lock JEEP CHEROKEE 1994 Service Owner's Guide

(2) If red warning light is illuminated, or if neither
warning light is illuminated, make several stops and
note pedal action and brake response.
(3) Check brake pedal response with transmission
in Neutral and engine running. Pedal should remain
firm under steady foot pressure. If pedal falls away,
problem is either in vacuum booster or master cylin-
der.
(4) During road test, make normal and firm brake
stops in 25-40 mph range. Note faulty brake opera-
tion such as pull, grab, drag, noise, fade, pedal pul-
sation, etc.
(5) Inspect suspect brake components and refer to
problem diagnosis information for causes of various
brake conditions.
COMPONENT INSPECTION
Fluid leak points and dragging brake units can
usually be located without removing any compo-
nents. The area around a leak point will be wet with
fluid. The components at a dragging brake unit
(wheel, tire, rotor) will be quite warm or hot to the
touch.
Other brake problem conditions will require compo-
nent removal for proper inspection. Raise the vehicle
and remove the necessary wheels for better visual ac-
cess.
During component inspection, pay particular atten-
tion to heavily rusted/corroded brake components
(e.g. rotors, caliper pistons, brake return/holddown
springs, support plates, etc.).
Heavy accumulations of rust may be covering se-
vere damage to a brake component. It is wise to re-
move surface rust in order to accurately determine
the depth of rust penetration and damage. Light sur-
face rust is fairly normal and not a major concern (as
long as it is removed). However, heavy rust buildup,
especially on high mileage vehicles may cover struc-
tural damage to such important components as
brakelines, rotors, support plates, and brake boosters.
Refer to the wheel brake service procedures in this
group for more information.
DIAGNOSING SERVICE BRAKE PROBLEMS
BRAKE WARNING LIGHT OPERATION
The red brake warning light will illuminate under
the following conditions:
²for 2-3 seconds at startup as part of normal bulb
check
²parking brakes applied
²low pedal caused by malfunction in front/rear
brake hydraulic circuit (differential switch valve ac-
tuated)
If the red light remains on after startup, first ver-
ify that the parking brakes are fully released. Then
check pedal action and fluid level. A red light indi-
cates that the valve in the differential pressureswitch has been actuated. If a problem is confirmed,
inspect the hydraulic system and wheel brake compo-
nents.
On models with ABS brakes, the amber warning
light only illuminates when an ABS component has
malfunctioned. The ABS light operates indepen-
dently of the red warning light. Refer to the antilock
brake section for more detailed diagnosis informa-
tion.
PEDAL FALLS AWAY
A brake pedal that falls away under steady foot
pressure is generally the result of a system leak. The
leak point could be at a brakeline, fitting, hose,
wheel cylinder, or caliper. Internal leakage in the
master cylinder caused by worn or damaged piston
cups, may also be the problem cause.
If leakage is severe, fluid will be evident at or
around the leaking component. However internal
leakage in the master cylinder will not be physically
evident. Refer to the cylinder test procedure in this
section.
LOW PEDAL
If a low pedal is experienced, pump the pedal sev-
eral times. If the pedal comes back up, worn lining
and worn rotors or drums are the most likely causes.
However, if the pedal remains low and/or the warn-
ing light illuminates, the problem is in the master
cylinder, wheel cylinders, or calipers.
A decrease in master cylinder fluid level may only
be the result of normal lining wear. Fluid level will
decrease as lining wear occurs. It is a result of the
outward movement of caliper and wheel cylinder pis-
tons to compensate for normal wear.
SPONGY PEDAL
A spongy pedal is most often caused by air in the
system. However, thin drums or substandard brake
lines and hoses will also cause a condition similar to
a spongy pedal. The proper course of action is to
bleed the system, or replace thin drums and suspect
quality brake lines and hoses.
HARD PEDAL OR HIGH PEDAL EFFORT
A hard pedal or high pedal effort may be due to
lining that is water soaked, contaminated, glazed, or
badly worn. The power booster or check valve could
also be faulty. Test the booster and valve as de-
scribed in this section.
BRAKE DRAG
Brake drag occurs when the lining is in constant
contact with the rotor or drum. Drag can occur at
one wheel, all wheels, fronts only, or rears only. It is
a product of incomplete brakeshoe release. Drag can
be minor or severe enough to overheat the linings,
rotors and drums.
