ABS System Operation

Type of operation
Under normal conditions, the standard brake system performs until locking of a wheel is detected by the ABS control unit.
During standard braking, inlet valves are normally opened and outlet valves are normally closed. When the ABS control unit detects a wheel lock, the hydraulic unit controls each valve to maintain, increase or decrease pressure.





The ABS control unit detects the speed and the deceleration rate of each wheel with wheel speed sensors. When braking the wheel speed is decreased and difference between wheel speed and vehicle speed is generated. If deceleration rate of some wheels exceeds expected value (Point A),ABS control unit determines wheel lock has occurred and opens the corresponding outlet solenoid valves to reduce brake fluid pressure. If locking pressure on the wheels is exceeded (Point B and wheel speed reaches point C, then the ABS control unit closes outlet valve of the corresponding wheels to increase fluid pressure.





For vehicles with ABS, the following symptoms can occur, but indicate normal operation of the ABS.

Principle of operation





1. Standard braking (Non-ABS braking)
When the brake pedal is depressed, the power brake unit closes the central valve of the master cylinder and generates pressure on the brake hydraulic pressure line. This opens the solenoid inlet valve and closes the outlet valve and transmits hydraulic pressure from the master cylinder to the wheel cylinders.

ABS braking





2.1 Maintaining pressure
After the ABS control unit detects wheel lock due to excessively increased pressure, the ABS control unit closes the inlet valve to avoid further increase in hydraulic pressure.





2.2 Decreasing pressure
If wheel lock continues, the ABS control unit opens the corresponding outlet valve so that brake fluids flow to the pressure accumulator. This reduces pressure only for wheels that are locked.





2.3 Increasing pressure
When wheels have been returned to normal speed from locking, the ABS control unit stops operating the solenoid valve and returns to standard braking operation.