Section 9 (Abs/Traction Control - Repair (E46)): Description

WARNING: This page does not describe the selected car, but rather 34 other vehicles, including the 2004 BMW M3, 2004 BMW 330xi, 2004 BMW 330i, 2004 BMW 330Ci, and 2004 BMW 325xi. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: For additional information, see BRAKE DISC in BRAKES.

CAUTION: See MAINTENANCE AND GENERAL INFORMATION in GENERAL INFORMATION.

Anti-Lock Brake System/Automatic Stability Control plus Traction (ABS/ASC+T) system consists of a hydraulic unit, slip control module, 4 wheel speed sensors, ASC switch, relays and wiring harness. System may also have brakelight switch, pedal travel sensor, auxiliary throttle position motor, auxiliary throttle position sensor and/or ASC diode. ASC+T portion of system interfaces with engine control module (DME). If vehicle has automatic transmission, ASC+T/DSC portion of system interfaces with automatic transmission control module to modify shift points.

To maintain maneuverability and stability of vehicle during hard braking, ABS prevents wheel lock-up. ABS activates at speeds greater than about 6 MPH and deactivates when speed drops below about 3 MPH. ASC+T optimizes vehicle stability and traction when accelerating or starting from a standstill. During control phase, engine torque is reduced by decreasing engine air intake and retarding ignition timing, and braking is applied to rear wheels. If more control is needed, ignition and fuel injection are gradually phased out. Brakes are only applied if vehicle is traveling at speed of less than 60 MPH. For speeds up to 24 MPH, braking of individual rear wheels has same effect as limited slip differential which increases traction. When moving from a standstill when both driven wheels have same level of traction, or in all circumstances at speeds more than 24 MPH, the brake applications have effect of increasing stability.

Dynamic Stability Control (DSC) is an enhancement to ASC+T system. DSC optimizes stability during cornering and avoidance maneuvers. DSC is integrated into ASC+T control module. DSC monitors steering angle, lateral acceleration, brake pressure and movement of vehicle around its vertical axis. If differences in wheel speeds occur, system avoids wheel spin by reducing engine torque. If necessary, system responds with application of brakes to rear wheels. If system detects instability in vehicle, braking action can also be directed to front wheels by DSC to stabilize vehicle.

ABS indicator light and ASC+T indicator lights on the instrument panel come on when ignition is turned on, then go out after about 2 seconds or when engine is started (bulb check). If a problem is detected in ABS system, ABS indicator light will come on and ABS/ASC+T systems will be disabled. Vehicle will then convert to conventional braking. ASC indicator light will flash during traction regulation. If a problem is detected, or ASC+T system is switched off, ASC+T indicator light will come on steadily. ASC+T system can be manually switched off which is advisable when snow chains are used or vehicle is being rocked.