Anti-lock Braking System (ABS), Traction Control

Braking on the snow without Anti-lock brakes Click for larger photo

Anti-lock Brakes warning light

ABS hydraulic control unit. Click for larger photo

The Anti-lock Braking System (ABS) is designed to help the driver maintain steering control during hard braking, especially in slippery conditions. The Anti-lock brakes come as standard equipment on most of today's vehicles.
If you want to know whether your vehicle is equipped with the Anti-lock Braking System, you can check your owner's manual. Typically, if the car has the ABS, you'd see the Anti-lock Brakes warning light on the instrument panel when you turn the ignition on.

Why do you need Anti-lock Brakes?

Imagine, you are driving a car without the Anti-lock Braking System (ABS) on a slippery road (e.g. after a rain or snow). Suddenly, you notice something on the road right in front of you. You hit the brakes hard and try to steer aside, but the car doesn't react to the steering anymore and just skids out of control. This happens, because all the wheels are locked up while you are holding down the brake pedal. As a result, you lose the ability to steer the vehicle (upper photo).

The ABS system is designed to help in situations like this. The Anti-lock Braking system prevents the wheels from locking up, helping you maintain steering control during hard braking. In a similar situation, driving a car equipped with the ABS, it would be easier for you to steer your vehicle while braking.
See the video at the Bosch website:
Anti-lock Brakes video clips and animation

Over the years, I myself had a few situations where the Anti-lock brakes saved me from an accident. Once I was driving on an icy road and the car with snow-covered windows backed out from the driveway right in front of me. I hit the brakes hard, but thanks to anti-lock brakes, I was still able to steer aside.

Important to know about ABS:

The ABS system can help to slightly reduce the braking distance in some situations. However, under certain conditions (e.g. on loose snow or gravel), the braking distance with the ABS may be actually longer. The main purpose of the Anti-lock Braking System is to help the driver maintain steering control during hard braking. Read more at the NHTSA website: Antilock Brake System (ABS)

Some older trucks may have a two-wheel ABS system that controls only rear wheels. The rear wheel ABS system does not help to maintain steering control during braking.
To gain full advantage of the ABS system you need to know what type of ABS you have in your vehicle and how to operate it correctly. Check your owner's manual for details.
Good tires play an important role; for example, if the car has worn-out or very wide sport tires, the ABS can hardly prevent it from skidding on the snow or ice.

In the early 90s, when the Anti-lock Brakes still was a fairly new technology, the person I knew had a bet with his friends that he can brake his car with the ABS on the ice and the car won't skid. That's what the seller told him when he bought the car. I know about this, because I bought this car a couple years later. A friend of mine recognized the car and told me the story, still laughing. What happened on that winter day was the car went into a skid on an icy road and hit the tree with its side. It was a minor collision, nobody was hurt, but the lesson was learned. It was Toyota Supra with very wide sport tires. By the way, it was a gorgeous-looking car; shame Toyota has discontinued it.

How Anti-lock braking system works

Major components of the typical Anti-lock Braking System include four wheel speed sensors (one at each wheel), an electronic control unit (the ABS computer) and a hydraulic control unit (lower picture). The ABS computer constantly monitors the signals from each wheel speed sensor. When it senses that any of the wheels is approaching lock up during braking, the ABS computer sends the signal to the hydraulic control unit, which modulates the braking pressure for a corresponding wheel(s) preventing it from locking up. When the ABS is activated, it feels like rapid brake pedal pulsation with a buzzing noise, which is normal.

Anti-lock braking system problems

If the Anti-lock Brakes warning light comes on while driving, there is a problem with your Anti-lock Braking System and you need to have your car checked out by your mechanic or a dealer as soon as possible. A mechanic will check your brakes and scan your ABS electronic control unit with the special scanner that will show a fault code. This code will help a mechanic to find a problem. The fault code doesn't say which part is defective, but it says what signal was out of range. For example, the fault code may say something like "No signal from the left front wheel sensor", so the sensor will need to be checked and replaced if needed. The problems with the ABS wheel sensors or bad connection at the sensor connectors are very common, although sometimes the ABS computer or the hydraulic control unit can fail too. Once the problem is repaired, typically, the ABS warning light will come off by itself after the Anti-lock Braking System will check itself during driving.

Traction Control

Mazda6 Traction Control Warning light

If the Anti-lock Braking system helps maintain control during hard braking, the Traction Control system uses similar approach to help the driver maintain control during hard acceleration, particularly on a slippery road. The Traction Control system (TCS or TRAC) reduces wheel slip and maintains traction of the drive wheels by individually applying brake to the slipping drive wheel and reducing the engine torque at the same time.
Different vehicles use different design, but the basic purpose of the Traction Control System remains the same - to prevent the drive wheels from slipping during acceleration.

A typical Traction Control system is based on vehicle's Anti-lock braking system (ABS) and uses many of the ABS system components. For example, a Traction Control system uses the wheel speed sensors of the ABS system to monitor the speed of all four wheels.

Why do you need Traction Control

Once again, to understand when the Traction Control may be helpful, let's look how a car would behave without Traction Control on a slippery road. Again, let's imagine that you are accelerating from a stop on an icy road in a typical front-wheel drive car without Traction Control. If you accelerate gently, you might be OK, the drive wheels won't spin; but if you press the gas pedal harder, one or both drive wheels could lose traction and begin to spin on the ice. When this happens, it's very hard to control a vehicle because the front wheels have very little traction when spinning.

The Traction Control is designed to help in this type of situations. Once the Traction Control System senses that any of the drive wheel(s) starts slipping, it reduces the engine torque and shortly applies the brake to the slipping wheel(s) to slow it down just enough to regain traction. This helps you to control your vehicle better during acceleration. When Traction Control system operates, you could probably feel that the engine power is reduced and hear some buzzing noise similar to that of ABS. The Traction Control warning light may also flash.

Again, as with any technology, a Traction Control system can only do so much. Good driving skills are essential to be safe on the road. A Traction Control system cannot help in all possible situations; for example, if a car is stuck in loose, wet snow, it might be easier to free it out by turning the Traction Control OFF and letting the wheels to spin freely to cut through the loose snow for a better grip. Good tires are also important for the Traction Control system to be effective. If you never used the Traction Control system before, check your owner's manual, it explains how to use it correctly. If your Traction Control warning light stays on during driving, have your vehicle checked out, your Traction Control system may have a problem.
See Traction Control video here: Bosch: Traction Control.