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You, a patchy icy road, and your car. There's only one brake pedal for four wheels, how do you stop as safely as possible?

Your car's 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. 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.

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 as well. This feature is called brake assist. The ABS will apply the maximum allowable brake pressure during a panic stop situation, which is what the ABS system is specifically designed for.

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ABS allows the driver to maintain steering control

The system helps the driver steer around danger instead of skidding into it. 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.

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 previously mentioned, the correct driver reaction is to hold steady or even increase brake pressure in order to achieve the shortest possible stopping distance. 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 sensor reads zero MPH incorrectly while braking, the system will engage and no fault codes 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.

ABS is designed to keep the wheels turning

C300BKAZMany 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, like a deserted parking lot. 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. During these types of conditions 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, which increases stopping distance.

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. Make sure to always keep in mind that stopping distances will be longer in these types of driving conditions.

Beyond ABS

Some vehicles like the Mercedes C300 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. Keep in mind 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 to help control a skid.

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About The Author: Bill Petrey

Bill PetreyBill is a Mercedes enthusiast, ASE certified technician and Senior Sales Associate at FCP Euro. He can be reached at billp@fcpeuro.com


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Written by :
Bill Petrey

Bill is an ASE certified technician and Senior Sales Associate at FCP Euro. He can be reached at billp@fcpeuro.com


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