5 - 8 BRAKESJ

Brake drag also has a direct effect on fuel economy.
If undetected, minor brake drag can be misdiagnosed
as an engine or transmission/torque converter prob-
lem.
Minor drag will usually cause slight surface char-
ring of the lining. It can also generate hard spots in
rotors and drums from the overheat/cool down pro-
cess. In most cases, the rotors, drums, wheels and
tires are quite warm to the touch after the vehicle is
stopped.
Severe drag can char the brake lining all the way
through. It can also distort and score rotors and
drums to the point of replacement. The wheels, tires
and brake components will be extremely hot. In se-
vere cases, the lining may generate smoke as it chars
from overheating.
An additional cause of drag involves the use of in-
correct length caliper mounting bolts. Bolts that are
too long can cause a partial apply condition. The cor-
rect caliper bolts have a shank length of 67 mm
(2.637 in.), plus or minus 0.6 mm (0.0236 in.). Refer
to the Disc Brake service section for more detail on
caliper bolt dimensions and identification.
Some common causes of brake drag are:
²loose or damaged wheel bearing
²seized or sticking caliper or wheel cylinder piston
²caliper binding on bushings or slide surfaces
²wrong length caliper mounting bolts (too long)
²loose caliper mounting bracket
²distorted brake drum or shoes
²rear brakeshoes binding on worn/damaged support
plates
²severely rusted/corroded components
²misassembled components.
If brake drag occurs at all wheels, the problem may
be related to a blocked master cylinder compensator
port or faulty power booster (binds-does not release).
The brakelight switch can also be a cause of drag.
An improperly mounted or adjusted brakelight
switch can prevent full brake pedal return. The re-
sult will be the same as if the master cylinder com-
pensator ports are blocked. The brakes would be
partially applied causing drag.
BRAKE FADE
Brake fade is a product of overheating caused by
brake drag. However, overheating and subsequent
fade can also be caused by riding the brake pedal,
making repeated high deceleration stops in a short
time span, or constant braking on steep roads. Refer
to the Brake Drag information in this section for
causes.
PEDAL PULSATION
Pedal pulsation is caused by components that are
loose, or beyond tolerance limits.
Disc brake rotors with excessive lateral runout or
thickness variation, or out of round brake drums arethe primary causes of pulsation. Other causes are
loose wheel bearings or calipers and worn, damaged
tires.
PULL
A front pull condition could be the result of:
²contaminated lining in one caliper
²seized caliper piston
²binding caliper
²wrong caliper mounting bolts (too long)
²loose caliper
²loose or corroded mounting bolts
²improper brakeshoes
²damaged rotor
²incorrect wheel bearing adjustment (at one wheel)
A worn, damaged wheel bearing or suspension
component are further causes of pull. A damaged
front tire (bruised, ply separation) can also cause
pull. Wrong caliper bolts (too long) will cause a par-
tial apply condition and pull if only one caliper is in-
volved.
A common and frequently misdiagnosed pull condi-
tion is where direction of pull changes after a few
stops. The cause is a combination of brake drag fol-
lowed by fade at the dragging brake unit.
As the dragging brake overheats, efficiency is so
reduced that fade occurs. If the opposite brake unit is
still functioning normally, its braking effect is mag-
nified. This causes pull to switch direction in favor of
the brake unit that is functioning normally.
When diagnosing a change in pull condition, re-
member that pull will return to the original direction
if the dragging brake unit is allowed to cool down
(and is not seriously damaged).
REAR BRAKE GRAB
Rear grab (or pull) is usually caused by contami-
nated lining, bent or binding shoes and support
plates, or improperly assembled components. This is
particularly true when only one rear wheel is in-
volved. However, when both rear wheels are affected,
the master cylinder or proportioning valve could be
at fault.
BRAKES DO NOT HOLD AFTER DRIVING
THROUGH DEEP WATER PUDDLES
This condition is generally caused by water soaked
lining. If the lining is only wet, it can be dried by
driving with the brakes lightly applied for a mile or
two. However, if the lining is both wet and dirty, dis-
assembly and cleaning will be necessary.
