The system measures wheel speed using two sensors on the front wheels and one sensor on the rear wheels. Based on the signals from individual sensors, the electronic device calculates the approximate average speed of the vehicle. By comparing the speed of the vehicle, the control unit determines the slippage of an individual wheel and determines how close the brakes are to lockup.
As soon as the wheel has shown a tendency to lock up, the ABS system closes off the pressure on the brake, maintaining this condition. This means that the pressure in the brake mechanism does not increase even if the pressure on the brake pedal is increased. If there is still a tendency to block, the pressure in this brake is reduced by the ABS system by opening the exhaust valve. This process continues until the wheel starts to accelerate again.
If the wheel starts accelerating again, then 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 with hard braking for each wheel for as long as the brake pedal is released, respectively, until the car comes to a complete stop (speed 3 km/h).
The safety system in the electronic control unit ensures that the ABS is deactivated in the event of a power failure or too low voltage (battery voltage less than 11 V); in this case, the ABS indicator lamp on the instrument panel lights up during the trip. The braking system then functions as usual. When braking, the car behaves as if there is no ABS system.
The hydraulic unit consists of a return pump as well as three solenoid 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 brake fluid pressure can increase to the value provided by the brake master cylinder.
Pressure is constant: the inlet and outlet valves in the solenoid valve device are closed. The brake fluid pressure does not change even if the pressure between the brake master cylinder and the solenoid valve increases.
Decrease in pressure: the inlet valve in the solenoid valve is open. The brake fluid flows out through a reservoir, from which it is pumped back into the brake master cylinder system by a return pump.
This is required so that fluid is not lost from the brake master cylinder. The operation of this pump is felt by noticeable pulsations of the brake pedal. Pump noise is attenuated by a brake circuit damper.
If the ABS indicator comes on while driving, this indicates that the ABS system is disabled.
1. Stop the car, turn off the engine and start it again.
2. Check battery voltage. If the voltage is less than 11V, please charge it.
Attention! If the ABS indicator lights up from the very beginning of the trip, and then goes out, you should look for the reason in recharging the battery that was initially too low.
3. Raise the car, remove the front wheels, inspect the electrical wires for external damage (frayed).
Further ABS checks must be carried out in a workshop. The control device tests the ABS system before each trip and remembers the malfunctions that have appeared, the codes of which remain in its memory even when the battery is disconnected. In company workshops, these errors are decoded using a special device, and then purposefully eliminated.