suspension BMW 528I TOURING 1997 E39 Level Control System Manual

Initial Print Date:3/02Revision Date:
Subject Page
Hydraulic Level Control System .........................................................
Air Suspension Systems (EHC) ...........................................................
Table of Contents
Level Control Systems

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.

4
Level Control Systems
Hydropneumatic Rear Leveling System
This module pertains to the hydropneumatic rear suspension system with the engine dri-
ven piston pump. The earlier system using the electro-hydraulic pump will not be dis-
cussed.
The self-leveling suspension system is designed to maintain vehicle ride height under
loaded conditions.
The system is fully hydraulic, utilizing a tandem oil pump to supply pressure to both the
suspension system and power steering system.
The system is installed on:
 E32 - 735 iL, 740iL and 750iL
 E34 - Touring 525i and 530i
 E38 - 740 iL and 750iL

6
Level Control Systems
Tandem Pump
The tandem pump consists of a vane pump for power steering and a radial pump for self-
leveling suspension. Both pumps are driven by the same shaft and have separate oil feeds
for each system.
The radial pump has the capacity to pump 102 liters of oil per minute. The maximum pres-
sure is limited to 1900 Psi (130bar) by the pressure regulator located on the control valve.

7
Level Control Systems
Oil Reservoir
The single reservoir stores fluid for both the self-leveling and power steering systems.
Incorporated in the reservoir, is a reed type level warning switch. If the fluid level drops and
the contact closes, a signal is sent to the check control module and a warning will be dis-
played in the instrument cluster matrix display.
The hydraulic fluid required for the self-leveling suspension system varies between models
and installed equipment. The following fluid is used:
 Pentosin CHF 7.1
 Pentosin CHF 11S
 Power Steering Fluid (BMW)
This type of fluid is used because of its
low viscosity and low noise characteris-
tics. It cannot be mixed with other fluid
types.
The self-leveling system fluid type is
always marked on the top of the hydraulic
reservoir. Always use the correct fluid
never mix with another fluid.
When filling the system, be aware of the
status of the level control system. If the
vehicle is loaded, the level will appear to
be low. Do not fill the system unless it is
in the resting state. Otherwise, an over-
flow situation could result.

9
Level Control Systems
Control Valve
The rotary control valve located on the rear suspension has three positions:
 Raise
 Level
 Lower
The control lever is attached to the rear stabilizer bar to sense vehicle loading. When the
vehicle is loaded (passenger or luggage) the suspension drops and the stabilizer bar twists.
The control lever is moved in the "raise" direction.
With the engine running, oil flows from the pump to the bottom of the rear struts. The body
of the car is lifted and the control lever returns to the level position.
In the level position, the oil
bypasses the valve and
returns to the reservoir
through the return line.
A minimum pressure of
440 Psi (30bar) is main-
tained at all times. If ser-
vice is required, a bleed off
valve is installed on the
control valve to drain the
systems pressure.

12
Level Control Systems
Camber Warning Switch (E32 AND E34 Touring)
The self-leveling suspension systems used in the E32 and the E34 Touring are equipped
with a camber warning switch. The switch is mounted on the right rear axle support and
connected through a rod to the semi-trailing arm.
The function of the switch is to warn the driver of an
overload condition or an unsafe driving situation.
If the switch detects a rear wheel camber of -3.5 deg.
A signal is sent to the check control module and dis-
played on the dash.
A time delay of 8 minutes is incorporated to prevent
the warning from being displayed while driving
through turns.
The E38 does not use the camber warning switch
due to the design of the rear suspension.

13
Level Control Systems
Air Suspension Systems (EHC)
Purpose of the System
Air Suspension systems were first introduced on the 1999 E39 Sportwagon. Since then
the E53 (X5) and E65/E66 (7Series) models were available with this new system. EHC are
available is two configurations, the Single Axle Air Suspension System and the Dual Axle
Air Suspension System. The Dual Axle (EHC II) system is only available on E53 X5 models
from the 2002 model year.
EHC systems can be found on the following vehicles:
 E39 Sportwagon 528i, 525i and 540i from the 1999 Model Year (Single Axle)
 E53 X5 3.0i and 4.4i from the 2000 Model Year (Single Axle)
 E53 X5 4.6iS from 2002 (Single Axle)
 E53 X5 3.0i and 4.4i from 2002 (Dual Axle EHC II)
 E65/E66 from 2003 Model Year (Single Axle)
There are some functional changes with these systems. On EHC, the entire axle load is
borne by the air suspension. The underlying control philosophy of EHC is “Control only
when absolutely necessary”. This means that brief changes in the ride height are not com-
pensated (such as potholes). This avoids any unnecessary control operations.
The advantages of air suspension are as follows:
 Control is independent of the vehicle engine
 Single-wheel control is possible
 Lateral locking is affected
 A distinction is made between load and drive states
 An inclined load is identified and corrected
 However, an inclination is not compensated
 Self diagnosis can be performed
 Diagnosis with DISplus or GT-1 is possible
 An automatic interruption of control takes place in case of cornering and wheel
changes.

14
Level Control Systems
EHC System Overview
EHC I Single Axle Air Suspension E39/E53
EHC II Dual Axle Air Suspension E53
1. Air Supply Unit
2. Rear Axle Air Bellows
3. Ride Height Sensors
4. Pressure Accumulator/
Valve Unit
5. Front Axle Air Bellows
6. Control Unit

16
Level Control Systems
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 fil-
ter is used to check the need for a regulating sequence.
The slow filter is used during the normal operation mode to prevent normal suspension trav-
el from causing the system to make adjustments.
The rapid filter is used during the pre-run and tailgate (LID) modes to ensure that the sus-
pension is adjusted quickly while the vehicle is being loaded or checked prior to operation.