ELECTRONIC HEIGHT CONTROL (EHC) AIR SUSPENSION SYSTEM
OVERVIEW
The self-leveling air suspension system is being introduced on US market E39 sport wagon
vehicles. The system is standard on the 540i and optional on the 528i sport wagons.
The mechanical design is identical to the standard 528i rear suspension however the coil
springs are replaced by air cushion springs. The air pressure in the air springs is managed by
the EHC system which automatically controls the ride height under all operating conditions.
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OVERVIEW OF EHC CONTROL SYSTEM
The control philosophy of EHC is to "Initiate a control sequence only when necessary". The
system offers the following advantages:
� The control system operates independently from the vehicle�s engine (no engine driven
hydraulic pump system as per previous self leveling systems).
� Individual control of the rear wheels is possible
� An uneven load is identified and compensated for
� Uneven road surfaces are identified and not compensated for
� Automatic control is interrupted when cornering
� The system is diagnosable using the DIS or MoDiC
The air suspension system consists of the following components:
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EHC SYSTEM COMPONENTS
CONTROL MODULE
The Control Module is mounted in the module carrier box in the luggage compartment on
the right side. It contains the processing electronics and final stages for operation of the
EHC system.
The control module receives the following
inputs for its processing functions:
� KL 30 & 31 (Power/Ground)
� KL 15
� Left & Right Ride Height Sensors
� K Bus for;
- Vehicle speed
- Engine running
- Door/tailgate - open/closed
The control module incorporates two filters (slow/rapid) for processing the input signals
from the ride height sensors. Depending on the operating mode, either the slow or rapid filter
is used to check the need for a regulating sequence.
The slow filter is used during the normal operation mode to prevent normal suspension travel
from causing the system to make adjustments.
The rapid filter is used during the pre-run and tailgate (LID) modes to ensure that the suspension
is adjusted quickly while the vehicle is being loaded or checked prior to operation.
E39 Features 06
EHC
CONTROL
MODULE
AIR SPRINGS
The air spring is made from a flexible rubber material. It
forms an air tight cavity which provides the calculated
spring rate required for the sport wagon.
As the spring compresses downward the bottom edge of
the rubber material rolls along the vertical surface of the
base mount cylinder.
Air is added or removed from the air spring through its top
port. The top port of each spring is connected to a reservoir and the air supply pipes. The reservoirs are required
to hold additional air due to the compact design of the
springs.
REAR AXLE LEVEL SENSORS
Hall effect sensors are mounted on the left
and right sides of the rear suspension for
ride height detection. They are pivoted by a
coupling rod through the rear axle swing
arms.
The hall sensors produce a varying voltage
input to the control module as the suspension height changes.
If the vehicle is equipped with Xenon headlights the right side sensor contains an
additional sensor for the automatic head
E39 Features 07
light level adjustment system.
WARNING DISPLAYS
If the system is faulted and off-line or set in the transport mode, the following is displayed:
Basic Cluster: A warning lamp indicator is illuminated used on the
basic cluster.
High Cluster: A message is posted in the high cluster matrix display.
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AIR SUPPLY SYSTEM (LVA)
The air supply system is identified as the LVA in the diagnosis program and in the repair
manual. It is mounted in the spare tire well compartment. The components are housed in
a sound deadening carrier, through rubber bushings, to prevent operating noises from
being transmitted through the vehicle�s interior.
With the exception of the compressor relay, individual replacement parts for the air supply
system are not available. If diagnosis determines a defect in any of the other air system
components, complete replacement is necessary.
The air supply system consists of the following components:
� Compressor assembly with;
- Piston compressor
- Electric motor
- Air dryer (desiccator)
- Pressure relief solenoid valve
- Pressure maintenance valve
- Check valves
� Compressor Relay (Replaceable)
� Solenoid Valve Block (2 - two way
valves)
� Lines - including distributor block
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E39 Features 05
AIR SUPPLY SYSTEM (LVA) OPERATION
The single stage piston compressor produces a maximum pressure of 13.5 Bar. The compressor is maintenance free - provided it is used in a dust free environment. This includes
the compressor�s intake air filter.
The compressor is driven by a DC motor that is controlled by the compressor relay through
the control module.
When the compressor is activated, the pressure builds up to a working pressure of 11.4
Bar (+0.8/-1.5 Bar). This is controlled through the pressure maintenance valve. The air
under pressure is fed through the dryer and check valve to the solenoid valve block for the
air springs.
There is one solenoid valve in the valve block for each air strut. This allows the system to
compensate for uneven loads in the vehicle and maintain the vehicles ride height at all
times.
