What is an Anti-lock Braking System (ABS)?

What is ABS?

ABS stands for anti-lock braking system. It is a safety feature that prevents wheels from locking up while braking to avoid skidding.

ABS consists of computer sensors and solenoid valves which monitor wheel speed. The ABS senses when wheels are about to lock during an emergency braking situation and releases enough brake force to maintain traction with the road surface. The ABS allows drivers to retain more control of the vehicle by modifying brake pressure.

Without the ABS, drivers can lose steering control of their vehicles while skidding and potentially overturn. ABS is particularly important for controlling wheel speed on loose gravel or slippery surfaces. However, ABS doesn’t necessarily shorten a vehicle’s stopping distance. On most surfaces, a vehicle equipped with ABS will likely stop faster than one without. But on slippery surfaces, ABS activated by hard braking may increase stopping distance while allowing drivers to keep steering control.

An overview of how ABS works

Before anti-lock brakes, drivers were taught to “pump their brakes” when they start to skid on slippery roads. Today, ABS does this for drivers, only faster and more accurately.

The ABS comprises wheel speed sensors, an electronic control unit (ECU), valves, and a pump.

1. The system relies on sensors that constantly monitor the speed of the vehicle’s wheels.

2. The sensors send data to the ECU. The ABS controller is always looking out for rapid deceleration, an early signal for wheel lock.

3. When the ABS controller detects rapid deceleration or an emergency stop, it uses a valve in the brake line to reduce pressure to the wheel. This causes acceleration.

4. The controller will then use a pump to increase pressure again until the wheel decelerates again. However, because of the ABS, the tire slows down at the same rate as the vehicle.

When the anti-lock brake system is working, drivers will feel the brake pedal pulsing. This is rapid opening and closing of the valves — or the brakes “pumping.”

Types of ABS

There are three basic types of anti-lock brake systems.

• Four channel, four sensor ABS. This system has a separate speed sensor and valve on all four wheels. It’s considered the best system because it monitors individual wheels, ensuring maximum braking force. The ABS will only pulse on the single wheel that’s about to lock up, giving drivers better traction control.

• Three channel, three sensor ABS. This setup is found on pickup trucks featuring four-wheel ABS. In this system, both front wheels have individual valves and speed sensors. The rear wheels share a valve and one speed sensor in the rear axle. Because the rear wheels are monitored together, both wheels must lock up to trigger ABS.

• One channel, one sensor ABS. This system has one valve and one speed sensor to monitor all four wheels and is located in the rear axle. It’s typically found on pickup trucks that have rear-wheel ABS.

U.S. federal mandate requires all new cars and minivans made in or after 2012 to have four-wheel ABS. To determine if your vehicle has ABS, consult an owner’s manual or look for an ABS light on the dashboard.