Basic Function

Level Control System On Rear Axle

Electronic Level Control:

a) Basic Function
The level control system on the rear axle comprises a hydro-pneumatic auxiliary suspension, which is automatically engaged at a given spring deflection. On sedans, for example, level control begins functioning with approximately two passengers seated and 90 lb. of luggage in the trunk. The control point selected for the loaded vehicle is the level which corresponds to the designed level of the pertinent vehicle model with the appropriate suspension version.

The level control system on the rear axle operates hydro-pneumatically and essentially comprises the following three main parts:

1. The suspension elements, including spring struts (5) and accumulators (4).

2. The pressure oil system, comprising the pressure oil pump (1) and oil reservoir (2).

3. The control equipment, comprising level control unit (3) with appropriate operating linkage.

On a vehicle with the level control system on the rear axle, the spring struts (5) are a partially supporting suspension system, i.e. they are used in addition to the rear springs to support the rear end of the vehicle. The spring struts are installed instead of rear shock absorbers and each is connected to a gas-filled accumulator (4). In addition to their supporting function, they act as shock absorbers (refer to Section b "Suspension elements" below). Suspension Elements

When the engine is running, the oil pump (1) draws hydraulic oil from the oil reservoir (2) via suction line (A) for delivery to the level control unit (3) via the pressure oil line (P).

The level control unit is connected with a torsion bar (77) on the rear axle via a lever-type linkage and allows the delivered oil to flow back unpressurised into the oil reservoir in the "neutral" or "discharge" positions. The circulation of hydraulic oil is interrupted when the rear end of the vehicle drops below a given vehicle level control point. At this point, the control disc of the level control unit closes the return flow duct to the oil reservoir, which in turn has moved the level control unit into the "filling" position. (refer to Section d "Control equipment" below). Control Equipment

The delivered oil, now subject to the relevant pressure, will flow through the check valve in the level control unit and through the pressure oil lines (S1, S2) and (P4) into the accumulator (4) and spring struts (5). The oil quantity supplied under pressure will lift the rear end of the vehicle until the specified vehicle level is attained. The level control unit will simultaneously switch over into the "neutral" position and re-establish the conditions of pressureless circulation described above.

When the vehicle is unloaded or its rear end is lifted above the level position, the level control unit is set to "discharge". In such a case, in addition to the oil supplied by the oil pump, the oil initially required for lifting the lowered rear end of vehicle will flow from the pressure reservoirs and spring struts back into the oil reservoir.

The oil discharge allows the vehicle level to drop until the level control unit has again attained the "neutral" position.

To make sure that the spring struts will be able to perform their additional function as shock absorbers independent of the position of the level control unit, the oil pressure in the spring struts does not drop below a given basic pressure.

Maintenance of this basic pressure is ensured by the fact that below a given pressure the discharge valve in the level control unit is pushed out of reach of the control disc by a spring.

For overload protection, the level control unit is also provided with a pressure release valve. When there is excessive pressure in the spring elements, the pressure release valve allows the oil supplied by the oil pump to flow back into the oil reservoir even when the level control unit is in the "filling" position.

An oil drain plug integrated into the level control unit is used for draining the basic pressure which is available independent of the positions of the level control unit.

Note
Adjusting the vehicle according to the specifications is a prerequisite for the correct functioning of the level control system as well as providing comfortable ride and optimal driving characteristics, both in ready-to-drive condition as well as under load.

In ready-to-drive condition, the rear end of the vehicle is supported only by the rear springs and the basic pressure in the suspension elements. The basic pressure is maintained by the drain valve in the level control unit, independent of the position of the lever. A rear end which is too low in the ready-to-drive position should never be set any higher by adjusting the connecting rod of the level control unit. This would merely result in overcompensation under load. It would also result in poor driving characteristics, oil-flow noises and knocking noises under deflection, as well as an excessively high trailer hitch during towing operations. Under high loads as well as during acceleration, this would also lead to a maximum pressure in the spring elements (limited by the level control unit) being reached, such that under these driving conditions the opening noise of the pressure relief valve would be heard as a whistling and knocking noise.

For this reason the rear spring level must be corrected when the ready-to-drive level is too low. However, a prerequisite for adjusting the ready-to-drive level is that the specified basic pressure in the suspension elements is present.