1987 BMW 525I TOURING Level Control System Manual

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.

51
Level Control Systems
Kerb (Curb) Mode
The Kerb mode prevents the inclination caused by the vehicle mounting an obstacle with
ine wheel from being compensated. Compensation would cause a renewed inclination of
the vehicle and result in a renewed control operation after the vehicle comes off the obsta-
cle.
The Kerb Mode is activated when the height difference between the left and right sides of
the vehicle is > 32mm for the E65/E66 with single axle air suspension and >24mm for the
E39/E53 with single axle air suspension and lasts longer than 0.9s. Twisting (also over both
axles) > 45mm must occur for the E53 with twin axle air suspension.
There must be no speed signal present. The system switches from single wheel control to
axle control.
The Kerb mode is quit when the difference between the left and right sides of the vehicle is
< 28mm for the single axle air suspension and lasts longer than 0.9s when the speed is
greater than > 1km/h.
If the system switches from Kerb mode to Sleep mode, this status is stored in the
EEPROM.
If the vehicle is loaded or unloaded in Kerb mode, the mean value of the axle is calculated
by the control unit. The value is calculated in the control unit from the changes in ride level
of the spring travel on the left and right sides.
A change in ride level is initiated if the mean value of compression or rebound at the axle is
outside the tolerance band of +/- 10mm. The left and right sides of the vehicle are raised
or lowered in parallel. The height difference between the two sides is maintained.
Curve
Since rolling motions have a direct impact on the measured ride level, an unwanted control
operation would be initiated during longer instances of cornering with an appropriate roll
angle in spite of the slow filtering of the Drive mode. The control operations during corner-
ing would cause displacement of the air volume from the outer side to the inner side of the
curve. Once the curve is completed, this would produce an inclination which would result
in a further control operation. The Curve mode prevents this control operation whereby
when cornering is recognized slow filtering is stopped and a potential control operation that
has started is terminated.
The Curve mode is activated for the E65/E66 single axle EHC and for the E53 twin axle
suspension for a lateral acceleration of > 2m/s
2and deactivated at < 1.5 m/s2.
The lateral acceleration is recorded by the rotation rate sensor.

52
Level Control Systems
Lift
The Lift mode is used to prevent control operations when a wheel is changed or during
work on the vehicle while it is on a lifting platform.
This mode is recognized when the permitted rebound travel at one or more wheels is
exceeded. For the E65/E66 the limit is > 55mm.
A “jack” situation is also recognized when the ride level is stored and the lowering speed
drops below the value of 2 mm/s for 3 seconds.
If the vehicle has been raised slightly and the permitted rebound travel has not yet been
achieved, the control operation attempts to readjust the ride height. If the vehicle is not low-
ered, a car jack situation is recognized after a specific period of time and this ride height is
stored.
A reset is performed if the vehicle is again 10 mm below this stored ride height.
Transport
The Transport mode is set and cleared by means of a diagnosis activation. It serves to
increase the ground clearance in order to ensure a safe transportation of vehicles on trans-
porter trucks. The nominal height of the vehicle is raised in this mode by 30 mm.
When the Transport mode is activated, the air suspension symbol is indicated in the vari-
able telltale in the instrument cluster and a text message is output in the Check Control
Display.
Control operations do not take place in this mode because the vehicle mass does not
change during transportation.
Belt
The Belt mode is set for mounting on the belt in order to avoid control operations.
When the Belt mode is activated, the air suspension symbol is indicated in the variable tell-
tale in the instrument cluster and a text message is output in the Check Control display.
The Belt mode is cleared by means of a diagnostic activation only. The Belt mode can no
longer be set.
New control units are supplied with the Belt mode set.
Control operations are not performed, the safety concept only operates with limited effect.
The Belt Mode is also known as “Band Mode” or “Assembly Line Mode”.

53
Level Control Systems
Operating Principle
Initialization/Reset Performance
When the control unit is powered up after a reset (such as an undervoltage < 4.5 V or by a
load cutoff), different tests and initializations are performed. This system is only enabled
after the tests have been successfully completed and starts to execute the control pro-
grams on a cyclical basis.
Occurring faults are stored and displayed.
Control Sequence
In an ongoing control operation, the high pass filter (fast filter) is always used to prevent the
controlled height from overshooting the nominal value. If a low pass filter (slow filter) were
used to calculate the ride height, brief changes of ride height would be consumed. The
low-pass filter is used while the vehicle is driven. This type of filtering filters out vibrations
which are excited by the road surface.
The high pass filter is used to respond quickly to ride level deviations from setpoint. These
take place while the vehicle is stationary in the event of large load changes.
Both sides of the vehicle are controlled individually, i.e. even the setpoint/actual value com-
parison for both sides is carried out individually. Exception: check for undershooting of the
minimum height in Pre-mode and Kerf mode: consideration of the left and right mean val-
ues in each case.
The following stipulations are applicable here:
 Raising before lowering
 Activation of all valves with control in the same direction
 Individual wheel deactivation
To ensure safe closing of the non-return valve in the air drier, the drain valve is actuated
briefly for 200ms after the control operation has ended.
The permissible ON period of the components is monitored while control up operation are
executed.

54
Level Control Systems
Safety Concept
The safety concept is intended to inhibit any system malfunction, particularly unintentional
control operations, through monitoring of signals and function relevant parameters. If faults
are detected, the system is switched over or shut down depending on the affected com-
ponent. The driver is informed of existing faults via the display. Detected faults are stored
for diagnostic purposes.
In order to ensure high system availability, existing faults, as far as possible, are cleared with
terminal 15 ON. This is done by resetting the fault counter to zero. However, the fault
memory content in the EEPROM is retained and can be read out for diagnostic purposes.
The system is then operational again. The fast troubleshooting helps to detect existing
faults before control operation can take place.
Only lowering is permitted if:
 The permissible supply voltage of 9 volts is undershot
 The permissible compressor running time of 480 seconds is exceeded.
A reset takes place if the voltage is in the OK range of 9 to 16 volts or after the compres-
sor pause time of 100 seconds has elapsed.
Only raising is permitted if:
 The permissible control down period of 40 seconds is exceeded
 The reset takes place the next time the vehicle is driven or after the next control up
operation.
No control operation takes place if:
 The permissible supply voltage of 16 volts is exceeded
The reset takes place as soon as the voltage is in the OK range.

55
Level Control Systems
Workshop Hints
Ride Height Measurement
When checking vehicle ride height with EHC, measure from the lower edge of the wheel
opening to the center of the wheel hub.
Diagnosis
Diagnostic items can be found in the “Control Unit Functions” path when using the DISplus
or GT-1. The functions available are Identifcation (ID page), Read/Clear Fault Memory, Read
Test Codes, Diagnosis Requests and Component Activation.
Service Functions
In the Diagnosis Program, there are numerous Service Functions that can be performed for
the E65/E66 EHC system. By entering into the “Function Selection” program and follow-
ing the “Chassis - Pneumatic Suspension” path all of the Service Functions are listed. The
Service functions include Ride-Level Offset, Transport Mode and Band Mode.

56
Level Control Systems
Review Questions
1. The EHC control module on the E65/E66 is connected to the Bus.
2. List the operating modes that are specific to the E65/E66 Air Suspension System:
3. Why is the accumulator needed on the dual axle EHC system?
4. List the components specific to the Dual Axle EHC system:
5. Which 2 Buses system are connected to the EHC control module on the Dual Axle EHC
system?
6. What components are replaceable on the LVA?
7. What are the 3 possible ride height setting on the Dual Axle EHC system?
8. What type of sensor is used for ride height measurement?