BMW 525I TOURING 1989 E34 Level Control System Manual

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.

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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”.

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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.

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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.

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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.

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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?

9. How much pressure is in the rear air strut on the E66 when it is stored?
10. List the correct color for the air hoses on the EHC I system:
Left Right
11. The “Band Mode” is also known as
and
12. What operating modes are specific to the Dual Axle EHC system?
13. What is the maximum system pressure on the EHC I system (E39/E53)?
14. What is unique about the rear air struts on the E66?
15. What is the maximum air pressure of the air supply unit on the E53 with Dual Axle Air
Suspension?
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Level Control Systems