Anti-Lock Brake System: Why It's Important and How It Works

The purpose of an anti-lock brake system (ABS) is to prevent the wheels from locking up or skidding during braking. If the brakes lock up, the vehicle takes longer to stop and the ability to steer the vehicle is greatly diminished. 

The ABS system prevents wheel lock-up by monitoring each wheel with a wheel speed sensor. When the wheel starts to skid, hydraulic pressure to the brake is temporarily reduced to prevent the wheel from skidding.

anti-lock brake system

What will I feel when the ABS engages?

As a driver, you will feel the brake pedal pumping and a "grinding" mechanical noise as the ABS pump and the valve assembly operates to meter the braking pressure to the affected wheel(s). A screeching noise may be heard from the tires as they skid for very short periods. No warning lights will illuminate during normal ABS operation.

How should I react when the ABS engages?

If the ABS engages in a panic situation, you probably won't have time to think of what to do and will keep your foot firmly planted on the brake pedal, which you should. There are some models that will do this for you—this feature is called brake assist. The ABS will apply the maximum allowable brake pressure during a panic stop situation.

The panic stop situation is what the ABS system is specifically designed for and its purpose is not necessarily to stop the vehicle faster, but to allow the driver to maintain steering control. The system helps the driver steer around danger instead of skidding into it.

What other types of driving conditions can cause the ABS to engage?

Anything that will cause a wheel to stop turning during braking will cause the ABS system to engage. Hitting a bump, crossing railroad tracks, or traversing any slick surface while braking can cause a wheel to stop turning and the ABS to engage. These non-panic situations are where drivers tend to release brake pressure, which results in longer stopping distances.

What could I do wrong when the ABS engages?

The most common mistake drivers make when the ABS engages is to release pressure on the brake pedal. This reaction will always increase stopping distance and gives the impression that something is wrong with the braking system.

Releasing brake pressure during an ABS stop prevents the system from performing its job and results in a loss of braking pressure to the wheels that have not lost traction. As stated above, the correct driver reaction is to hold steady or even increase brake pressure in order to achieve the shortest possible stopping distance.

What if my ABS engages when it shouldn't?

It's possible for the ABS to engage when it shouldn't. The most common reason for this is a faulty wheel speed sensor. If a wheel speed senor reads 0 MPH incorrectly while braking, the system will engage and no fault code(s) will be stored. If you suspect your ABS system is engaging when it should not, road testing your vehicle with a qualified technician may be the best way to confirm this.

Peculiarities of ABS

Many technicians recommend that owners get the "feel" of their ABS system before a panic situation arises. A good way to do this is to find a SAFE open area—a deserted parking lot for example. From a speed of about 30 MPH, apply the brakes hard, as you would in a panic situation, taking note of what happens when the ABS engages. Knowing what to expect when the ABS engages can be beneficial if the system engages unexpectedly.

There are some situations when you could be better off without ABS; some examples would be loose gravel, powdery snow, etc. What happens in these types of conditions is this—when a wheel begins to skid in the loose material, the gravel or snow begins to build up in front of the tire, which helps slow the vehicle. The ABS is designed to keep the wheels turning, which will prevent build up in front of the tires—this increases stopping distance.

The ABS systems are not designed to be turned on and off by the driver. The best reaction to a skid in these types of conditions would be to allow the ABS system to work by holding firm or increasing pressure on the brake pedal as described above. Always keep in mind that stopping distances will be longer in these types of driving conditions.

Beyond ABS

Some vehicles have incorporated other systems into their basic ABS—tire pressure monitoring, traction control, and stability control. Some tire pressure monitoring systems use the speed of the wheels to calculate low tire pressure—a low tire will be slightly smaller, resulting in a slightly faster wheel speed. The traction control systems look for wheel spin from the drive wheels on acceleration and can reduce engine power or apply the brake on the affected wheel to stop the spinning (some systems do both). The stability control systems add a steering wheel position sensor and yaw rate sensor, which adjusts engine output and applies braking force to a specific wheel (or wheels) to help control a skid.

Key component — ABS valve assembly

The anti-lock brake system (ABS) valve assembly is operated by the ABS control module. The valves open and close as necessary when the ABS is active, providing the correct brake fluid pressure to each wheel to prevent wheel lockup during braking.

abs valve assenbly

Additional key parts

ABS wheel speed sensor, ABS differential sensor, ABS control module

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