interrogates the engine control system to determine the current torque output, determines how much the torque
output the current conditions will allow, and signals this requirement to the engine control system, which reduces the
torque by partially closing the throttle. With execution of the torque reduction, the brake system reduces brake pres-
sure to make the transition smooth, while maintaining forward progress. By reducing engine power, braking effec-
tiveness is maintained and the system can operate throughout the normal vehicle speed range. That is why the
system is identified as providing “all-speed” traction control.
With AWD, where front-wheel slip can occur, the degree of throttle intervention is relatively less than with rear-wheel
drive. The difference in speed capability and the degree of throttle intervention between rear-wheel drive and all-
wheel drive is due to the fact that non-driven front wheels on a rear-wheel drive vehicle give the system an accurate
vehicle speed reference on which to base responses. With AWD, the possibility that the front wheels may also be
slipping makes appropriate corrective action more difficult to determine, thus limiting the effective speed range. Off-
setting this is the fact that loss of traction is less likely with AWD because torque is transmitted through all four
wheels to begin with. In actual driving situations on snow or ice, the rear-wheel drive and AWD systems respond in
essentially the same way up to the 45 mph (72 km/h) limit of the AWD system.
When severe wheel slippage is detected (as on snow-covered roads), the Winter Mode feature of All-Speed Traction
Control causes the transmission to up-shift to higher gears at lower speeds than normal. Once a slippery launch
condition is detected, the transmission will remain in Winter Mode for a minimum of three minutes. After that, if the
road is providing normal traction, the system returns to providing normal up-shifts.
ELECTRONIC STABILITY PROGRAM - MK25
To determine whether the car is responding properly to cornering commands, ESP uses steering wheel angle, yaw
(turning) rate and lateral acceleration sensors (combined into Dynamics Sensor). Using signals from these sensors,
in addition to individual wheel speed sensor signals, the system determines appropriate brake and throttle actions.
Once initiated, ESP operates much like All-Speed Traction Control, except that the goal is directional stability. If the
vehicle yaw response, or rate of turning, is inconsistent with the steering angle and vehicle speed indications, the
ESP system applies the brakes and, if necessary closes the throttle, to restore control. This occurs whether the
vehicle is turning too rapidly (oversteering) or not rapidly enough (understeering).
ESP notifies the active brake booster electronically of the need for maximum output. A solenoid opens a valve in the
booster that immediately applies maximum boost to the master cylinder. A travel sensor in the booster detects the
rate and travel of the brake pedal. Above the threshold of an “extreme” brake application, ESP activates the sole-
noid, thus applying maximum available hydraulic pressure to the brakes. In this type of situation, stopping distance
is determined by the ABS system based on available traction, rather than the amount of force that the driver can
exert on the brake pedal. This becomes the deciding factor in how quickly the vehicle will stop.
ELECTRONIC STABILITY PROGRAM - MK25E
To determine whether the car is responding properly to cornering commands, ESP uses steering wheel angle, yaw
(turning) rate and lateral acceleration sensors (combined into Dynamics Sensor). Using signals from these sensors,
in addition to individual wheel speed sensor signals, the system determines appropriate brake and throttle actions.
Once initiated, ESP operates much like All-Speed Traction Control, except that the goal is directional stability. If the
vehicle yaw response, or rate of turning, is inconsistent with the steering angle and vehicle speed indications, the
ESP system applies the brakes and, if necessary closes the throttle, to restore control. This occurs whether the
vehicle is turning too rapidly (oversteering) or not rapidly enough (understeering).
ELECTRONIC VARIABLE BRAKE PROPORTIONING
Upon entry into EVBP the inlet valve for the rear brake circuit is switched ON so that the fluid supply from the
master cylinder is shut off. In order to decrease the rear brake pressure, the outlet valve for the rear brake circuit
is pulsed. This allows fluid to enter the low pressure accumulator (LPA) in the Hydraulic Control Unit (HCU) resulting
in a drop in fluid pressure to the rear brakes. In order to increase the rear brake pressure, the outlet valve is
switched off and the inlet valve is pulsed. This increases the pressure to the rear brakes. This back-and-forth pro-
cess will continue until the required slip difference is obtained. At the end of EVBP braking (brakes released) the
fluid in the LPA drains back to the master cylinder by switching on the outlet valve and draining through the inlet
valve check valve. At the same time the inlet valve is switched on in case of another brake application.
The EVBP will remain functional during many ABS fault modes. If both the red BRAKE and amber ABS warning
indicators are illuminated, the EVBP may not be functioning.
5 - 390
BRAKES - ABS SERVICE INFORMATION
LX