Caution: On models with a speed control system, the ABS unit performs the functions of the anti-lock braking system (ABS) and the vehicle speed control system (ASR).
Models without vehicle speed control system
The anti-lock braking system is factory-installed on most vehicles and can be installed as an optional extra on the remaining vehicles. The system consists of a hydraulic unit with three hydraulic solenoid valves, one for each front brake and one valve for the rear brakes, an electrically driven pump, three wheel sensors and an electronic control unit (ECU). The anti-lock braking system prevents the wheels from locking during heavy braking. This ensures that the vehicle remains controllable under full braking. Based on the signals from the wheel sensors, the ECU can determine the vehicle speed. By comparing the speed of an individual wheel with the average vehicle speed, the control unit determines whether the wheel is locked. When the wheel is about to lock, the brake fluid pressure is too high, so the pressure in the caliper is reduced by a signal to the control unit by activating the appropriate solenoid valve, which isolates the caliper of the corresponding brake from the master cylinder.
If the wheel speed continues to decrease, the ECU signals the return pump to pump fluid from the corresponding caliper into the master cylinder. Once the wheel starts to rotate at normal speed, the pump switches off and the electromagnetic valve opens, causing brake fluid to be supplied to the caliper in normal mode and the wheel to be braked. The operation of the anti-lock system during hard braking is repeated on each wheel until force is applied to the pedal or the vehicle stops. The anti-lock system cycle is repeated up to 10 times per second. When the anti-lock system is operating, a pulsation is felt on the brake pedal.
Any malfunction in the anti-lock system causes it to be switched off. In this case, the ABS indicator light on the instrument panel lights up and the brake system operates normally. Troubleshooting of the anti-lock system must be done at a service station.
Models with Automatic Speed Regulator (ASR)
On models with the speed control system, the hydraulic system consists of four electromagnetic valves, an electrically driven return pump, a valve that switches the hydraulic device from the anti-lock system to the speed control system functions, and a speed control system sealing valve. The vehicle is equipped with two electronic control units (ECU), one of which controls the hydraulic unit, and the other controls the actuators of the throttle position. An additional pump and pressure accumulator are also installed. In order to create and maintain the pressure in the system necessary for the braking functions during the operation of the speed control system.
The vehicle speed control system prevents the wheels from losing traction with the road surface. The system adjusts the rotation speed of the rear wheels by braking the wheel or closing the throttle valve depending on the vehicle speed. In some cases, both the wheel braking and the throttle valve closing may be performed.
The wheel is braked as follows. When the wheel loses traction with the road surface, the wheel begins to rotate at an increased speed, and the hydraulic device supplies brake fluid under pressure from the pressure accumulator to the caliper of the corresponding wheel and brakes the wheel. After the wheel returns to normal rotation speed, the hydraulic device returns the fluid to the pressure accumulator.
The throttle valve is controlled as follows. After the wheel loses traction, the second electronic control unit activates the throttle valve actuator, which, by closing the throttle valve, reduces engine speed. As soon as the wheel starts to rotate at normal speed, the throttle valve actuator returns it to its original position.
If you detect any faults in the vehicle's speed control system, you must contact a service station.