Originally Posted by WHealy

Edmunds definitions :

Traction Control

Traction control deals specifically with lateral (front-to-back) loss of friction during acceleration. In other words, when your car accelerates from a dead stop, or speeds up while passing another vehicle, traction control works to ensure maximum contact between the road surface and your tires, even under less-than-ideal road conditions. For example, a wet or icy road surface will significantly reduce the friction (traction) between your tires and the pavement. And since your tires are the only part of your car that actually touches the ground, any resulting loss of friction can have serious consequences.

Traction control is part of a series of three braking technology developments that began appearing in vehicles in the mid-eighties. (Note: Many German vehicle manufacturers call traction control by its original German name: ASR traction control. ASR stands for "Acceleration Slip Regulation." It's the same technology we're talking about here, but with a fancier name that most Americans have never heard of.) In chronological order, these developments are: anti-lock brakes, aka ABS (1978), traction control (1985), and stability control (1995). All three technologies come from the laboratories of Robert Bosch Company in Germany, and all address the issue of improving contact (traction) between your car's tires and the road.

Traction control works at the opposite end of the scale from ABS -- dealing with acceleration rather than deceleration. Still, since many of the same principles apply to both systems, it might be best to visualize it as sort of ABS in reverse. ABS works by sensing slippage at the wheels during braking, and continually adjusting braking pressure to ensure maximum contact between the tires and the road. You can actually hear the system working (a grinding sound) and feel it (the pedal pulsing).

As we mentioned above, ABS and traction control operate similarly. In fact, the ABS control unit is the basic "building block" for traction control and stability control. By adding modules and sensors, the system can be expanded to include these newer technologies.

In the case of traction control, the basic ABS system -- as well as other components in the vehicle -- requires some modification. To begin with, the old-style accelerator cable is typically replaced by an electronic drive-by-wire connection (although some older systems still use a mechanical accelerator cable), meaning the mechanical hook-up between the accelerator pedal and the throttle ceases to exist. Instead, a sensor converts the position of the accelerator pedal into an electrical signal, which the control unit (similar to the one used in ABS) uses to generate a control voltage. The standard ABS hydraulic modulator is also expanded to include a traction control component.

All these parts work together to activate the traction control system.

Let's say you're at a stoplight on wet pavement. The light turns green and you press too firmly on the accelerator pedal. There is slick asphalt under your tires and the wheels begin to spin. The traction control system instantaneously kicks in, sensing that the wheels have begun to slip. Within a fraction of a second, this data is fed back to the control unit, which adjusts throttle input and applies braking force to slow the wheels (some older systems also retarded engine spark). The wheels are thus prevented from spinning and the car maintains maximum traction.

It's really that simple. Again, think of it as ABS in reverse.

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DSC

This brings us to our present topic: stability control. The third "building block" in modern braking systems, stability control incorporates everything ABS and traction control do plus a yaw-sensing feature that works to increase traction during potential side-skidding situations. In other words, whereas both ABS and traction control work on the longitudinal (front-to-back) axis of the vehicle, stability control operates on the lateral (side-to-side) axis. Bosch's Electronic Stability Program (ESP), the first such system on the market, began appearing in 1995 Mercedes-Benz S-Class sedans. It has since become a popular feature on many upscale vehicles.

According to Bosch's Kosmider, "The platform for ABS, traction control and stability control is essentially the same. We simply add sensors to get the desired effect."

In addition to discrete electrical components, ceramic sensors and solenoid valves, stability control systems typically utilize wheel-speed sensors, steering-angle sensors and a hydraulic modulator. The key component is, however, something called a rotational speed sensor (also known as a yaw-rate sensor). Yaw can be described as "the movement of an object turning on its vertical axis."

The yaw-rate sensor determines how far off-axis a car is "tilting" in a turn. This information is then fed into a microcomputer that correlates the data with wheel speed, steering angle and accelerator position, and, if the system senses too much yaw, the appropriate braking force is applied.

As you can imagine, stability control systems are particularly effective in inclement driving conditions, where the roadway may be covered with rain, ice or snow and the normal friction between the tires and the road is reduced.

There are a couple of things you may want to know about stability control. First, the system will do most of the "thinking" for you. Depending on the particular driving situation, the system may activate an individual wheel brake or any combination of the four, as well as control the throttle, until the vehicle is once again stable.

Second, the system is fully independent of the driver's actions. Even if the car is free-rolling (no acceleration or braking input from the driver), the stability control system will kick in and perform its duty. All you need to do is steer.

Remember, though, that stability control, like any technology, is not fool-proof. All vehicles must ultimately obey the laws of physics. Members of our road test staff report spinning sedans with activated stability control systems on dry pavement. Be sure to make your driving decisions based on the appropriate criteria — visibility, road conditions, speed, the condition of the tires and brakes — and not to rely on any technology to correct for unsafe driving.

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Originally, TC and DSC systems only came on automatic transmissions. Now I've never seen this documented anywhere, but I assume that they bridged the gap to manuals by using traction control with DCS reducing speed by braking as opposed to throttle control. But that's just a theory.