Braking with ABS control at rear axle, function

W42.30-0023-06

1 Brake cylinder on front axle

2 Brake cylinder on rear axle

13.04 2-circuit service brake valve with front axle control

16.01 Air admission relay valve (front axle)

18.05 Trailer control valve for 2-line brake

26.07 Mechanical ALB controller

26.09 Pneumatic ALB controller

A10 ABS control unit

B30 Left front axle rpm sensor (12.08)

B31 Right front axle rpm sensor (12.08)

B32 1st left rear axle rotation rate sensor (12.08)

B33 1st right rear axle rotation rate sensor (12.08)

V1 Supply, circuit 1

V2 Supply, circuit 2

V3 Supply, circuit 3

Y16 Left front axle ABS solenoid valve (45.01)

Y17 Right front axle ABS solenoid valve (45.01)

Y18 Left rear axle ABS solenoid valve (45.01)

Y19 Right rear axle ABS solenoid valve (45.01)

Requirement

 Vehicle speed > 6 km/h

Function sequence

1 The left front axle rpm sensor (B30), right front axle rpm sensor (B31), left 1st rear axle rpm sensor (B32) and right 1st rear axle rpm sensor (B33) continuously measure the rotational speed of the wheels.

2 The ABS control unit (A10) evaluates the rpm signals from the rpm sensors and, in this way, continuously receives new information on the current slip, deceleration and acceleration values at the wheels. By comparing the circumferential speeds of the individual wheels, the electronics determine a reference speed which roughly corresponds to the actual vehicle speed.

3 The brake pedal is depressed.

4 Brake pressure is fed from the 2-circuit service brake valve with front axle regulation (13.04) to the brake cylinders at the front axle (1) via the left front axle ABS solenoid valve (Y16) and right front axle ABS solenoid valve (Y17).

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5 The brake cylinders at the rear axle (2) are actuated via the mechanical ALB controller (26.07) or via the pneumatic ALB controller (26.09), and brake pressure is applied to the left rear axle ABS solenoid valve (Y18) and right rear axle ABS solenoid valve (Y19) depending on the load.

 The ABS solenoid valves at the front and rear axle are not actuated by the ABS control unit (A10). The compressed air can flow through the solenoid valves without resistance.

6 The pressure in the brake cylinders increases and the vehicle is braked. The rpm sensors at the wheels on the front and rear axle signal a constant deceleration of the circumferential speeds to the ABS control unit (A10).

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7 The pressure in the brake cylinders continues to increase. The ABS control unit (A10) detects (e.g. from the signal from the right 1st rear axle rpm sensor (B33)) that the wheel slip at the right rear wheel is such that the wheel may begin to lock.

8 The ABS control unit (A10) triggers ABS control for the right rear wheel.

9 The right rear axle ABS solenoid valve (Y19) is actuated by the electronics of the ABS control unit (A10) and switches to "pressure hold". The connection to the brake cylinder of the right rear wheel is interrupted and the pressure applied to the brake cylinder is maintained. The pressure in the left brake cylinder and at the brake cylinders on the front axle (1) continues to increase irrespective of this.

 Triggering of ABS control also causes the ABS control unit (A10) to interrupt operation of the engine brake and retarder (if fitted) and, on all-wheel drive vehicles, to deactivate the interaxle differential lock in the transfer case.

10 The ABS control unit (A10) continues to monitor the behavior of the right rear wheel and switches the right rear axle ABS solenoid valve (Y19) to "pressure reduction" if the wheel still has a tendency to lock.

11 The brake cylinder of the right rear wheel is then vented via the right rear axle ABS solenoid valve (Y19) until the ABS control unit (A10) detects that the tendency to lock no longer exists.

 If the braking operation has not yet been completed, "pressure reduction" is followed by a subsequent gradual "pressure buildup" to increase the braking power. This, in turn, is followed by "pressure hold" and, if necessary, another "pressure reduction" phase. The brake pressure can be adapted to the current requirements 3 to 5 times per second until the braking operation has been completed or until the vehicle has come to a standstill.

12 When the brake pedal has been released, the brake cylinders are vented via the relay valve, ALB controller and service brake valve.