Auto Service World
Feature   February 1, 2001   by Auto Service World

COUNTERTALK FOR THE COUNTERPERSON: ABS, Traction Control and Stability Control


ABS, which debuted a couple of decades ago now (an amazing fact in itself), has taken over as more than just a means to stop wheel lock-up under heavy braking or on slippery surfaces. It has evolved to become an important part of vehicle control in a variety of conditions.

The first evolution of ABS to perform other functions was traction control. On mid- to high-horsepower applications, this generally took the form of low speed traction control. When activated, this type of traction control would compare the wheel speed sensor signals of the drive wheels and undriven wheels, and apply braking force to the drive wheels if a predetermined difference in wheel speed was detected. Depending on the vehicle, this would be either at the front wheels or the rear. In addition, particularly in high-horsepower cars, engine horsepower was reduced to prevent excessive heat buildup in the brake system and all sorts of other nasty outcomes.

A good example of this type of evolution is the Chevy Corvette. Starting with the 1990 model–the ZR-1 model may ring a bell–GM’s flagship performance vehicle featured a Bosch 2S Micro ABS system. This system was a 3-way circuit, one for each front wheel and a common one for the rear wheels. It also featured a lateral acceleration switch, essentially a mercury switch affected by cornering forces. The lateral accelerometer circuit provides a signal to the Electronic Brake Control Module (EBCM). This information is used by the EBCM to modify the pressure applied to the rear brakes to prevent loss of control at speeds above 50 km/h.

The 1992 to 1994 Corvette built on this and was equipped with Bosch 2S Micro ABS with Acceleration Slip Regulation (ASR). The ABS/ASR system on this vehicle functions differently to control braking or wheel spin under acceleration. The ABS uses a 4-way hydraulic circuit, one for each wheel. However, when ABS is activated in response to a signal from the wheel speed sensors and the Electronic Brake And Traction Control Modulator (EBTCM), the modulator valve assembly activates both rear wheels simultaneously, regulating them as one circuit.

To prevent wheel slip during acceleration, the ASR function uses, in order, spark-timing retard, throttle close down, and rear brake application. The EBTCM monitors the difference between the front wheel speed and rear wheel speed to detect rear wheel slip during acceleration.

The EBTCM directs the Engine Control Module (ECM) to retard spark timing. If there is still wheel slip, the EBTCM will use throttle close down. If wheel slip persists, and speed is less than about 80 km/h, rear brakes will be applied, with each rear wheel’s brake being controlled individually.

While traction control debuted as an enhancement to performance vehicles, its evolution has taken it to the realm of the SUV.

On BMW’s X5, a sophisticated traction control system takes the place of the locking differentials. Instead of relying on the differential to feed torque to the gripping wheels when off road, the job goes to the brakes. The ADB-X system replaces all lock differentials in the BMW all-wheel-drive system. Contrary to conventional four-wheel-drive systems on the market, BMW’s all-wheel-drive system does not require any mechanical locks, but rather uses the benefits of the company’s Dynamic Stability Control (DSC), that combines an additional function with BMW all-wheel drive: the electronic ADB-X differential lock. As soon as ADB-X recognizes that a wheel is about to spin, it slows the wheel automatically by applying the brakes. This, in turn, re-directs the power flow to the wheels which still have traction and appropriate grip on the road, at the same time reinstating traction on the wheel that is spinning. ADB-X offers the same effects as conventional differential locks without their usual disadvantages such as friction losses, extra weight and additional space requirements. The system is the same as that found on the new 3-series all-wheel-drive sedans.

Ford has also brought stability control to its line. Its Interactive Vehicle Dynamic (IVD) technology, currently offered on the Lincoln LS, will appear on the 2001 Windstar when the system is rolled out mid-model year. Ford Motor Company’s AdvanceTrac system is designed to enhance stability in extreme driving conditions when drivers misjudge speed or road conditions. The system uses a combination of braking to the offside rear wheel and engine torque reduction to help counter the vehicle’s momentum, as monitored by steering angle and dynamics sensors. IVD is currently available on European Focus models and Lincoln LS models with automatic transmissions. The system soon will be available on Ford Focus in North America.

Interactive Vehicle Dynamics integrates and manages the antilock brake system, all-speed traction control and yaw stability assist. In addition to providing the normal antilock braking and traction control functions, IVD has a yaw stability function that proportions brake pressure (above or below the driver’s applied pressure) to each wheel individually, thereby delivering enhanced steering control at the dynamic limits for any traction surface while braking, accelerating or coasting. The IVD system could be activated when a vehicle is taking a turn too quickly or when it encounters a slippery road surface.

The system compares the driver’s intended path (which is derived from the steering wheel angle) to the actual vehicle path, which is inferred from a yaw rate sensor, a lateral accelerometer and wheel-speed sensors. If the actual path is different from that intended path (e.g., when the vehicle is understeering or oversteering), the IVD controller applies braking at selected wheel(s) and reduces engine torque if required to make the best use of available traction to keep the vehicle on the intended path and minimize loss of control.

While the maintenance of the vehicle control systems behind these traction control systems will require some training, it should not go unnoticed that these systems all put added stress and wear on the braking system. Theoretically, it would actually be possible to wear the brakes out while barely ever using them in the conventional sense. At the very least, it puts the proper operation of the vehicle braking system into an even more critical safety realm.


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