tronic variable brake proportioning (EVBP) to bal-
ance front-to-rear braking when the brakes are
applied in the partial braking range. For more infor-
mation on EVBP, (Refer to 5 - BRAKES - ABS -
DESCRIPTION).

Included in the ABS system is an integrated con-

trol unit (ICU) and four wheel speed sensors. These
components are described in detail in this section.

The antilock brake system prevents wheel lockup

under braking conditions on virtually 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 capa-
bility. This allows the driver to retain greater control
of the vehicle during braking.

DESCRIPTION - ELECTRONIC VARIABLE
BRAKE PROPORTIONING

This vehicle, equipped with ABS, uses 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).

OPERATION

OPERATION - ANTILOCK BRAKE SYSTEM

There are a few performance characteristics of the

Mark 20e Antilock Brake System that may at first
seem abnormal, but, in fact, are normal. These char-
acteristics are described below.

5 - 46

BRAKES - ABS

NORMAL BRAKING

Under normal braking conditions, the ABS func-

tions the same as a standard base brake system with
a vertically split (front-rear) master cylinder and con-
ventional 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 three wheel cir-
cuits (rear brakes are controlled together by the
same set of solenoid valves; front brakes are con-
trolled separately) 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 per-
ceived at the very end of an ABS stop and is consid-
ered normal.

During an ABS stop, the brakes hydraulic system

is still split front-to-rear. However, the brake system
pressure is further split into three control channels.
During antilock operation of the vehicle’s brake sys-
tem, the front wheels are controlled independently
and the rear wheels are controlled together.

The system can build, hold and release pressure at

each front wheel and both rear wheels, depending on
signals generated by the wheel speed sensors (WSS)
at each wheel and received at the controller antilock
brake (CAB).

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.

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. Neither the
red BRAKE warning indicator lamp, nor the amber
ABS warning indicator lamp, illuminate and no fault
codes are stored in the controller antilock brake
(CAB).

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 DRBIII

t scan tool to detect and verify premature

ABS cycling.

BRAKES - ABS

5 - 47

BRAKES - ABS (Continued)

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.

CAUTION

The ABS uses an electronic control module, the

CAB. This module is designed to withstand normal
current draws associated with vehicle operation.
Care must be taken to avoid overloading the CAB
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 DRBIII

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 24-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 con-
nector should be disconnected 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.

5 - 48

BRAKES - ABS

BRAKES - ABS (Continued)

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.

DIAGNOSIS AND TESTING - INSPECTION AND
ROAD TEST

(1) Visually inspect the ABS for damaged or dis-

connected components and connectors.

(2) Check the vehicle battery voltage. Ensure that

battery voltage is not above 15 volts or below 11
volts. Repair as necessary before continuing.

(3) Verify the brake lamps are operational. If they

are not, repair them prior to continuing.

(4) Connect the DRBIII

t scan tool to the Data

Link Connector located under the instrument panel
to the right of the steering column. If the DRBIII

does not power-up, check the power and ground sup-
plies to the connector.

(5) Turn the ignition key to the ON position. Select

ANTILOCK BRAKES.

(6) Read and record any Diagnostic Trouble Codes

(DTCs). If any DTCs are present, refer to the appro-
priate chassis diagnostic information.

NOTE: Diagnostic trouble codes (DTCs) are kept in
the controller’s memory until either erased by the
technician using the DRBIII

, or erased automati-

cally after 3500 miles or 255 ignition key cycles,
whichever occurs first. DTCs are retained by the
controller even if the ignition is turned off or the
battery is disconnected. More than one DTC can be
stored at a time. When accessed, the number of
occurrences and the DTC that is stored are dis-
played.

If no problems are observed, it will be necessary to

road test the vehicle.

Many ABS conditions judged to be a problem by

the driver may be normal operating conditions. To
become familiarized with the normal operating char-
acteristics of this antilock brake system, (Refer to 5 -
BRAKES - ABS - OPERATION).

WARNING: CONDITIONS THAT RESULT IN TURNING
ON THE RED BRAKE WARNING INDICATOR LAMP
MAY INDICATE REDUCED BRAKING ABILITY.

Before road testing a brake complaint vehicle, note

whether the red BRAKE warning indicator lamp,
amber ABS warning indicator lamp, or both are
turned on. If it is the red BRAKE warning indicator,
there is a brake hydraulic problem that must be cor-
rected before driving the vehicle (Refer to 5 -
BRAKES - BASE - DIAGNOSIS AND TESTING). If
the red BRAKE warning indicator is illuminated,
there is also a possibility that there is an ABS prob-
lem and the amber ABS warning indicator is not able
to illuminate. The red BRAKE warning indicator
lamp is then turned on by default.

If the amber ABS warning indicator is on, road test

the vehicle as described below. While only the amber
ABS warning indicator is on, the ABS is not func-
tional. The ability to stop the car using the base
brake system should not be affected.

(7) Turn the key to the OFF position and then

back to the ON position. Note whether the amber
ABS warning indicator lamp continues to stay on.

(8) If the amber ABS warning indicator lamp stays

on, shift into gear and drive the car to a speed of
approximately 25 km/h (15 mph) to complete the ABS
Start-Up and Drive-Off Cycles (see Antilock Brake
System - Operation). If at this time the amber ABS
warning indicator lamp stays on, refer to the Appro-
priate Diagnostic Information.

(9) If the amber ABS warning indicator lamp goes

out at any time, drive the vehicle a short distance.
Accelerate the vehicle to a speed of at least 64 km/h
(40 mph). Bring the vehicle to a complete stop, brak-
ing hard enough to cause the ABS to cycle. Repeat
this action several times. Using the DRBIII

t, read

and record any Diagnostic Trouble Codes (DTCs). If
any DTCs are present, refer to the Appropriate Diag-
nostic Information.

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.

BRAKES - ABS

5 - 49

BRAKES - ABS (Continued)

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