engine BMW 750IL 1993 E32 Level Control System Manual

44
Level Control Systems
Control Unit
The EHC control module is located in the right rear luggage compartment area in the mod-
ule carrier next to the battery. On the E65/E66, the control module is connected to the K-
CAN S. The EHC control module receives the following information:
 Vehicle Ride Height
 Load Cutout Signal
 Terminal 15 ON/OFF
 Vehicle Speed
 Lateral Acceleration
 “Engine Running” Signal
 Flap Status (Doors/Trunk)
The Control unit decides on a case by
case basis whether a control opera-
tion is required in order to compen-
sate changes in load. It prevents
intervention in the case of other caus-
es. This makes it possible to adapt
the frequency, specified height, toler-
ance thresholds and battery load opti-
mally by means of the control opera-
tion to the relevant situation.
In addition to handling the self levelling
suspension, the control module moni-
tors the system components as well
as storing a displaying faults. The
control module has full diagnostic
capability.
The EHC module is a 26 pin module
with an ELO type connector. The
module is connected to the K CAN S.
The majority of the input messages
are from the K CAN S Bus.
EHC Module Location

49
Level Control Systems
Post Mode
The Post-mode is adopted in order to compensate any inclination or to adjust the ride
height after driving and between the Pre-mode and Sleep mode.
The Post-mode is limited in time to 1 minute. The Post-mode is only executed if the engine
has been running before the system switches into this mode. If the engine has not been
previously running, the system switches directly from Pre-mode into Sleep mode.
The control operation is performed in a narrow tolerance band of +/- 6mm and is terminat-
ed at +/- 4mm. The fast signal filter is used.
In the event of an inclination (Kerb Mode), the control operation takes place for the nominal
heights applicable in this situation.
Pre-Mode
The Pre-mode is activated by the “Load Cutoff” signal (e.g. by opening the door or unlock-
ing with the remote control). The Pre-mode then stays set for 16 minutes and is restarted
with a change in status.
The ride height of the vehicle is monitored and evaluated with a wide tolerance band.
In Pre-mode, the vehicle is only controlled un to the nominal height if the level is significantly
below the nominal height. The control tolerance band is -40mm from the mean value for
the single axle air suspension and -20mm for the dual axle system. This control tolerance
ensures that the vehicle is only controlled up in the case of large loads in order to increase
the ground clearance prior to departure. Small loads give rise to small compression travel
and this is compensated only when the engine is started. This control setting helps reduce
the battery load.
With the single axle air suspension, the vehicle is controlled down when the mean value
derived from both ride height signals is > 0mm and one side is in excess of +10mm. With
twin axle air suspension, the vehicle is controlled down when one side is >15mm.
In this mode, only the mean value of the two height signals is considered when deciding
whether there is an need for control operation.
The control operation is executed as long as pressure is available in the accumulator. When
the accumulator is empty and the engine is turned off, the control operation is driven direct-
ly by the compressor. User-activated changes of ride level and filling of the accumulator are
not possible.
Control operations which were started in other modes are continued with the inner toler-
ance bands applicable to these modes.
There is no inclination identification in Pre-mode.

50
Level Control Systems
Normal
The normal mode is the starting point for the vehicle’s normal operating state. It is obtained
by way of the engine running signal.
Ride level compensation, changing the vehicle’s ride height and filling the accumulator are
possible. The compressor starts up as required.
A narrower tolerance band than that in Pre-mode cab be used because the battery capac-
ity does not have to be protected. The fast filter is used with a narrow tolerance band of
+/- 10mm. In this way, ride level compensation takes place outside a narrow band of 10+/-
10mm. The faster filter allows the system to respond immediately to changes in ride level.
Evaluation and control are performed separately for each wheel.
When a speed signal is recognized, the control unit switches into Drive mode. When the
vehicle is stopped, the control unit remains in Drive mode. The system switches back into
Normal mode when a door or the boot (trunk) lid is also opened. If none of the doors or
the boot lid is opened, the vehicle cannot be loaded or unloaded.
This prevents a control operation happening when the vehicle (for example) is stopped at
traffic lights and the ride height is above the mean axle due to the pitching motion on the
rear axle.
Drive
The Drive mode is activated for E39/E53 single axle air suspension when a speed signal of
>4km/h is recognized. The Drive mode is recognized from >1km/h for the E65/E66 single
axle air suspension and for the E53 twin-axle air suspension system.
Low pass filters are used. In this way, only changes in ride height over a prolonged period
of time (1000 seconds) are corrected. These are merely the changes in ride height, caused
by vehicle compression and a reduction in vehicle mass due to fuel consumption. The high
pass (fast) filter is used during the control operation. The slow filters are recognized at the
end of the control operation. The slow filters are re-initialized at the end of the control oper-
ation. The markedly dynamic height signals caused by uneven road surfaces are filtered
out.