Under what circumstances does ABS activate?

[Ribeiro]

ABS activates during vehicle braking. During braking, it automatically controls the braking force to prevent wheel lock-up, maintaining a state where the wheels are both rolling and slipping (with a slip ratio of around 20%) to ensure maximum traction between the wheels and the road surface. Here is a detailed explanation of ABS: 1. During braking, ABS quickly determines the lock-up state of each wheel based on speed signals from the wheel speed sensors. It then closes the normally open input solenoid valve for the wheel that is starting to lock up, keeping the braking force constant. If the wheel continues to lock up, the normally closed output solenoid valve is opened, causing the braking pressure on that wheel to rapidly decrease as the brake fluid is diverted to the reservoir, preventing complete wheel lock-up due to excessive braking force. This keeps the braking state at the optimal point (with a slip ratio S of 20%), ensuring the best braking performance and maximum driving safety. 2. The brake fluid in the front chamber of the master cylinder is under dynamic pressure, which pushes the reaction sleeve to the right. The reaction sleeve then pushes the booster piston, causing the brake pedal pushrod to move to the right. Therefore, when ABS is active, the driver may feel vibrations in the brake pedal and hear some noise. After the vehicle decelerates, once the ABS computer detects that the wheel lock-up state has disappeared, it closes the main control valve, returning the system to normal braking operation. If the accumulator pressure drops below the safety limit, the red brake warning light and the amber ABS warning light will illuminate. In this case, the driver must apply greater force to the brake pedal in a deep-press manner to achieve effective braking on both the front and rear wheels.