FAQs: Anti-Lock Braking System (ABS)

February 15, 2005

Questions

Does my vehicle have ABS? 

What is ABS? How does it work? 

How do you know ABS is working? 

My car has ABS. How should I use my brakes? 

Do cars with ABS stop more quickly than cars without it? 

How should I steer during an emergency stop in an ABS-equipped vehicle?

What is the difference between rear-wheel ABS and four-wheel ABS? 

What is Electronic Stability Control? Is that the same as ABS? 

My vehicle has ABS. How do I check my brake fluid?

 


 

Introduction

ABS stands for Anti-lock Braking System. Most newer car models offer ABS as either standard or optional equipment. Anti-lock braking systems function differently from “regular” brakes (i.e., systems without the anti-lock feature), so it is important to know how to use these brakes. Safe, careful, controlled steering is extremely important in emergency braking situations with ABS. Contrary to popular belief, ABS does not appreciably shorten stopping distances. Improper steering when using ABS can result in unexpected and undesired vehicle responses.

 

Does my vehicle have ABS?

Please consult your owner's manual. It is very important to know what safety equipment your vehicle does and does not have, so read that manual carefully. If you do have ABS, your instrument panel will have an amber ABS indicator light. If this light is illuminated anytime besides when you first start your car, this indicates a problem with your anti-lock braking system, so you should have a qualified technician inspect the system.

If you have a pickup truck, van, or SUV, and you do have ABS, make sure you know whether you have two-wheel ABS or four-wheel ABS.

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What is ABS? How does it work?

ABS stands for anti-lock braking system. Anti-lock brakes are designed to prevent skidding, and four-wheel anti-lock brakes help the driver to maintain steering control during maximal braking efforts.

ABS uses electronic controls and sensors to control braking force during hard braking, while the driver steps on the brakes firmly without pumping them . Maximum brake force is achieved when the tires are turning at a certain speed relative to the vehicle's speed (i.e., when the wheels aren't locked and skidding). ABS senses the rotation of the wheels and “pumps” the brakes for the driver, “trying” to keep the wheel speed close to the speed needed for optimal braking performance. More importantly, ABS prevents the vehicle from skidding out of control, and four-wheel ABS allows the driver to maintain the ability to steer around hazards in emergency braking situations.

The big difference between driving a vehicle with ABS and driving one without ABS is that with ABS, you should never pump the brakes . When you're braking hard with ABS, you concentrate on braking hard, and the electronics will do the “pumping” for you.

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How do you know ABS is working?

First, ABS will only feel different from ordinary brakes when you're braking hard. During “ordinary” braking (i.e., not a “panic” stop), you should brake in exactly the same manner whether you have ABS or not, and the brakes should respond in the same way.

When you initiate hard braking, the anti-lock braking system may be activated. When this happens, you may experience rapid pulsations or vibrations of the brake pedal, it may feel like the brakes are “pushing back at you,” or the pedal may suddenly drop. Also, the valves in the ABS controller may make a noise that sounds like “grinding” or “buzzing.” These are not signs that the ABS is not working, they are signs that the ABS is working, so if you're braking hard, and you this happens, just keep the brake pedal pressed down.

One way to test your ABS brakes, and also familiarize yourself with the feeling of how they work, is to drive your car ( fairly slowly ) on a loose gravel lot (without any people or traffic ) and apply the brakes hard.

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My car has ABS. How should I use my brakes?

During non-emergency situations, you should apply the brakes in the same manner whether you have ABS or not, and the vehicle should respond in exactly the same way. When you're coming to an “ordinary” stop, you should not be braking hard enough to activate the anti-lock braking system.

During emergency situations, when you're trying to stop in the shortest distance possible, you should just step on the brake pedal as hard and as fast as you can. Do not pump the brakes! If your vehicle has ABS, pumping the brakes will not help you in any way, and can even be counterproductive. Just step on the pedal and let the electronics “pump” for you

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Do cars with ABS stop more quickly than cars without it?

Under most conditions, a vehicle with a good anti-lock brake system can stop in a slightly shorter distance than an average driver could accomplish in the same vehicle without ABS; however, that is not the main purpose of ABS, and the difference is generally not great enough to notice or be of any real use. ABS certainly doesn't improve your stopping distance nearly enough to justify driving faster or following more closely!

ABS can actually lengthen stopping distances on some surfaces, such as loose snow, dirt, or gravel, because of effects that occur during locked-wheel skidding that are unique to those surfaces.

The importance of ABS to you as a driver is that it helps you to retain control of your vehicle during a maximal braking effort. Without ABS, if you brake hard enough to lock your wheels, you will not be able to steer the vehicle, and the vehicle may skid sideways or spin around. Two-wheel ABS will prevent the vehicle from spinning out of control, and four-wheel ABS will actually allow you to retain the ability to steer your vehicle while you're still braking.

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How should I steer during an emergency stop in an ABS-equipped vehicle?

