An anti-lock braking system (ABS) is a safety system that prevents wheels from locking when braking. It's been used in modern vehicles for the past 25 years.  Here's how an ABS works: Wheel speed sensors: Attached to each wheel, these sensors measure the wheel's rotational speed. Valves: There is a valve in the brake line of each brake controlled by the ABS. These valves allow, block, and release pressure on the brakes by assuming three different positions. Pumps: Filled with hydraulic fluid that applies pressure to the brake drums or calipers. The pump restores the pressure to the... Show more

An anti-lock braking system (ABS) is a safety system that prevents wheels from locking when braking. It's been used in modern vehicles for the past 25 years. 

Here's how an ABS works:
Wheel speed sensors: Attached to each wheel, these sensors measure the wheel's rotational speed.
Valves: There is a valve in the brake line of each brake controlled by the ABS. These valves allow, block, and release pressure on the brakes by assuming three different positions.
Pumps: Filled with hydraulic fluid that applies pressure to the brake drums or calipers. The pump restores the pressure to the hydraulic brakes after the valves have released it.
ECU: This unit calculates the slip ratio and adjusts the signals to an adequate value.
Actuator: This unit sets the brake pressure based on the signals from the ECU. 

ABS improves steering control, shortens the braking distance, and prevents abnormal tire wear. 
ABS is not active below 7 km/hr. At lower speed only normal braking is available. During ABS operation the brake fluid returns to the master cylinder and the driver will feel pulsations at the brake pedal which help to indicate that ABS is in operation. 
 

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