BRAKE FLUID CONTAMINATION
There are two basic causes of brake fluid contami-
nation. The first involves allowing dirt, debris, or
other liquid materials to enter the cylinder reservoirs
JBRAKES 5 - 9

BRAKE BLEEDINGÐBRAKE FLUID AND LEVELÐBRAKELINES AND HOSES
INDEX
page page
Brake BleedingÐXJ/YJ with ABS Brakes....... 14
Brake BleedingÐXJ/YJ with Standard Brakes . . . 13
Brake Fluid Contamination.................. 13
Brake Fluid Level........................ 13Brakeline Charts......................... 15
Brakelines and Hoses..................... 15
Combination Valve....................... 15
Recommended Brake Fluid................. 13
RECOMMENDED BRAKE FLUID
The only brake fluid recommended for Jeep vehi-
cles with standard or antilock brakes, is Mopar brake
fluid, or an equivalent fluid meeting SAE J1703 and
DOT 3 standards.
Use new brake fluid only to top off the master
cylinder or refill the system. Never use re-
claimed fluid, fluid not meeting the SAE/DOT
standards or fluid from an unsealed container.
Do not use fluid from any container that has
been left open for any length of time. Fluid in
open containers can absorb moisture.
BRAKE FLUID LEVEL
Always clean the master cylinder and cover before
checking fluid level. If not cleaned, dirt from the
cover could enter the fluid. Also check the cover seal
and replace it if torn or distorted.
Correct fluid level is to within 6 mm (1/4 in.) of the
reservoir rim, or to the fill mark on models with a
plastic reservoir. Refer to the Antilock Brake section
for fluid levels on models equipped with ABS brakes.
BRAKE FLUID CONTAMINATION
Oil in the fluid will cause brake system rubber
seals to soften and swell. The seals may also become
porous and begin to deteriorate.
If fluid contamination is suspected, drain off a sam-
ple from the master cylinder. A suction gun or simi-
lar device can be used for this purpose.
Empty the drained fluid into a glass container.
Contaminants in the fluid will cause the fluid to sep-
arate into distinct layers. If contamination has oc-
curred, the system rubber seals, hoses and cups must
be replaced and the system thoroughly flushed with
clean brake fluid.
BRAKE BLEEDINGÐXJ/YJ WITH STANDARD
BRAKES
Use Mopar DOT 3 brake fluid, or an equivalent
meeting SAE/DOT standards J1703-F and DOT 3, to
fill and bleed the system.
On standard brake models, bleeding can be per-
formed either manually or with pressure equipment.
However, if pressure equipment is used, it will be
necessary to hold the front brake metering valveopen in order to bleed the front brakes. The valve
can be held open with a tension clip tool or by hand.
It will also be necessary that a suitable size pressure
tank hose adapter be available for use on the master
cylinder.
MANUAL BLEEDING PROCEDURE
(1) If master cylinder has been overhauled or a
new cylinder will be installed, bleed cylinder on
bench before installation. This shortens time needed
to bleed system and ensures proper cylinder opera-
tion.
(2) Wipe master cylinder reservoir and cap clean
with shop towels.
(3) Remove cover and fill master cylinder reservoir
with Mopar, or equivalent DOT 3 brake fluid.
(4) Open all caliper and wheel cylinder bleed
screws.
(5) Close bleed screws after fluid begins flowing
from each bleed screw.
(6) Top off master cylinder reservoir again.
(7) Use following bleed sequence:
²master cylinder
²right rear
²left rear
²right front
²left front
(8) Observe following brake bleeding precautions:
²Do not pump brake pedal at any time while bleed-
ing. Air in system will be compressed into small bub-
bles that are distributed throughout hydraulic
system. This will make a second and third bleeding
operation necessary.
²Bleed only one wheel brake unit at a time and use
a bleed hose to bleed each wheel brake unit (Fig. 7).
²Attach one end of bleed hose to bleed screw and in-
sert opposite end in glass container partially filled
with brake fluid (Fig. 7). Glass container makes it
easier to see air bubbles as they exit the bleed hose.
²Be sure end of bleed hose is immersed in fluid. Im-
mersing hose end in fluid prevents air from being
drawn back into cylinder and brakeline.
(9) Bleed master cylinder first. Have helper oper-
ate brake pedal while bleeding each master cylinder
fluid outlet line.
JBRAKES 5 - 13

STANDARD MASTER CYLINDER
INDEX
page page
General Service Information................ 20
Master Cylinder Installation................. 20Master Cylinder Overhaul.................. 20
Master Cylinder Removal.................. 20
GENERAL SERVICE INFORMATION
The service information in this section covers the
standard (non-ABS) master cylinder only. The center
feed master cylinder used with the ABS system is
covered in the antilock brake component service sec-
tion.
