 affected wheel increases when the electric compressor delivers air into the air suspension bellows through 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 steering column tube module control unit.
• Wheel speeds: detected by 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 the rear axle electronic level control receives front left and front right wheel speed signals from the ESP® control unit, with v > 10 km/h. Level control functions are in operation again.

The output stage of the rear axle electronic level control unit is actuated through XENTRY Diagnosis, irrespective of the given locking status and actuations are always possible.

	Function schematics
	Function schematic for level control on the rear axle	Model 206, 214, 236, 254 with code 480 (Self-levelling suspension)	PE32.22-P-2500-97B