ELECTRONIC VARIABLE BRAKE PROPORTIONING
Vehicles equipped with ABS use electronic variable brake proportioning (EVBP) to balance front-to-rear braking. The
EVBP is used in place of a rear proportioning valve. The EVBP system uses the ABS system to control the slip of
the rear wheels in partial braking range. The braking force of the rear wheels is controlled electronically by using the
inlet and outlet valves located in the integrated control unit (ICU).
EVBP activation is invisible to the customer since there is no pump motor noise or brake pedal feedback.
OPERATION
ANTILOCK BRAKE SYSTEM WITH TRACTION CONTROL
ABS
There are a few performance characteristics of the MK25 Antilock 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 diagonally
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 a
brake application, the brake system enters the ABS mode. During ABS braking, hydraulic pressure in the four wheel
circuits is modulated to prevent any wheel from locking. Each wheel circuit is designed with a set of electric sole-
noids to allow modulation, although for vehicle stability, both rear wheel solenoids receive the same electrical signal.
Wheel lockup 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 pressure at all 4 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 either rear wheel does not exceed that of the highest slip rear wheel in order to maintain
vehicle stability.
The system can build and release pressure at each wheel, depending on signals generated by the wheel speed
sensors (WSS) at each wheel and received at the Antilock Brake Module (ABM).
NOISE AN BRAKE PEDAL FEEL
During ABS braking, some brake pedal movement may be felt. In addition, ABS braking will create ticking, popping,
or groaning noises heard by the driver. This is normal and is due to pressurized fluid being transferred between the
master cylinder and the brakes. If ABS operation occurs during hard braking, some pulsation may be felt in the
vehicle body due to fore and aft movement of the suspension as brake pressures are modulated.
At the end of an ABS stop, ABS is turned off when the vehicle is slowed to a speed of 3–4 mph. There may be a
slight brake pedal drop anytime that the ABS is deactivated, such as at the end of the stop when the vehicle speed
is less than 3 mph or during an ABS stop where ABS is no longer required. These conditions exist when a vehicle
is being stopped on a road surface with patches of ice, loose gravel, or sand on it. Also, stopping a vehicle on a
bumpy road surface activates ABS because of the wheel hop caused by the bumps.
TIRE NOISE AND MARKS
Although the ABS system prevents complete wheel lockup, some wheel slip is desired in order to achieve optimum
braking performance. Wheel slip is defined as follows: 0 percent slip means the wheel is rolling freely and 100
percent slip means the wheel is fully locked. During brake pressure modulation, wheel slip is allowed to reach up to
25–30 percent. This means that the wheel rolling velocity is 25–30 percent less than that of a free rolling wheel at
a given vehicle speed. This slip may result in some tire chirping, depending on the road surface. This sound should
not be interpreted as total wheel lockup.
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BRAKES - ABS - SERVICE INFORMATION
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