Anti-Lock - Traction Control: Description

NOTE: For additional information on brake system, see DISC article.

CAUTION: See ANTI-LOCK BRAKE SAFETY PRECAUTIONS article in GENERAL INFORMATION.

Anti-Lock Brake System (ABS) consists of a hydraulic unit with 4 solenoid switching valves, 4 speed sensors, a control unit, relays and wiring harness. See Fig 1 .

ANTI-LOCK light on instrument panel comes on when ignition is turned on. ANTI-LOCK light goes out once engine is started, indicating ABS is ready for operation.

ABS activates at speeds greater than 8 MPH. When vehicle speed is greater than 3 MPH, the built-in electronic test monitor will start checking ABS. If fault is found, warning light will come on. When ABS light is on, ABS is switched off. Vehicle will then convert to conventional braking.

If battery voltage is less than 10.5 volts with ignition on and test speed is exceeded, ABS will remain off until alternator increases voltage output to more than 10.5 volts. ABS warning light will then go out.

Traction control system, called Automatic Stability Control (ASC), controls slip of drive wheels, regardless of road surface conditions. During control phase, engine torque is reduced by Digital Motor Electronics (DME) system and throttle control system. If this is not sufficient, drive wheel which is spinning too fast is additionally braked using wheel brake in conjunction with a hydraulic actuator.

ASC light in instrument cluster comes on constantly when ignition is switched on as bulb check. System is switched on after engine start and can be switched off as necessary. ASC light is off when system is switched on. If ASC light is flashing, ASC control is active. If ASC light remains on, system has switched off automatically because of a fault or was switched off manually.

The ABS, ASC or ASC+T warning light may come on intermittently, and normal troubleshooting procedures do not indicate any defect. This can happen when the vehicle is driven over 50 mph on cars that have recently had two new tires, snow tires or chains installed. Consequently, if running snow tires on only two wheels or chains, ASC switch should be turned off.

Check all four tires on the car to make sure they are all the same brand, style and size. Also note if there are any new tires with substantially more tread than the other tires. Differences in tires may cause enough variance in rolling diameters to confuse the ABS control unit, which may give a false signal to the ABS warning light. This is especially true if snow tires have been mounted on the rear wheels only.

Fig 1: Identifying Anti-Lock Brake System Components (Typical)

Courtesy of BMW OF NORTH AMERICA, INC.