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.
ANTILOCK BRAKE SYSTEM (ABS)
There are a few performance characteristics of the MK25e anti-lock brake system that may, at first, seem abnormal
but in fact are normal. These characteristics are described below.
NORMAL BRAKING
Under normal braking conditions, the ABS functions the same as a standard base brake system with a front/rear
split master cylinder and conventional vacuum assist.
ABS BRAKING
ABS operation is available at all vehicle speeds above 3-5 mph. If a wheel locking tendency is detected during
brake application, the brake system enters ABS mode. During ABS braking, hydraulic pressure in the four wheel
circuit is modulated to prevent any wheel from locking. Each wheel circuit is designed with a set of electronic sole-
noids to allow brake pressure to be modulated at each wheel individually. For vehicle stability reasons, both rear
wheel solenoids receive the same electrical signal input during ABS. Wheel lock-up may be perceived at the very
end of an ABS stop and is considered normal.
During an ABS event, the integrated control unit (ICU) regulates hydraulic brake pressure at all four of the vehicle’s
wheels.
The hydraulic pressure at each front wheel is controlled independently (relative to the amount of slip at each wheel)
in order to maximize the braking force generated by the front brakes. The rear wheels are controlled such that the
hydraulic pressure at each wheel is the same and does not exceed the pressure appropriate for the wheel with the
highest slip in order to maintain vehicle stability.
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.
ELECTRONIC STABILITY PROGRAM
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
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BRAKES - ABS SERVICE INFORMATION
KJ