The system measures the wheel speed using two sensors on the front wheels and one sensor on the rear wheels. Using signals from individual sensors, the electronic device calculates the estimated average speed of the vehicle. By comparing the vehicle speed, the control device determines the degree of slippage of an individual wheel and determines how close the brakes are to locking.
As soon as the wheel shows a tendency to lock, the ABS system closes the pressure on the brake, maintaining its current state. This means that the pressure in the brake mechanism does not increase even when the pressure on the brake pedal increases. If the tendency to lock continues, the pressure in this brake is reduced by the ABS system by opening the release valve. This process continues until the wheel begins to accelerate again.
If the wheel starts accelerating again, the ABS increases the pressure in its brake system, but it does not reach the pressure value in the general brake system. This process is repeated during sharp braking for each wheel until the brake pedal is released, respectively, until the car comes to a complete stop (speed 3 km/h).
A safety system in the electronic control unit ensures that the ABS is switched off if the power supply is faulty or the voltage is too low (battery voltage less than 11 V); in this case, the ABS indicator light on the instrument panel lights up while driving. The brake system functions as usual. When braking, the car behaves as if the ABS system is not present.
The hydraulic unit consists of a return pump and three electromagnetic valves – one for the front wheels and one for the rear brakes.
Pressure increase: The inlet valve in the solenoid valve device is open, the fluid pressure in the brake mechanism can increase to the value provided by the brake master cylinder.
Constant pressure: The inlet and outlet valves in the electromagnetic valve unit are closed. The fluid pressure in the brake mechanism does not change even if the pressure between the master cylinder and the electromagnetic valve increases.
Pressure reduction: The inlet valve in the solenoid valve is open. Brake fluid flows out through the reservoir, from where it is pumped back into the master cylinder system by the return pump.
This is required to prevent fluid loss from the main brake cylinder. The operation of this pump is felt by noticeable pulsations in the brake pedal. Pump noise is dampened by a brake circuit damper.
If the ABS indicator comes on while driving, it indicates that the ABS system is disabled.
1. Stop the car, turn off the engine and start it again.
2. Check the battery voltage. If the voltage is less than 11V, charge it.
Attention! If the ABS indicator lights up from the very beginning of the trip and then goes out, the reason should be sought in recharging the initially too low battery voltage.
3. Raise the car, remove the front wheels, inspect the electrical wires for external damage (abrasions).
Further ABS checks should be carried out in a workshop. The control unit tests the ABS system before each trip and remembers any malfunctions that occur, the codes of which remain in its memory even when the battery is disconnected. In branded workshops, these errors are decoded using a special device and then purposefully eliminated.