Basic Function
A. General Function
Level control on rear axle is a hydro-pneumatic auxiliary suspension which engages automatically starting at a given deflection. On sedans, e.g., the level control will start approximately at a load of 2 persons on front seats and approx. 40 kg luggage in trunk. In each respective case, the control point for the loaded vehicle is the level which conforms to the design position of the respective vehicle model with the respective spring variant.
The level control system on rear axle operates hydro-pneumatically and comprises essentially the following three main components:
1. The suspension elements, comprising spring struts (5) and pressure reservoirs (4).
2. The pressure oil system, comprising pressure oil pump (1) and fluid reservoir (2).
3. The control unit, comprising level controller (3) with respective actuating linkage.
Sedans And Coupes:
T-Sedans:
On vehicle with level control on rear axle, spring struts (5) are partially supporting members, that is, they are used for carrying rear end of vehicle in addition to rear springs. The spring Struts are installed instead of rear shock absorbers and are connected to a gas-filled pressure reservoir (4) each. In addition to level control they are simultaneously serving as shock absorbers. (refer to section B. "Suspension elements" below). Suspension Elements
With the engine running, the pressure oil pump (1) sucks hydraulic oil from fluid reservoir (2) via suction line (A), so that the oil will flow to level controller (3) via pressure oil line (B1).
The level controller is connected by means of a linkage to torsion bar (10) on rear axle. When set to positions "neutral" or "return flow" the delivered oil will flow back without pressure into fluid reservoir. This circuit of the hydraulic oil is interrupted when rear end of the vehicle drops below vehicle level, since the control disk of the level controller is now keeping the return flow duct to fluid reservoir closed, which in turn results in moving the level controller to position "filling". (refer to section D "Control equipment" below). Control Equipment
The oil delivered in this manner is now flowing into pressure unit (4) and spring struts (5) via check valve in level controller and pressure oil lines (B2) and (B3) while maintaining the required pressure. The resulting pressure increase will raise rear end of vehicle until the specified vehicle level has been attained. The level controller will simultaneously change to position "neutral" and once again establish the condition of pressureless circuit delivery described further above.
When the vehicle is unloaded or when its rear end is raised beyond level position, the level controller is set to "return flow". In such a case, in addition to the oil delivered by the pressure oil pump, the oil initially required for increasing the pressure and also for raising the lowered rear end of the vehicle will return to fluid reservoir from pressure reservoirs and spring struts.
As a result of the outflowing oil, the vehicle level will drop until the level controller is once again in "neutral" position.
Oil pressure in spring struts should not drop below a given basic pressure, so that the spring struts can fully meet their additional jobs of shock absorbers acting independent of positions of level controller. Making sure of that basic pressure is attained by means of a compression spring which pushes the return flow valve in level controller out of reach of control disk when the available pressure is below a given mark.
As a protection against overloading the system, the level controller is additionally provided with a pressure relief valve, which permits the oil delivered by the pressure oil pump to flow back into the fluid reservoir when the pressure in the pressure element is too high, even in "filling" position of level controller.
A distributor with bleed screw located in line system between level controller and suspension elements or a level controller with integrated bleed screw (2nd version starting March 1977) serves for draining the basic pressure resulting independent of the position of the level controller.
Note
A prerequisite for correct operation of level control as well as for obtaining good riding comfort and optimal riding characteristics is a vehicle level set according to specifications both in condition ready-for-driving and under load.
In condition ready-for-driving the rear end of the vehicle is carried by the rear springs and the basic pressure in the suspension elements only. The basic pressure is maintained by drain valve in level controller independent of position of lever. If rear end of vehicle is too low in condition ready-for-driving, raising of rear end should never be attempted by adjusting connecting rod of level controller, since in such a case the vehicle level would be continuously regulated at too high a level under load. The result would be poor riding characteristics, oil flow and knocking noises during deflection, as well as an essentially too highly placed ball head when operating a trailer. In addition, under high load as well as during acceleration, the max. pressure in suspension elements limited by level controller, would be attained so that during such driving conditions the opening noise of the pressure relief valve would be constantly heard as a whistling and knocking noise. If the level is too low in ready-for-driving condition, corrections should therefore be made on rear springs. A prerequisite for evaluating the ready-for-driving level is, however, that the full pressure in suspension elements is available. Prior to measuring the semi-trailing arm position in ready-for-driving condition, the level controller should therefore be moved for a short moment in upward direction into "filling" position and then downwards in "emptying" position with the connecting rod disconnected and the engine running (for "checking vehicle level").