and all Right-Hand-Drive (RHD) vehicles feature an
Antilock Brake System (ABS) designated MK20e.
This antilock brake system is available with or with-
out traction control. Later production LHD vehicles
use an ABS designated MK25e. Like the Mark 20e,
the system is available with or without traction con-
trol. The differences are mainly found in the Inte-
grated control Unit (ICU). (Refer to 5 - BRAKES/

HYDRAULIC/MECHANICAL/ICU

(INTEGRATED

CONTROL UNIT) - DESCRIPTION)

The purpose of the antilock brake system is to pre-

vent wheel lockup under braking conditions on virtu-
ally any type of road surface. Antilock braking is
desirable because a vehicle that is stopped without
locking the wheels retains directional stability and
some steering capability. This allows the driver to
retain greater control of the vehicle during braking.

DESCRIPTION - 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 propor-

BRAKES - ABS

5 - 85

tioning valve. The EVBP system uses the ABS sys-
tem 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 out-
let 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 feed-
back.

DESCRIPTION - TRACTION CONTROL SYSTEM

Traction control reduces wheel slip and maintains

traction at the driving wheels at speeds below 56
km/h (35 mph) when road surfaces are slippery. The
traction control system reduces wheel slip by braking
the wheel that is losing traction.

A Traction Control Off Switch that is a momentary

contact type switch is located on the steering column
upper shroud.

HYDRAULIC SHUTTLE VALVES

Two pressure relief hydraulic shuttle valves are

included on vehicles with traction control. These
valves are located inside the HCU and cannot be ser-
viced separately from the HCU.

TRACTION CONTROL LAMP

The traction control function lamp is located in the

transmission range indicator display of the instru-
ment cluster.

OPERATION

OPERATION - ANTILOCK BRAKE SYSTEM

There are a few performance characteristics of the

MK20e and MK25e Antilock Brake Systems that may
at first seem abnormal, but in fact are normal. These
characteristics are described below.

NORMAL BRAKING

Under normal braking conditions, the ABS func-

tions 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 5–8 km/h (3–5 mph). If a wheel locking ten-
dency 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
solenoids 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 nor-
mal.

During an ABS stop, the brakes hydraulic system

is still diagonally split. However, the brake system
pressure is further split into four control channels.
During antilock operation of the vehicle’s brake sys-
tem, the wheels are controlled independently and are
on separate control channels.

The system can build, hold and release pressure at

each wheel, depending on signals generated by the
wheel speed sensors (WSS) at each wheel and
received at the Controller Antilock Brake (MK20e) or
Antilock Brake Module (MK25e).

NOISE AND 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 5–6 km/h (3–4
mph). There may be a slight brake pedal drop any-
time that the ABS is deactivated, such as at the end
of the stop when the vehicle speed is less than 5
km/h (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, stop-
ping 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.

Complete wheel lockup normally leaves black tire

marks on dry pavement. The ABS will not leave dark
black tire marks since the wheel never reaches a
fully locked condition. However, tire marks may be
noticeable as light patched marks.

5 - 86

BRAKES - ABS

BRAKES - ABS (Continued)

START-UP CYCLE

When the ignition is turned on, a popping sound

and a slight brake pedal movement may be noticed.
The ABS warning indicator lamp will also be on for
up to 5 seconds after the ignition is turned on. These
conditions occur as part of an ABS self-diagnosis test
performed by the antilock brake system. The popping
noise is the result of brief activation of the solenoids
inside the integrated control unit (ICU).

DRIVE-OFF CYCLE

When the vehicle is first driven off, a humming

may be heard or felt by the driver at approximately
25–40 km/h (15–25 mph). This is caused by brief acti-
vation of the ABS pump motor on the ICU and is a
normal function of ABS as the system is performing
a diagnosis check.

PREMATURE ABS CYCLING

Symptoms of premature ABS cycling include: click-

ing sounds from the solenoid valves; pump/motor
running; and pulsations in the brake pedal. Prema-
ture ABS cycling can occur at any braking rate of the
vehicle and on any type of road surface.

