housings where wheel speed sensors mount; and
loose wheel speed sensor mounting bolts.
After diagnosing the defective component, repair or
replace it as required. When the component repair or
replacement is completed, test drive the vehicle to
verify that premature ABS cycling has been cor-
rected.
OPERATION - 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.
OPERATION - TRACTION CONTROL SYSTEM
The CAB (MK25) or ABM (MK25e) monitors wheel
speed. During acceleration, if the CAB or ABM
detects front (drive) wheel slip and the brakes are
not applied, it enters traction control mode. Traction
control operation proceeds in the following order:
(1) Close the normally open traction control (TC)
valves.
(2) Start the pump/motor and supply volume and
pressure to the front (drive) hydraulic circuit. The
pump/motor runs continuously during traction con-
trol operation.
(3) Open and close the build and decay solenoid
valves to maintain minimum wheel slip and maxi-
mum traction.
The cycling of the build and decay valves during
traction control is similar to that during antilock
braking, except the valves work to control wheel spin
by applying the brakes, whereas the ABS function is
to control wheel skid by releasing the brakes.
Two pressure relief shuttle valves allow pressure
and volume to return to the master cylinder reservoir
when not consumed by the build and decay valves.
These valves are necessary because the pump/motor
supplies more volume than the system requires.
If the brakes are applied at anytime during a trac-
tion control cycle, the brake lamp switch triggers the
control module to switch off traction control.
The traction control function indicator in the
instrument panel illuminates during a traction con-
trol cycle.
The traction control system is enabled at each igni-
tion cycle. It may be turned off by depressing the
traction control OFF switch button. The traction con-
trol function indicator illuminates immediately upon
depressing the button. Pressing this button again or
turning off and restarting the vehicle will enable the
traction control system.
CAUTION
The ABS uses an electronic control module. This
module is designed to withstand normal current
draws associated with vehicle operation. Care must
be taken to avoid overloading the circuits.
CAUTION: In testing for open or short circuits, do
not ground or apply voltage to any of the circuits
unless instructed to do so for a diagnostic proce-
dure.
CAUTION: These circuits should only be tested
using a high impedance multi-meter or the desig-
nated scan tool as described in this section. Power
should never be removed or applied to any control
module with the ignition in the ON position. Before
removing or connecting battery cables, fuses, or
connectors, always turn the ignition to the OFF
position.
CAUTION:
The
CAB
(MK25)
47-way
connector
should never be connected or disconnected with
the ignition switch in the ON position.
CAUTION: The ABM (MK25e) 47-way connector
should never be connected or disconnected with
the ignition switch in the ON position.
CAUTION: This vehicle utilizes active wheel speed
sensors. Do not apply voltage to wheel speed sen-
sors at any time.
CS
BRAKES - ABS
5 - 51
BRAKES - ABS (Continued)