Electronic rear axle level control, function

P32.33-2014-09

1 Steering angle sensor signal

2 Wheel speed signals

3 Level sensor signals

4 Mileage

5 Country code

6 System/fault messages

7 Outside temperature

8 Atmospheric pressure

9 Vehicle data

10 Status of door/rear wall door contact

11 Wake-up signal for door/rear wall door contact

12 Load current, AIRmatic compressor unit motor

13 Circuit 87

14 Intake air temperature for regulation of AIRmatic compressor unit

15 Signal, circuit 61

16 Actuation of level valves

M14/9 Right rear door CL [ZV] motor

N3/10 ME control unit

N10 Front SAM ECU

N10/8 Rear SAM control unit

N22 AAC [KLA] control and operating unit

N22/7 Comfort AAC [KLA] control and operating unit

N47-5 ESP control unit

N71 Headlamp range adjustment control unit

N80 Steering column module

N93 Central gateway control unit

N97 Rear axle level control system control unit

Y36/8 Electronic rear axle level control valve unit

CAN B CAN class B bus

CAN-C CAN class C

17 Actuation of AIRmatic pressure reduction valve

A1 Instrument cluster

A9/1 AIRmatic compressor unit

A72 Rear door locking unit

B14 Outside temperature display temperature sensor

B22/7 Left rear level sensor

B22/10 Right rear level sensor

L6/1 Left front rpm sensor

L6/2 Right front rpm sensor

L6/3 Left rear rpm sensor

L6/4 right rear rpm sensor

M14/5 Right front door CL [ZV] motor

M14/6 Left front door CL [ZV] motor

M14/8 Left rear door CL [ZV] motor

Conditions of operation

 Ignition On or

 Engine running

Function

The electronic rear axle level control compensates for load effects at the rear axle. To do this the system has two air spring bellows at the rear axle which take over the complete static and dynamic supporting share.

If the AIRmatic compressor unit delivers air via the electric rear axle level control valve into the left air spring bellows or right air spring bellows, then the vehicle level increases.

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Conversely the vehicle level reduces if air is released via the AIRmatic pressure reduction valve (A9/1y1) in the AIRmatic compressor unit.

 Depending on the load in normal driving mode the AIRmatic compressor unit operates at a pressure between 6 and 10 bar or 87 psi and 145 psi.

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Wake-up input

With aid from the right front door rotary tumbler microswitch (M14/5s1), left front door rotary tumbler microswitch (M14/6s1), left rear door rotary tumbler microswitch (M14/8s1), right rear door rotary tumbler microswitch (M14/9s1), rear door locking unit and the activities of the transmitter key (A8/1) when the ignition is switched off (circuit 87 Off) the rear axle level control system control unit is "woken up" by the front SAM control unit via a hardware line in order to check the current vehicle level and to correct it if necessary.

The status of the rotary tumbler microswitch and the rear door locking unit is evaluated via the SAM control unit and reported on to the front SAM control unit via CAN-B.

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When the ignition is switched on the front SAM control unit transmits a wake-up signal to the rear axle level control system control unit, as long as at least one door or the load compartment is open or an opening and closing process is in progress via the transmitter key.

As the actuation of the AIRmatic compressor unit would heavily load the on-board electrical system when the engine is not running, as a rule the AIRmatic compressor unit is not actuated in wake-up mode.

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Lock position

So that when the vehicle is lifted intentionally, e.g. with a jack, air is not released continuously from the bellows, in order to lower the vehicle it is necessary to detect this reduction in load on the wheels and from it to derive a blocking position. The blocking position prevents the actuation of the electronic rear axle level control valve unit and the AIRmatic compressor unit.

If the blocking position is set by the rear axle level control system control unit, there is no display in the instrument cluster and no fault storage. The blocking position is automatically erased by the rear axle level control system control unit under certain conditions (amongst others left front axle wheel speed or right front axle wheel speed > 0 km/h or 0 mph) and the level control functions go into operation again. Output stage actuations via the diagnosis are then performed independently of the locking position status, i.e. they are always possible.