BAS brake application, function

P42.31-2064-06

30 Positive

31 Ground

87 ABS Voltage supply

A1e41 ESP warning lamp

A7/7 BAS brake booster

A7/7b1 BAS diaphragm travel sensor

A7/7s1 BAS release switch

A7/7y1 BAS solenoid valve

L6/1 Left front rpm sensor

L6/2 Right front rpm sensor

N47-5 ESP control unit

X11/4 Data link connector

CAN-C Controller Area Network bus Class C (engine compartment) (CAN-C)

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

The ESP control unit receives input signals from the following components:

 ME-SFI [ME] control unit (N3/10) (Controller Area Network bus, class C) (engine compartment)

 BAS diaphragm travel sensor

 BAS release switch

Normal operation

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

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The system is ready for braking. The 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 ESP to form output signals for the following components:

 BAS solenoid valve

 ESP warning lamp (via CAN-C)

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

 normal operation,

 BAS pressure buildup

 BAS pressure reduction

is to be activated.

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Brake application in normal operation

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

On brake application, outside 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 the foot pressure.

P42.31-0212-76

Illustration shows partial braking

a Atmospheric pressure

b Reduced vacuum

c Vacuum

A7/7 BAS brake booster

A7/7s1 BAS release switch

A7/7y1 BAS solenoid valve

BAS pressure buildup

The ESP control unit uses the brake pedal actuation speed and the vehicle speed to detect whether the preconditions exist for BAS activation.

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

BAS is activated when the following conditions exist simultaneously:

 speed > 8 km/h

 BAS release switch operated

 no current faults detected

 system enabled (after self-test)

 pedal speed threshold exceeded

P42.31-0213-76

Illustration shows pressure buildup

a Atmospheric pressure

c Vacuum

A7/7 BAS brake booster

A7/7s1 BAS release switch

A7/7y1 BAS solenoid valve

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 servo assistance.

BAS pressure reduction

The BAS release switch informs the ESP control unit that emergency brake application has ended. The BAS solenoid valve is no longer activated and the maximum brake servo assistance is switched off. The normal brake servo assistance remains unchanged.

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

 BAS release switch not operated

 speed < 3 km/h

 operating time > 20 s

 A fault is detected that causes the ESP warning lamp to be activated.

P42.31-0214-76

Illustration shows pressure reduction

a Atmospheric pressure

c Vacuum

A7/7 BAS brake booster

A7/7s1 BAS release switch

A7/7y1 BAS solenoid valve

Brake application with BAS, pressure reduction

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