Brake application at front axle, function

W42.25-0130-09

1.1 Proportioning valve

1.2 Relay valve

4.05 Four-circuit protection valve with integral pressure limiter

12.07 Brake wear sensor

12.08 Rpm sensor

13.05 Brake value sensor

V1 Supply, circuit 1

V2 Supply, circuit 2

a Supply pressure (V1, V2)

b Redundancy circuit (pneumatic control pressure)

c Brake pressure

16.07 Proportioning relay valve

20.02 Single-circuit diaphragm brake cylinder

30.07 Pressure limiting valve
(4.6 to 5.5 bar)

33.07 3/2-way valve for auxiliary braking effect

45.01 ABS solenoid valve

When de-energized and the vehicle is operating with a semitrailer, the maximum supply pressure is available via the pressure limiting valve (30.07) at the brake value sensor (13.05) and at the proportioning relay valve (16.07).

If the trailer recognition control unit detects that no trailer is attached, the pressure limiting valve (30.07) is energized and limits the maximum brake pressure at the front axle. The pressure limit adapted for the semitrailer/tractor combination which is necessary when operating without a semitrailer is given on an information plate inside the vehicle. If the tire size is changed, the assigned pressure limit must be taken into account.

Limiting of the front axle brake pressure by the pressure limiting valve (30.07) in tractor vehicles without semitrailer is intended to improve braking stability under high braking deceleration rates.

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The effect of this facility on the maximum achievable braking deceleration is negligible.

When the brake pedal is depressed, the brake value sensor (13.05) measures the pedal travel and relays it to the control unit. In addition, the stop lamp switches are closed, indicating to the control unit that the brake lights must be actuated.

The BS/EPB control unit then actuates the proportioning valve (1.1) in the proportioning relay valve (16.07) and electropneumatic control pressure operates the relay valve (1.2) such that the calculated brake pressure is sent to the single-circuit diaphragm brake cylinders (20.02) via the ABS solenoid valves (45.01).

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The wheel speed sensors (12.08) provide the control unit with feedback on the achieved braking effect and the wheel slip. This feedback is used in conjunction with the position of the brake pedal to regulate the brake pressure precisely.

In the event of a fault in the electronics system, the redundant (pneumatic) control pressure modulated by the brake value sensor (13.05) actuates the relay valve (1.2) in the proportioning relay valve (16.07), ensuring the auxiliary braking effect required by law.