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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.
CORNER BRAKING CONTROL (CBC)
Corner Braking Control (CBC) was an additional feature first added to the ASC/5 system of
the E39 at introduction. CBC improves stability control while braking through curves.
As the vehicle enters a curve, the weight of the car shifts to the outside of the curve. With
non CBC equipped vehicles if the driver brakes while driving through a curve an equal
hydraulic force is applied to each wheel. Though the pressure is equal, the braking effort
is unequal at the tire footprint due to the increased weight of the vehicle on the outside
curve.
With a CBC equipped sys-
tem, the feature regulates
the apply pressure to the
wheels based on the turn-
ing angle (curve recogni-
tion). When the driver
brakes in the same situa-
tion, an unequal hydraulic
pressure is applied to each
side of the vehicle.
Though the apply pressure
is unequal, the dynamic of
the vehicle’s weigh transfer
compensates for the
unequal apply pressure, as
a result braking effort is
equal at the tire footprint.
The ASC control module
monitors the speed signals
from all four wheels to
determine when this regu-
lation is required.
This is a feature found on
vehicles equipped with the
Dynamic Stability Control
System (DSC), however
this ASC/5 system does
not incorporate full DSC
control.
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