Operating unit, function - GF42.46-P-4200-03SL

WARNING: This page is about a different car, the 2009 Mercedes-Benz E550 and 2008 Mercedes-Benz E550. However, it is still accessible from the selected car via links, so may be relevant.

Functional diagram

Fig 1: Operating Unit Function Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Function during normal braking

During a normal brake application the operating unit (1) is isolated from the wheel brakes by the closed separation valves in the hydraulic unit. Primarily it now has the task of conveying pedal feel to the driver.

The simulation of the pedal feel is generated in the primary circuit by the brake pressure simulator (2). This happens in 3 stages:

The brake fluid is pressed against the force of a soft spring (2a) via the piston 1 (2d).
If the brake pedal is operated further, then the harder spring (2b) is compressed via the piston 2 (2e). The pressure of this spring increases progressively.
Finally the back pressure of the rubber buffer (2c) still acts in the case of very heavy brake applications.

Function if the SBC system fails

If a fault occurs in the system, the operating unit functions like a normal tandem brake master cylinder without additional brake power assistance at the front axle circuit of the brake system. The floating piston (1a) and primary piston (1b) push on the front left brake and the front right brake, respectively.

The inflow to the brake pressure simulator (2) is cut off by displacing the floating piston (1a).