air condition BMW 525I TOURING 1989 E34 Level Control System Manual

3
Level Control Systems
Level Control Systems
Purpose of the System
The primary function of Level Control Systems is to maintain the height of the vehicle as
closely as possible to a predetermined level under all load conditions.
This constant level allows the suspension system to maintain the alignment geometry.
Camber and Toe in are minimally affected when the ride height is consistent. In addition,
the headlight range stays consistent throughout the various operating conditions.
The Level Control System is designed to operate in the event of static changes such as
when passengers are entering or exiting the vehicle or quasi-static such as s when the fuel
tank is emptying while driving.
The Level Control System come in various configurations such as hydraulic (hydropneu-
matic) or pneumatic only systems.
Hydropneumatic systems use high pressure hydraulic fluid which is dampened by a gas
cushion from a nitrogen charged accumulator. These system use and electro-hydraulic
pump or an engine driven piston pump. These systems are installed as follows:
Hydropneumatic Level Control System with electro-hydraulic pump- This
type of system is used on the early 5 Series vehicles (E12 and E28), the 6 Series
(E24) and the early 7 Series (E23). This system uses an electric motor, pump and
an expansion tank which is connected by hydraulic lines to the rear spring struts.
The rear spring struts are also connected to a pair of pressure accumulators which
are “Nitrogen Charged”. There is a control switch which mounted on the rear axle
which monitors the position of the stabilizer bar. Changes in ride height are detect-
ed and the system is regulated to maintain the correct level. During prolonged dyna-
mic movements during acceleration and braking, the level control system is disabled
by an acceleration sensor (mercury switch) and brake light input to the hydraulic
control unit.
Hydropneumatic Level Control System with engine driven piston pump- This
system can be found on the 7 Series (E32 and E38) and the 5 Series Touring (E34).
This configuration is similar in operation to the previous system with a few changes.
There hydraulic pressure now comes from an engine driven piston pump. This
pump is mounted in tandem with the power steering (radial type) pump. The pres-
surized fluid is sent to a control valve which distributes the fluid to the rear spring
struts and pressure accumulators. The control valve is attached to the rear sway
bar by a lever, changes in ride height will move the lever which will influence fluid flow
to the spring struts.
The next generation of BMW Level Control Systems evolved into pneumatic only systems
which are referred to as EHC. EHC will be discussed later in this module.

27
Level Control Systems
Two Axle Air Suspension (E53 EHC II)
Purpose of the System
The two axle air suspension system (EHC2) offers advantages over the single-axle air sus-
pension with respect to ride comfort and off-road capability.
Lowering the entire body makes it easier to enter, exit, load and unload the vehicle.
The vehicle's off-road capability was improved by providing the possibility for increasing the
ground clearance of the body.
The driver can now choose between three different ride levels which can be set with a rock-
er switch, as required. Automatic ride-height control for payload compensation and
automatic inclination compensation continue to be fitted.
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The automatic payload compensation facility for the single-axle air suspension did not per-
mit driver control. The driver could not actively control the system to make it easier to enter
and exit or load the vehicle.
Ride level was compensated via the rear axle only.
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The new system allows the ride-height control system to be controlled actively by the dri-
ver.
The twin axle air suspension allows both axles to be lowered evenly and in parallel.
As a result, it is easier for the occupants to enter, exit, load and unload the vehicle.
On the E39, the load of the complete rear axle was born for the first time by air suspension
in combination with the optional ride height control system. The system was controlled
automatically under all operation conditions, and there was no possibility for driver inter-
vention on the X5, the rear axle previously had single axle air suspension only. The air sup-
ply unit and the control unit were adopted from the E39. The air springs were adapted to
the X5.
There is a standard version and a sports version.
The ride-height control system (EHC) was supplied as standard in combination with the
M62 engine and is available as an optional extra in combination with the M54 engine.
EHC2 is optional on both the M62 and M54 versions of the X5 and not available on the
4.6is X5.

35
Level Control Systems
Principle of Operation
Ride Height Control Operations
In addition to the automatic ride-height control system for payload compensation, the dri-
ver can set three different vehicle ride heights.
 Off-road (+25 mm), high ground clearance to a max. speed of 50 km/h
 Standard (0 mm), normal ride level
 Access (-35 mm), for entry and exit, loading and unloading to a max. speed of 35 km/h
or can be activated in Standard mode at road speeds < 25 km/h
The various heights are selected by scroll rocker. Light emitting diodes indicate the present
ride height setting.
Ride height can be adjusted from terminal 15 and with the doors closed. The hood and tail-
gate may be open.
The system also controls inclination automatically, like the single axle air suspension.
All control operations are executed without stopping at intermediate levels. The vehicle is
configured pneumatically in such a way that the front and rear axles can be lowered in par-
allel in any load situation. Depending on the load situation, either the front axle or the rear
axle is slightly quicker. On account of the different control speeds, a difference in height
between the two axles is possible during all control operations. If a max. permissible thresh-
old is exceeded, the quicker axle is stopped briefly.
The various levels can be preselected while travelling. Changeover between ride levels is
effected at the speed threshold values defined in the control unit. The control unit monitors
the change-over.
As soon as the driver sets a new target ride level by pressing a button or when a change-
over is initiated automatically by a specific driving condition, the LED for the current ride
level remains lit and the LED for the target ride level begins to flash.
When the new level is reached, the LED for the previous level goes out and the LED for the
new level reached stays lit permanently.

39
Level Control Systems
Workshop Hints
If a threshold level is exceeded on all 4 wheels when the vehicle is stationary, the control
unit assumes that the vehicle has been raised on a workshop platform.
There are three possible reset conditions for workshop platform recognition:
 The original level values are undershot at all four wheels,
 A selection is made by button,
 A speed of >40 km/h is recognized for 3 s.
Vehicle jack
If the lowering speed at a wheel is too low during the lowering operation, the system
assumes that the wheel is jacked up. However, the downward velocity must be less than
a certain preprogrammed speed threshold. If the system detects a jacked wheel, it stores
the height of this wheel.
Car jack recognition is reset when the stored ride height is again undershot. When a trav-
elling speed of 40 km/h is maintained for at least 3 s, another control attempt is performed.
The car jack recognition can also be reset by button selection.
Please note that the system also controls ride height in diagnostic mode. For this reason,
Belt Mode must be activated before carrying out work on the system or before setting the
vehicle ride height.
Belt Mode:
Heights are fixed and are not compensated. If Belt Mode is set, the function LED is off. The
text message "ride-height control system inactive" appears in the instrument cluster.
Transport Mode:
The Transport Mode setting is for transportation purposes. When the ignition is turned on,
the message "ride-height control system inactive" appears. Heights are increased or
decreased depending on ignition key status, e.g. ride height is reduced when the vehicle is
lashed to a ship or train and raised when the "Engine on" signal is generated and when the
vehicle is transported on a transporter truck.
The correct ride height is set to ± 5 mm via "Activate components." The left and right ride
levels are set separately at the rear axle. The ride levels are then set at the front axle. The
left and right air springs are adjusted jointly for this purpose.
Following this, the new ride height for the front and rear axles is stored via the "Offset func-
tion."
Before replacing components, the system must be depressurized! This is done in the diag-
nostics via "control unit functions," "Component activation," "Pressure-relieve front axle/rear
axle." Repeat the activation procedure 6 times.