power steering fluid BMW 750IL 1991 E32 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.

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.