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

GF32.33-P-0004A Electronic rear axle level control, function
- except CODE (489a) Airmatic (semi-active air suspension)

Component Identification:

Pneumatics Function Diagram:

Function prerequisites
^ Ignition ON or
^ Engine running

Function
The electronic rear axle level control system changes the vehicle level at the rear axle when the vehicle is loaded or unloaded. To do this, the system features the left rear axle damping valve unit (Y53) and the right rear axle damping valve unit (Y54) at the rear axle, which are used to counteract both the static and dynamic load components. The vehicle level is adjusted at the rear axle via the left rear axle damping valve unit (Y53) and the right rear axle damping valve unit (Y54).

If the AIRmatic compressor unit (A9/1) supplies air to the left rear axle damping valve unit (Y53) or the right rear axle damping valve unit via the left rear level control valve (Y36/3) or via the right rear level control valve (Y36/4) (Y54) the vehicle level is raised. The vehicle level is lowered if air is released from the left rear axle damping valve unit (Y53) or from the right rear axle damping valve unit (Y54) via the relief valve (22) in the AIRmatic compressor unit (A9/1).

During normal operation, the AIRmatic compressor unit (A9/1) operates with a pressure of between 6 and 10 bar depending on the vehicle load.

Wake-up input
When the ignition is switched off (circuit 87 off), the driver-side SAM control unit with fuse and relay module (N10/1) "wakes" the rear axle level control system control unit (N97) via a hardware line. The door contact switches (S17/3, S17/4, S17/5, S17/6), the rear-end door locking unit (A72) and opening/closing operations using the transmitter key are also involved in this process. The control unit is woken so that it can check the current vehicle level and lower the vehicle to the design level if necessary.

The AIRmatic compressor unit (A9/1) is not normally actuated in the wake-up mode when the engine is switched off as this would add a large load to the on-board electrical system. (A9/1)

The status of the door contact switches (S17/3, S17/4, S17/5, S17/6) and the status of the rear-end door locking unit (A72) are passed to the driver-side SAM control unit with fuse and relay module (N10/1) by the rear SAM control unit with fuse and relay module (N10/2) and rear-end door control unit (N111), respectively.

The driver-side SAM control unit with fuse and relay module (N10/1) then wakes the rear axle level control system control unit (N97) using a signal passed via the hardwire line.

When the ignition is switched off, the driver-side SAM control unit with fuse and relay module (N10/1) sends a wake-up signal for 5 seconds to the rear axle level control system control unit (N97) if a door or the load compartment is opened, or if the driver uses the transmitter key. The lock function of the rear axle level control system control unit (N97) is activated during this time.

When the ignition is switched on, the driver-side SAM control unit with fuse and relay module (N10/1) sends a wake-up signal to the rear axle level control system control unit (N97) provided at least one door or the load compartment is open or the driver has used the transmitter key to open or close a door/the load compartment.

The rear axle level control system control unit (N97) is activated by a wake-up signal at the wake-up input. The unit remains in operation only throughout the duration of the wake-up procedure even if the wake-up signal at the wake-up input still remains after the wake-up procedure has been completed. The rear axle level control system control unit (N97) can only be woken again after switching off the wake-up signal and then switching it on again.

The wake-up mode is interrupted when circuit 61 is switched on (engine running). The rear axle level control system control unit (N97) then switches to the normal operating mode. The wake-up input is also used together with the vehicle speed signal to detect any change in load. The level control algorithm is changed accordingly.

The electronic rear axle level control system is actuated when:
Circuit 87 is ON
^ The vehicle level is too high and is lowered to the specified level.
^ The vehicle level is too low and is raised to the specified level.

Circuit 61 is on (rear axle level control system control unit (N97) not in diagnostic mode)
^ The initial test is started.
^ The algorithm for level control is active.

Lock position
It is important that the system automatically detects if the wheels of the vehicle are unloaded and that it sets a lock position. A lock position is required so that when the vehicle level is raised intentionally (e.g. using a jack) air is not continuously released from the left rear axle damping valve unit (Y53) or from the right rear axle damping valve unit (Y54) to lower the vehicle. The lock position prevents actuation of the left rear level control valve, the right rear level control valve and the AIRmatic compressor unit (A9/1). If the rear axle level control system control unit (N97) has set the lock position, an error message is not displayed in the instrument cluster (A1) and an error is not stored. The rear axle level control system control unit (N97) automatically deletes the lock position under certain conditions (such as left front axle wheel rpm or right front axle wheel rpm > 0 km/h) and the level control functions are once again operational. The end limits are actuated via diagnosis irrespective of the lock position status, i.e. actuation is always possible.