MASTER CYLINDER REMOVAL
(1) Disconnect brake lines at master cylinder.
(2) Remove cylinder mounting nuts and remove
master cylinder.
(3) Remove cylinder cover and drain fluid.
MASTER CYLINDER INSTALLATION
(1) Bleed master cylinder on bench before installa-
tion. Refer to overhaul assembly procedure in this
section for bleeding method.
(2) Install cylinder on brake booster studs and in-
stall cylinder attaching nuts. Tighten nuts to 21 NIm
(15 ft. lbs.).
(3) Connect brakelines to cylinder.
(4) Fill and bleed brake system.
MASTER CYLINDER OVERHAUL
CYLINDER DISASSEMBLY
(1) Remove cylinder cover and drain fluid.
(2) Examine cylinder cover seal. Discard seal if
torn or distorted.
(3) Clamp cylinder in vise (Fig. 1).
(4) Press primary piston inward with wood dowel
or phillips screwdriver and remove snap ring (Fig. 2).
(5) Remove and discard primary piston (Fig. 3).
Piston is serviced only as an assembly.
(6) Remove secondary piston (Fig. 4). Apply air
pressure through rear outlet port to ease piston out
of bore. Cover small ports at bottom of rear reservoir
with towel to prevent air leakage.
(7) Discard secondary piston. Do not disassemble
piston as components are only serviced as assembly.
CLEANING AND INSPECTION
Clean the cylinder with Mopar brake cleaning sol-
vent or clean brake fluid. Remove cleaning residue
with compressed air.
Inspect the cylinder bore. A light discoloration of
the bore surface is normal and acceptable but only if
the surface is in good condition.Replace the cylinder if the bore is scored, corroded,
or pitted.Do not hone the cylinder bore in an at-
tempt to restore the surface. Replace the cylin-
der if the bore is corroded or if doubt exists
about cylinder bore condition.
Fig. 1 Cylinder Mounted In Vise
Fig. 2 Removing/Installing Piston Snap Ring
5 - 20 BRAKESJ

(7) Carefully move master cylinder aside and re-
move booster.
POWER BRAKE BOOSTER INSTALLATION
(1) Install check valve and grommet in booster.
Also install spacer on booster, if equipped.
(2) Position booster on dash panel and install
booster mounting bolts/nuts.
(3) Working inside vehicle, install nuts on booster
mounting studs.
(4) Attach booster push rod to brake pedal. Secure
push rod with new bolt and nuts.
(5) Tighten booster mounting bolts/stud nuts to 41
Nzm (30 ft. lbs.) on XJ and 34 Nzm (25 ft. lbs.) on YJ.
(6) Tighten pedal push rod bolt inner nut to 34
Nzm (25 ft. lbs.) torque. Then tighten outer locknut to
8Nzm (75 in. lbs.) torque.
(7) Install master cylinder on booster studs.
Tighten attaching nuts to 21 Nzm (15 ft. lbs.).
(8) Connect vacuum hose to booster, top off master
cylinder fluid level and check brake operation.
Fig. 3 Master Cylinder Attachment (Typical)
Fig. 4 Booster Push Rod Attachment
Fig. 5 Booster Check Valve And Hose
JBRAKES 5 - 23

(2) Verify that left/right automatic adjuster lever
and cable are properly connected.
(3) Insert brake gauge in drum. Expand gauge un-
til gauge inner legs contact drum braking surface.
Then lock gauge in position (Fig. 6).
(4) Reverse gauge and install it on brakeshoes
(Fig. 6). Position gauge legs at shoe centers as
shown. If gauge does not fit (too loose or tight), ad-
just shoes.
(5) Pull shoe adjuster star wheel away from ad-
juster lever.
(6) Turn adjuster star wheel (by hand) to expand
or retract brakeshoes. Continue adjustment until
gauge outside legs are light drag-fit on shoes (Fig. 7).
(7) Repeat adjustment at opposite brakeshoe as-
sembly.
(8) Install brake drums and wheels and lower ve-
hicle.
(9) Make final adjustment as follows:
(a) Drive vehicle and make one forward stop fol-
lowed by one reverse stop.
