ABS prevents the wheels from locking up when braking. Due to this, the braking distance is reduced, especially with poor grip on the road surface (e.g. in rain or snow). Thanks to this, the car does not lose control even with full braking.
The anti-lock braking system works as follows. Wheel speed is measured by speed sensors, one for each wheel. Based on the signals from the sensors, the computer calculates the average speed, approximately corresponding to the speed of the car. By comparing the rotational speed of each individual wheel with this calculated value, the computer recognizes the slip condition of one or more wheels.
As soon as the wheel becomes prone to blocking, that is, the pressure of the brake fluid in the caliper of this wheel is too high in relation to the traction force of the wheel with the road, the braking system, on a signal from the computer, begins to prevent the pressure in the caliper of this wheel from increasing (the pressure remains constant even with more force on the brake pedal). If the tendency to lock up remains, the computer reduces fluid pressure in the caliper of that wheel, but as soon as wheel speed increases again, the pressure stops decreasing. When a certain wheel speed is reached, the computer increases the pressure without exceeding the total pressure in the brake system for a given time.
This process is repeated many times for each wheel. ABS works until the foot is removed from the brake pedal, and also shortly before the car comes to a complete stop (at about 4 km/h).
To ensure the normal operation of the ABS on all-wheel drive vehicles, the rear wheel brakes are automatically disengaged by a viscous coupling if necessary.
The emergency shutdown system disables the ABS in case of failure (e.g. broken wire) or at low voltage in the on-board network (below 10.5 V). In this case, the ABS warning lamp lights up on the instrument panel. At the same time, the braking system works as if there is no ABS on the car.
When the ABS warning lamp comes on while driving, the system is deactivated.
1 - computer
2 - ABS warning lamp
3 - speed sensor (in all-wheel drive vehicles, the rear sensors are located on the left and right on the rear axle gearbox)
4 - hydraulic block
5 - overvoltage protection relay
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