Pressure is drained through the left or right solenoid valve (energized open) the pressure
relief solenoid valve, restricter, check valve and dryer back to the inlet side of the compressor pump.
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EHC SYSTEM OPERATION
A fully functional EHC system is controlled by one of three different modes of operation.
The operation mode is selected by the control module based on current conditions provided by the monitored input signals. The main modes of operation are:
� Pre-Run/Post-Run Mode
� Normal Mode
� Tailgate Mode
Two special operating modes are also included in the control module programming.
� New/replacement mode (pre ZCS encoded). This mode provides basic operation.
� Transport Mode - Transport mode is set at the factory and raises the vehicle 30mm to
prevent vehicle damage during transportation. It must be deactivated with the DIS/
MoDiC prior to customer delivery.
PRE-RUN/POST-RUN MODE
The Pre-Run mode is activated when the vehicle is parked and the control module is in the
sleep mode. Opening a door or the tailgate initiates a system wake up and the control module comes on-line.
The control module performs a self-check of the control electronics and sensors. If no fault
is found, the system will check the ride height and institute a rapid regulation if the height
varies by more than 40mm.
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NORMAL MODE OPERATION
Once the rear lid is closed, KL 15 switched ON and the engine started, the system switches
into the normal operation mode. In the normal mode, the control module will constantly
monitor the input signals from the ride height sensors and will activate a correction if the
ride height deviates by at least 10mm.
TAILGATE OPERATING MODE
The tailgate operating mode is activated if the gate is opened with KL - 15 On and the
engine running. The difference between this mode and the normal operating mode is the
response time is rapid instead of slow .
SPECIAL OPERATING MODES
� ASSEMBLY LINE MODE (New control module)
The assembly line mode refers to control module manufacturing. New control modules
are stored in a deactivated state. The control programming is not active and must first
be ZCS encoded.
After installing a replacement control module, it must be coded using the DIS or MoDiC.
The instrument cluster fault display will remain illuminated until the control module is
coded.
� TRANSPORT MODE
The vehicle rolls off the factory assembly line with the EHC control module in the transport mode. The transport mode inflates the air springs to a higher position (approx.
30mm higher) than the normal mode in order to avoid damage during transit.
The system will not respond to any inputs that would alter the height of the vehicle. The
fault indicator (base cluster) is illuminated or instrument cluster matrix display (high cluster) provides the message �Leveling System� to draw attention to the transport mode
setting.
The Service Functions section of the diagnosis program is used to activate/deactivate
the transport mode. Once the transport mode is deactivated, check the vehicle Ride
Height Offset making sure the vehicle rear axle height is to specification.
CONTROL INTERRUPTS
CORNERING
To prevent unnecessary suspension adjustments while driving through corners, a �control
interrupt� is built into the system. Above 30MPH the control module monitors the left/right
ride height sensors for a difference of 30mm. Exceeding this difference will put the system
into a control interrupt and no adjustment will take place. The control interrupt last for a
duration of 5 minutes.
VEHICLE LIFTING
The ride height control is interrupted when the vehicle is raised on a lift or with a jack. The
system monitors the ride height sensor inputs and when the height limit of 90 mm is
exceeded, the control is switched OFF until the vehicle is lowered again.
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EHC SERVICE INFORMATION
DIAGNOSIS/CODING
� The EHC control module is connected to the diagnostic link. The EHC control module
activates the fault display in the instrument cluster to alert the operator of the off-line status
of the system. The EHC control module stores up to three electrical/electronic
faults.
� Diagnosis/troubleshooting of EHC is carried out using the fault symptom troubleshooting
program of the MoDiC or DIS. The EHC system has an extensive diagnosis program.
� Replacement control modules are shipped in the factory mode. The control modules
must be ZCS encoded using the DIS or MoDiC to activate the operating parameters.
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DIS/MODIC SERVICE FUNCTIONS PROGRAM
The Service Functions program of the DIS/MoDiC provides the Transport Mode activation/
deactivation and Ride Height Offset functions (see next page).
Once the transport mode has been released, or if the system requires left to right side
height adjustment, the ride height "OFFSET" must be carried out to ensure that the vehicles
suspension has a base ride height level starting point.
The "HEIGHT OFFSET" is adjusted using the DIS or MoDiC. The procedure is as follows:
� Place the vehicle on a level surface unloaded.
� Access the Height Offset program in the service function menu.
� Measure the base ride height from the lower edge of the wheel housing to the center of
the wheel hub.
� Check measured height against the specifications listed
� Use the DIS/MoDiC to correct the ride height if the value differs from the listed specification.
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