Without ABS, once your front wheels lock, your vehicle will skid straight ahead, regardless of how you steer. Drivers who are accustomed to driving vehicles without ABS may sometimes turn the steering wheel sharply when performing an emergency stop. This is dangerous in a vehicle with four-wheel ABS. If you need to steer the car while braking to avoid an obstacle in front of you, carefully steer according to where you want your vehicle to go, and avoid sudden large steering inputs (i.e., don't suddenly jerk the steering wheel half of a turn while stomping on the brakes). If you do that, you may end up driving off of the road or into oncoming traffic.

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What is the difference between rear-wheel ABS and four-wheel ABS?

Rear-wheel ABS, found on some pickup trucks, vans and SUVs, is designed to maintain directional stability and prevent the vehicle from skidding sideways, which can happen if the rear wheels lock. It will not enable steering control during a panic stop, because you can't steer if your front wheels are locked. If you're driving a vehicle with rear-wheel ABS, you should step firmly on the brake if you need to stop suddenly; however, if you feel the wheels begin to lock at a time when steering is necessary, you'll need to release some pressure from the brake in order to regain the ability to steer.

Four-wheel ABS, the type usually found on ABS-equipped passenger cars and some light trucks, is designed to maintain “steerability” during maximal braking. In an emergency braking situation, drivers of vehicles equipped with four-wheel ABS should step firmly on the brake pedal and hold it down. If necessary, you can try to steer around an obstacle in the road during a maximal braking effort, which wouldn't work without four-wheel ABS.

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What is Electronic Stability Control? Is that the same as ABS?

Electronic Stability Control (ESC) is a technology built upon the basic electronic sensors and control that comprise ABS; however, its purpose is different from that of ABS. ESC acts to keep the vehicle “on track” when sensors detect that the vehicle is in danger of turning more or less than the driver's steering inputs dictate.

The driver does not need to do anything to activate ESC, the sensors continuously monitor the driver's steering and the vehicle's path, and act when necessary. ESC prevents the vehicle from understeering (i.e., the front tires lose traction and the vehicle turns less than the driver intends) or oversteering (i.e., the rear tires lose traction and the vehicle turns further than the driver intends) by braking one or more wheels individually, and/or reducing the engine throttle, which help to keep the vehicle on its intended path.

The Insurance Institute for Highway Safety provides a detailed explanation of ESC in their January 3, 2005 Status Report, including a brief summary of their recent study which found ESC to be very effective in reducing single-vehicle crashes.

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My vehicle has ABS. How do I check my brake fluid?

The brake fluid in most ABS-equipped vehicles is stored in the ABS master cylinder, and checking the fluid level is done in the same manner as checking the fluid level in a vehicle without ABS.

Some anti-lock braking systems draw fluid from the master cylinder and store it in a special hydraulic accumulator, in which case, an ordinary visual inspection might lead you to believe that the fluid level is low, even if it is really properly filled. In some such multi-chamber fluid reservoir systems, you must discharge all of the fluid from the accumulator, by pumping the brakes 30 to 40 times with the ignition turned off (this is the only time you should pump your brakes if you have ABS!), before you can accurately check your brake fluid level. If your vehicle has ABS, this pumping method is a good rule of thumb to follow when checking your brake fluid level.

Finally, if you are unsure of whether or not you are doing everything properly, you may have a service facility check your brake fluid level for you. You can find more information about checking your brake fluid, with or without ABS, on theCalifornia State Automobile Association Web site.

What are the differences between ABS, Electronic Brake Force Distribution, and Emergency Brake Assist?

Electronic Brake Force Distribution (EBD) and Emergency Brake Assist (EBA) are both special functions of ABS, and have recently been appearing in more new vehicles.

EBD is a system that balances front and rear braking force. This is useful for vehicles with high center of gravity, which will experience greater dynamic weight transfer during hard braking, and for vehicles that are carrying a heavy load that shifts the weight distribution relative to that taken into account by the manufacturer when designing the brake system.

In the most efficient braking , the front wheels and rear wheels contribute equally to the total braking force between the tires and the ground. Deviations from this even distribution require greater traction from the tires contributing the greater proportion of the total braking force than would be needed in the case where the front and rear contribute equally. This decreases brake efficiency and makes any particular level of deceleration more difficult for the vehicle to manage. EBD seeks to keep brake efficiency as high as possible.

EBA assists the driver in hard braking. In some cases, an unsuspecting driver faced with an emergency may apply the brakes very quickly, but not press hard enough to stop in the shortest distance possible. EBA detects how fast the driver initially applies the brakes, and if the system detects that the driver is attempting an “emergency-stop,” it generates a maximal braking effort, which the unassisted driver may not have been able to achieve. EBA doesn't actually enhance the braking capability of the vehicle; however, it stops the vehicle in a shorter distance than a panicking driver might actually accomplish. The system does not “override” the driver, though. If the driver releases the brake pedal, the system deactivates.

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References

ABS Education Alliance

California State Automobile Association

Insurance Institute for Highway Safety

National Highway Traffic Safety Administration

Royal Society for the Prevention of Accidents (UK)