tires BMW X5 2005 E53 DSC System Workshop Manual

3
TRACTION CONTROL SYSTEM OVERVIEW (ASC)
THEORY OF TRACTION CONTROL
Traction Control Systems are referred to as Automatic Stability Control (ASC), Automatic
Stability + Traction Control (ASC+T), Dynamic Stability Control (DSC) or All Season Traction
(marketing term). The traction control system has gone through several changes since its
introduction in Model Year 1988. While each new version offered new innovations, the prin-
ciple of traction control remains the same for any of these versions.
The link between the vehicle and its driving environment is established by the frictional con-
tact between the tires and the road surface. The ability to control the vehicle while driving,
is lost as a result of excessive wheel spin or slip if either rear tire exceeds its ability to:
• Maintain traction
• Accept side forces to maintain directional control (lateral locating forces)
The primary function of the traction control system is to maintain traction and stability of the
vehicle regardless of the road surface condition. This is achieved by reducing the drive
torque applied to the rear wheels or pulsing the rear wheel brakes to eliminate wheel slip
depending on the version of traction control installed.
The electronics for traction control operation are incorporated in the ABS control module
and share many of the same components and sensors. The module determines the speed
of the vehicle through a calculated average of the four wheel speed sensor inputs. Wheel
slip/spin is then recognized by comparing the speed of the driven wheels to the front
wheels.
A critical slip ratio of greater than 5% between the wheels will cause the traction control regu-
lation to begin. This slip ratio is established when the control module detects a wheel speed
difference of 2 MPH or higher.