Premature ABS cycling is a condition that needs to

be correctly assessed when diagnosing problems with
the antilock brake system. It may be necessary to use
a scan tool to detect and verify premature ABS
cycling.

Check the following common causes when diagnos-

ing premature ABS cycling: damaged tone wheels;
incorrect tone wheels; damaged steering knuckle
wheel speed sensor mounting bosses; loose wheel
speed sensor mounting bolts; excessive tone wheel
runout; excessively large tone wheel-to-wheel speed
sensor air gap, or a damaged speed sensor head face.
Give special attention to these components when
diagnosing

vehicle

exhibiting

premature ABS

cycling.

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 traction control module monitors wheel speed.

During acceleration, if the module detects front
(drive) wheel slip and the brakes are not applied, the
module enters traction control mode. Traction control
operation proceeds in the following order:

(1) Close the normally open isolation 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 valves to

maintain minimum wheel slip and maximum trac-
tion.

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.

If the brakes are applied at anytime during a trac-

tion control cycle, the brake lamp switch triggers the
controller to switch off traction control.

HYDRAULIC SHUTTLE VALVES

Two pressure relief hydraulic 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.

TRACTION CONTROL LAMP

The traction control system is enabled at each igni-

tion cycle. It may be turned off by depressing the
Traction Control OFF Switch button when the igni-
tion is in the ON position. The traction control func-
tion lamp illuminates immediately upon depressing
the button.

The traction control function lamp illuminates dur-

ing a traction control cycle.

BRAKES - ABS

5 - 87

BRAKES - ABS (Continued)

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 (MK20e) 25-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.

CAUTION: Use only factory wiring harnesses. Do
not cut or splice wiring to the brake circuits. The
addition of aftermarket electrical equipment (car
phone, radar detector, citizen band radio, trailer
lighting, trailer brakes, etc.) on a vehicle equipped
with antilock brakes may affect the function of the
antilock brake system.

CAUTION: When performing any service procedure
on a vehicle equipped with ABS, do not apply a
12-volt power source to the ground circuit of the
pump motor in the HCU. Doing this will damage the
pump motor and will require replacement of the
entire HCU.

CAUTION: An attempt to remove or disconnect cer-
tain system components may result in improper
system operation. Only those components with
approved removal and installation procedures in
this manual should be serviced.

CAUTION: If welding work is to be performed on the
vehicle using an electric arc welder, the CAB
(MK20e) or ABM (MK25e) connector should be dis-
connected during the welding operation.

CAUTION: Many components of the ABS System
are not serviceable and must be replaced as an
assembly. Do not disassemble any component
which is not designed to be serviced.

CAUTION: Only the recommended jacking or hoist-
ing positions for this vehicle are to be used when-
ever it is necessary to lift a vehicle. Failure to raise
a vehicle from the recommended locations could
result in lifting a vehicle by the hydraulic control
unit mounting bracket. Lifting a vehicle by the
hydraulic control unit mounting bracket will result
in damage to the mounting bracket and the hydrau-
lic control unit.

CAUTION: Brake fluid will damage painted surfaces.
If brake fluid is spilled on any painted surface,
wash off with water immediately.

STANDARD PROCEDURE - ANTILOCK BRAKE
SYSTEM BLEEDING

The base brake’s hydraulic system must be bled

anytime air enters the hydraulic system. The ABS
though, particularly the ICU (HCU), should only
need to be bled when the HCU is replaced or
removed from the vehicle. The ABS must always be
bled anytime it is suspected that the HCU has
ingested air. Under most circumstances that require
the bleeding of the brakes hydraulic system, only the
base brake hydraulic system needs to be bled.

When bleeding the ABS system, the following

bleeding sequence must be followed to insure com-
plete and adequate bleeding.

(1) Make sure all hydraulic fluid lines are installed

and properly torqued.

(2) Connect the scan tool to the Data Link Connec-

tor. The connector is located under the lower steering
column cover to the left of the steering column.

(3) Using the scan tool, check to make sure the

CAB (MK20e) or ABM (Mk25e) does not have any
fault codes stored. If it does, clear them using the
scan tool.

5 - 88

BRAKES - ABS

BRAKES - ABS (Continued)

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