2003 BMW 745LI Pressure sensor

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.

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

28
Level Control Systems
System components
The X5 Two Axle Air Suspension System (EHC2) utilizes the air supply unit from EHC
mounted in the luggage compartment, with the following components added or modified:
 Air Supply Unit (with redesigned compressor and drier)
 Pressure Accumulator
 Valve Unit
 Ride Height Sensor
 Air Suspension Strut
 Control Unit
 Switch Assembly
Air Supply Unit
As on vehicles with single axle air suspension, the air supply unit is located in the luggage
compartment under the spare wheel. As with the single axle air suspension, the auxiliary
tanks for the rear air spring bellows are located in the luggage compartment.
The air supply unit is configured similarly to the single axle air supply unit for the ride-height
control system of the E39, which is currently in production.
The functions are implemented by activating a compressor and various valves in the air
supply unit and on the air accumulator valve unit.
The maximum pressure of the air supply unit is 21 bar.
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
Pneumatic System of EHC2

30
Level Control Systems
Compressor
Compressor operation is the same as in EHC with the following technical improvements:
 Addition of a temperature sensor (Located on the compressor cylinder head)
Temperature sensor switches off the compressor at temperatures above 110
0C.
 Extended compressor ON time (180 seconds)
 Redesigned air drier to compensate for additional air volume.
Pressure Accumulator
The twin axle air suspension system now features a pressure accumulator which forms an
air accumulator valve unit in combination with the valve. The air accumulator valve unit is
located beneath the vehicle floorpan in the right-hand sill area.
Top: Air Drying
Bottom: Dissipation of water to airCompressor with Drier
Pressure Accumulator Unit which
includes:
Pressure Accumulator
Pressure Sensor
Valve Unit
Front

31
Level Control Systems
The pressure accumulator decreases the load on the compressor and significantly reduces
the time required for large changes in ride height.
The EHC 2 control module monitors system pressure via a pressure sensor mounted on
the accumulator. Normal system pressure is 15.7 +/- 0.7 bar. Minimum system pressure
is 9 bar.
The pressure accumulator's charge is sufficient to fill the four suspension struts once from
the Access position to the normal position and compensate for vehicle load up to maximum
gross weight.
Valve Unit
In the valve unit, four bellows valves and the pressure accumulator valve are activated.
The bellows valves and the pressure accumulator valve are solenoid valves which are
closed under spring pressure when de-energized.
Accumulator/Valve Unit
1. Air Lines
Yellow-Black Front
Red-Blue Rear
2. Pressure Accumulator
3. Connecting Cable
4. Pressure Sensor
5. Valve Unit
Valve Unit
1. Connections for Air Lines
2. Pressure Accumulator valve
Pneumatic Layout of Control Valve
1. Pressure Accumulator
2. Accumulator Pressure Sensor
3. Pressure Accumulator Valve
4. Bellows Valves
5. From the Air Supply Unit
NW Size of opening/tubing size in mm.

33
Level Control Systems
Air Suspension Strut
Rear
Minor modifications were made to the rear axle air springs. The air springs and the dampers
are configured separately at the rear axle.
Front
The air suspension replaces the steel suspension at the front axle, i.e. the spring bellows is
attached to the damper. The front air suspension strut and the impact absorber form a
complete unit.
Control Unit
The EHC2 Control Unit is located behind the glovebox adjacent to the General Module.
The connector is a black 54 pin connector.
Inputs received directly into the control unit are:
Ride Height Level Sensor (X4) Up and Down requests from the switch assy.
General Module (Load cutout signal) CAN Bus Inputs
K Bus Inputs Pressure Sensor
Compressor Temperature
O Ou
ut
tp
pu
ut
ts
s
i
in
nc
cl
lu
ud
de
e:
:
Air Unit Control (Activation) Front and Rear Axle Valves
LED’s for Switch Unit Compressor Relay
Pressure Accumulator
Air Suspension Strut
1. Auxiliary Spring
2. Air Bellows
3. Damper