BAS brake application, function - GF42.31-P-2000GZ

MODEL 164

Fig 1: BAS Brake Application Function Diagram

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

The functions of the BAS brake booster (A7/7) are controlled using the components of the electric/electronic system.

The ESP control unit (N47-5) receives input signals from the following components:

ME control unit (N3/10) (Controller Area Network bus class C) (engine compartment) (CAN-C)
BAS diaphragm travel sensor (A7/7b1)
BAS release switch (A7/7s1)

Normal operation

In this case, the brake application is not at the higher pedal speed (that requires activation of BAS). The BAS solenoid valve (A7/7y1) in the BAS brake booster is de-energized in the basic position.

The system is ready for braking. Service brake and ABS function normally. This mode is also set if there is a fault in the BAS.

The input signals are processed in the ESP control unit to form output signals for the following components:

BAS solenoid valve
ESP warning lamp (A1e41) (via CAN-C)

Based on input signals the ESP control unit decides which of the 3 modes

BAS Normal mode,
BAS pressure buildup,
BAS pressure reduction

is to be activated.

Brake application in normal operation

The BAS brake booster is constantly supplied with the intake manifold vacuum via the vacuum line. If the brake is not actuated, equal vacuum exists on both sides of the diaphragms. The compression spring holds the vacuum piston in its starting position.

On brake application, atmospheric air is fed to the back of the diaphragms via a duct. A pressure difference that increases with the braking is created between the front and rear chambers of the booster and the resulting force acts in the direction of the master brake cylinder and supplements foot pressure.

Illustration shows partial braking

Fig 2: Identifying BAS Brake Booster And BAS Release Switch - Illustration Shows Partial Braking

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

BAS pressure increase

The ESP control unit recognizes that the preconditions exist for activation of BAS from the speed of the brake pedal and the vehicle speed.

The BAS solenoid valve is activated by the ESP control unit and the brake booster produces the maximum brake assistance power.

BAS is activated when the following conditions exist

simultaneously:

Speed > 8 km/h
BAS release switch is operated
no current fault is detected.
the system is enabled (after self-test)
The activation threshold of the pedal speed has been exceeded

Illustration shows pressure buildup

Fig 3: Identifying BAS Brake Booster And BAS Release Switch - Illustration Shows Pressure Buildup

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Brake application with BAS, pressure buildup

The activation conditions for a BAS brake application are fulfilled, the BAS solenoid valve is opened. Atmospheric pressure is applied to the back of the power diaphragm. The pressure difference between the front and back of the diaphragm provides maximum brake power assistance.

BAS pressure reduction

The ESP control unit receives information from the BAS release switch that the emergency brake application has terminated. The BAS solenoid valve is no longer activated and the maximum braking assistance force is switched off. The normal braking assistance force remains unchanged.

BAS is switched off when one of the following conditions exists:

BAS release switch (A7/7s1) is not actuated
Speed <3 km/h
Duration of activation >20 s
A fault is detected that causes the ESP warning lamp to be activated.

Illustration shows pressure release

Fig 4: Identifying BAS Brake Booster And BAS Release Switch - Illustration Shows Pressure Release

Courtesy of MERCEDES-BENZ OF NORTH AMERICA.

Brake application with BAS, pressure release

The BAS release switch is switched passively. The solenoid valve is switched off and only creates sufficient brake pressure to correspond to the position of the brake pedal.

BAS brake booster component description	GF42.31-P-4004GZ
Description of components in ESP control unit	GF42.45-P-5118GZ