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 detected and the system is regulated to maintain the correct level. During prolonged dynamic 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 pressurized 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.