(b) Repeat procedure 8-10 times to actuate self
adjuster components and equalize adjustment.
(c)Bring vehicle to complete standstill at
each stop. Incomplete, rolling stops will NOT
activate adjuster mechanism.
WHEEL CYLINDER REMOVAL
(1) Raise vehicle and remove wheel.
(2) Disconnect brakeline at wheel cylinder.If cyl-
inder brakeline fitting is hard to break loose,
spray generous amount of Mopar Rust Pene-
trant between fitting and line and around fitting
threads in wheel cylinder. Note that it may re-
quire a few minutes for penetrant to work.
(3) Remove brakeshoes.
(4) Remove bolts attaching wheel cylinder to sup-
port plate and remove cylinder.
WHEEL CYLINDER OVERHAUL (Figs. 8 and 9)
(1) Remove links.
(2) Remove dust boots.
(3) Remove cups and pistons. Discard cups.
(4) Remove and discard spring and expander.
(5) Remove bleed screw.
(6) Clean cylinder, pistons and links with Mopar
brake cleaner.
(7) Inspect cylinder bore and pistons. Light discol-
oration of bore is acceptable. However, replace cylin-
der if bore and pistons are scored, pitted, or corroded.
Do not hone cylinder bores or polish pistons.
Replace cylinder as an assembly if bore is dam-
aged.
(8) Install bleed screw.
Fig. 5 Adjuster Screw Components (9-Inch Brake)
Fig. 6 Adjusting Gauge To Brake Drum
Fig. 7 Adjusting Brakeshoes To Gauge
Fig. 8 Wheel Cylinder (9-Inch Brake)
5 - 36 BRAKESJ

(9) Coat cylinder bore, pistons, cups and expander
with brake fluid and reassemble cylinder compo-
nents. Be sure piston cup lips face expander.
WHEEL CYLINDER INSTALLATION
(1) Apply small bead of silicone sealer around cyl-
inder mounting surface of support plate.
(2) Start brakeline in wheel cylinder fitting by
hand.
(3) Align and seat wheel cylinder on support plate
(Fig. 10).
(4) Install cylinder mounting bolts (Fig. 10).
Tighten bolts to 10 Nzm (90 in. lbs.) torque.
(5) Tighten brakeline fitting to 15 Nzm (132 in.
lbs.) torque.
(6) Install brakeshoes. Adjust shoes to drum with
brake gauge.
(7) Install brake drums and lower vehicle.
(8) Fill master cylinder and bleed brakes.
SUPPORT PLATE REPLACEMENT
The support plate should cleaned and inspected
whenever the drum brake components are being ser-
viced.
Check the support plate for wear, or rust through
at the contact pads and replace the plate if neces-
sary. Be sure to lubricate the contact pads with Mo-
par multi-mileage grease before shoe installation.
Lubrication will avoid noisy operation and shoe bind.
(1) Raise vehicle and remove wheel/tire assembly.(2) Remove brake drum, brakeshoes, and wheel
cylinder.
(3) Remove axle shaft as described in Group 3.
(4) Remove support plate attaching nuts and re-
move support plate.
(5) Clean axle tube flange. If gasket is not used on
flange, apply thin bead of silicone adhesive/sealer to
flange.
(6) Position new support plate on axle tube flange.
(7) Apply Mopar Lock N9Seal, or Loctite 242 to
support plate attaching nuts. Then install and
tighten nuts.
(8) Apply light coat of Mopar multi-mileage grease
to contact pads of new support plate.
(9) Install wheel cylinder and brakeshoes.
(10) Adjust brakeshoes to drums. Refer to proce-
dure in this section.
(11) Bleed brakes.
(12) Install wheel and tire assembly.
(13) Adjust parking brake cable tensioner. Refer to
procedure in Parking Brake section.
(14) Lower vehicle and verify proper service brake
and parking brake operation.
BRAKE DRUM REFINISHING
Brake drums can be machined to restore the brak-
ing surface. Use a brake lathe to clean up light scor-
ing and wear.
CAUTION: Never refinish a brake drum if machining
will cause the drum to exceed maximum allowable
brake surface diameter.
Brake drums that are warped, distorted, or se-
verely tapered should be replaced. Do not refinish
drums exhibiting these conditions. Brake drums that
are heat checked or have hard spots should also be
replaced.
