1 Hydraulic unit
2 Speed sensor
3 Brake booster
4 Master cylinder
5 Expansion tank with brake fluid
The ABS system prevents the wheels from locking during heavy braking, thanks to which the vehicle's controllability is fully maintained.
The ABS system is ready to function when the ignition is switched on and speeds from 5-7 km/h. It controls all braking processes in the blocking area after speeds over 12 km/h.
RPM sensors on each wheel measure the speed at which the wheels are spinning. Based on the signals from the individual wheels, the control unit calculates an average speed that roughly corresponds to the speed of the vehicle. As a result of comparing the rotational speed of an individual wheel with the average rotational speed of all wheels, the ECU determines a wheel spin condition and can thereby determine that the wheel is approaching a locked state.
As soon as blocking begins, that is, when the pressure in the brake caliper is still too high in relation to the grip of the wheel with the road surface, the control unit maintains a constant fluid pressure based on signals from the speed sensors. This means that the pressure in the brake caliper does not increase even when the brake pedal is pressed harder. If the danger of blocking has not been eliminated, the brake fluid pressure is reduced by opening the bleed valve. At the same time, it opens just enough so that the wheel begins to rotate again, then a constant pressure is again maintained.
If the wheel speed exceeds a certain value, the hydraulic system again increases the pressure, but without exceeding the braking pressure, which is created by the driver when the brake pedal is pressed.
This process is repeated for each wheel until the brake pedal is released or until the vehicle almost stops (5-7 km/h).
The safety circuit in the electronic control unit ensures that the ABS system is disabled in the event of a failure (e.g. broken wire) or when the supply voltage drops (battery voltage below 10.5V), which is indicated by the ABS warning light on the dashboard. In this case, the behavior of the car when braking is the same. as if the ABS system had not been installed.
The hydraulic unit consists of a suction pump, as well as solenoid valves for the brakes of the front and rear wheels.
Pressure build-up: the inlet valve in the solenoid valve is open. The brake fluid pressure in the brake caliper can rise to a value determined by the brake master cylinder.
Pressure stabilization, inlet and outlet valves in the solenoid valve are closed. The brake fluid pressure in the brake caliper does not change, even if the pressure difference between the brake master cylinder and the solenoid valve increases.
Depressurization: The bleed valve in the solenoid valve is open. The brake fluid flows through the accumulator into the bleed pump, which pumps out the fluid, overcoming the pressure in the brake master cylinder.
This is required so that all the brake fluid is not pumped out of the master brake cylinder. The operation of the pump is felt by the pulsations of the brake pedal. The noise of the pump is damped by dampers in each brake circuit. When the ABS warning light comes on:
- Stop the car, turn off the engine and start it again.
- Check battery voltage. If the voltage is less than 10.5 V. charge the battery.
Caution: If the ABS warning light comes on at the beginning of a trip and then goes out, this means that the battery voltage was initially low and then increased due to the charge from the alternator.
- Raise the car, remove the wheels, check the integrity of the electrical wires of the speed sensors (no fraying).
- Have the ABS system further checked at a workshop.