Anti-lock Braking System (ABS) (06.09)
Brake system (06.00)
6-9-2 Workshop Manual Issue 4, November 2008
difference between the ideal and actual response within a
tolerance band. This directly influences the forces on the
tires to generate a corrective yaw moment to reduce the side
forces of the tires where appropriate
System Overview
The DSC system embraces capabilities far beyond that of
ABS, or ABS and traction control combined, while relying on
the components of these systems. It also incorporates these
additional sensors for measur ing the vehicle’s motion and
brake system pressure:
Ya w r a t e s e n s o r - located centrally on the transmission
tunnel.
Lateral acceleration sensor - integrated with the yaw rate
sensor.
Steering angle sensor - located on the upper steering
column.
Pressure transducer - located on master cylinder.
The ABS / DSC module supports data exchange with other
vehicle electronic systems via the CAN network; the module
also enables diagnostic interrogation using WDS.
The following components register driver demand and the
ABS / DSC module processes their signals as a basis for
defining an ideal response:
PCM - position of accelerator pedal.
Brake master cylinder pressure transducer - driver’s
braking effort.
Steering angle sensor - position of steering wheel.
There are many supplementary parameters also included in
the processing calculations these include the coefficient of
friction and vehicle speed. The ABS / DSC module monitors
these factors based on signals transmitted by the sensors for:
• Wheel speed
• Lateral acceleration
• Brake pressure
• Yaw rate
Using these parameters, the function of the ABS / DSC
module is to determine the current vehicle status based on
the yaw-rate signal and the slip as estimated by the ABS /
DSC module. It then maintains the vehicle response within
a tolerance of the ‘normal’ behaviour, which is easily
controlled by the driver.
In order to generate the desired yaw behaviour the ABS /
DSC module controls the selected wheels using the ABS
hydraulic system and engine control system. In the event of
engine intervention, the ABS / DSC module calculates the
torque which should be supplied by the engine to the
wheels, and relays this request signal to the PCM which
implements the torque request.
The PCM receives signals from the DSC system via the CAN
bus and reduces engine torque as follows:
• The throttle is positioned to provide the requested engine target torque. • During the transient phase of torque reduction caused by
mechanical and combustion delays, other alternative
torque reduction methods are used to provide a quicker
response.
• The ignition is retarded and / or the fuel is cut-off at the injectors at selected cylinders.
• Ignition and fuelling are reinstated when the engine torque reaches the requested value
Operation Summary
• DSC is switched ‘ON’ when the engine is started
• When the system is operating , the DSC light in the instrument cluster will flash, at the rate of twice a second
• DSC can be switched ‘OFF / ON’ by pressing the DSC switch
• The DSC light in the instru ment cluster will illuminate
continuously when the system is switched ‘Off’.
• ‘DSC OFF’ will be displayed in the message centre right to indicate the system has been switched ‘OFF’
• A malfunction in the traction control system will be indicated to the driver by the following:
• The DSC light in the instru ment cluster will illuminate
continuously
• The message ‘DSC NOT AVAILABLE’ will be displayed in the message centre right
• If vehicle speed control is engaged it will automatically disengage when traction control is operating .
Traction Control
Traction control is a function of DSC, and is operated in
association with DSC. Traction control prevents excessive
wheel-spin at standing starts, or during acceleration. Wheel-
spin is usually caused by ex cessive use of the accelerator
pedal, or slippery, loose or bumpy road surfaces. To prevent
excessive wheel-spin and mainta in vehicle stability such
situations are overcome by the intervention of the traction
control system by:
• Braking the driven-wheel when it starts to slip
• And / or adapting the engine torque to a level corresponding to the traction available on the road
surface.
Functional Description
Traction control uses the ABS electronic and mechanical /
hydraulic hardware with additional valves to control the
braking pressure at the calipe rs. An engine interface also
enables the engine to respond to torque reduction requests
from the traction control. As with ABS, the signals from the
wheel-speed sensors are supplied to the ABS module, where
they are used to calculate the wheel-slip of the individual
wheels. Traction control intervention is initiated if the slip at
one of the wheels is excessive.
