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.
38
Level Control Systems
All control operations in advance / overrun mode are executed as long as pressure is avail-
able in the accumulator. When the accumulator is empty and the engine is turned off, con-
trol operations are directly driven by the compressor. User-activated changes of ride level
and filling of the accumulator are not possible.
Terminal 15
As soon as the ignition is turned on (terminal 15), the user is allowed to lower the ride level
as required.
However, it is still not possible to raise the ride level or fill the accumulator.
Ride level is compensated outside a narrow tolerance range of 10 mm upwards and 10 mm
downwards.
Engine "on"
Ride level compensation, raising and lowering the vehicle's ride height as well as filling the
accumulator are permitted when the engine is running. The compressor also starts up dur-
ing every control operation.
Ride level is still compensated outside the narrow tolerance range of ±10 mm.
As long as the vehicle is stationary, high speed filtered ride level signals are used to detect
a change of load. This allows the system to react immediately to changes in ride level.
As soon as the vehicle is travelling, it changes over to low speed filtered ride-level signals.
The system no longer reacts to bump movements caused by road surface unevenness. A
mean value is formed over a prolonged period of time, i.e. payload is only altered by the
progressive emptying of the fuel tank.
The high speed filter is not used until the vehicle is stationary again and a lid is opened. If
no lid is opened, the vehicle logically cannot be loaded or unloaded.
SleepWake-Up
Power Down
Watch Dog
Ignition “ON”Advance/
Overrun
VA = Load Cutout Signal
Sleep = Temporary Power Down of Control Unit
Wake-up = Activating the Control Unit
Watch Dog = Monitoring
Engine Off
Engine On
Engine Running
Terminal 15 “ON” VA up
VA Down
Terminal 15 “OFF”