If the brake drums are heavily coated with rust,
clean and inspect them carefully. Rust damage on
high mileage drums can be severe enough to require
replacement.
The maximum allowable diameter for the drum
braking surface is usually indicated on the drum
outer face (Fig. 11).
WHEEL NUT TIGHTENING
The wheel attaching lug nuts must be tightened
properly to ensure efficient brake operation. Over-
tightening the nuts or tightening them in the wrong
sequence can cause distortion of the brake rotors and
drums.
Impact wrenches are not recommended for tighten-
ing wheel nuts. A torque wrench should be used for
this purpose.
A light coat of LPS Anti-Corrosion spray lube
around the hub face and on the studs will cut down
on rust/corrosion formation.
Fig. 9 Wheel Cylinder (10-Inch Brake)
Fig. 10 Wheel Cylinder Mounting
JBRAKES 5 - 37

ABS SYSTEM OPERATION
INDEX
page page
Acceleration Switch....................... 41
Combination Valve....................... 42
Electronic Control Unit (ECU)............... 41
General Information....................... 39
Hydraulic Control Unit (HCU)................ 39
Ignition Switch........................... 42Master Cylinder.......................... 40
Pedal Travel Sensor...................... 41
Power Brake Booster..................... 40
System Relays.......................... 42
System Warning Lights.................... 42
Wheel Speed Sensors..................... 41
GENERAL INFORMATION
The Jeep antilock brake system (ABS) is an elec-
tronically operated, all-wheel brake control system.
Major components include the master cylinder, vac-
uum power brake booster, ECU, hydraulic control
unit (HCU) and various control sensors (Fig. 1). The
ABS brake system is available on XJ and YJ models.
The antilock hydraulic system is a three channel de-
sign. The front wheel brakes are controlled individually
and the rear wheel brakes in tandem (Fig. 2).
The antilock system is designed to retard wheel
lockup during periods of high wheel slip when brak-
ing. Retarding wheel lockup is accomplished by mod-
ulating fluid pressure to the wheel brake units.
The ABS electronic control system is separate from
other electrical circuits in the vehicle. A specially
programmed electronic control unit (ECU) is used to
operate the system components.
System components include:
²electronic control unit (ECU)
²wheel speed sensors and axle shaft tone rings²hydraulic control unit (HCU)
²tandem master cylinder with central valves
²vacuum power brake booster
²pedal travel sensor
²acceleration switch
²main relay and pump motor relay
²ABS warning light
²pump motor sensor
HYDRAULIC CONTROL UNIT (HCU)
The hydraulic control unit (HCU) consists of a
valve body and pump/motor assembly (Fig. 3).
The valve body contains the electrically operated
solenoid valves. It is the solenoid valves that modu-
late brake fluid apply pressure during antilock brak-
ing. The valves are operated by the antilock
electronic control unit (ECU).
Fig. 1 Antilock Components (XJ Shown)
Fig. 2 AntiLock System Basic Layout
JBRAKES 5 - 39

The HCU provides three channel pressure control
to the front and rear brakes. One channel controls
the rear wheel brakes in tandem. The two remaining
channels control the front wheel brakes individually.
During antilock braking, the solenoid valves are
opened and closed as needed. The valves are not static.
They are cycled rapidly and continuously to modulate
pressure and control wheel slip and deceleration.
The pump/motor assembly provides the extra vol-
ume of fluid needed during antilock braking. The
pump is connected to the master cylinder reservoir
by supply and return hoses.
The pump is operated by an integral electric motor.
The DC type motor is controlled by the ECU.
The pump mechanism consists of two opposing pis-
tons operated by an eccentric cam. One piston sup-
plies the primary hydraulic circuit. The opposite
piston supplies the secondary hydraulic circuit. In op-
eration, one piston draws fluid from the master cyl-
inder reservoir. The opposing piston then pumps
fluid to the valve body solenoids. The pump cam is
operated by the electric motor.
MASTER CYLINDER
A new style tandem master cylinder is used with the
ABS system (Fig. 4). It is a center feed design. The pri-
mary and secondary pistons each contain a central
valve which is a unique feature. The valves are used in
place of the conventional piston and seal assemblies.
The valves close and open the cylinder pressure cham-
bers during brake application and release.
The only repairable components on the ABS master
cylinder are the reservoir, reservoir grommets and
the connecting hoses. The cylinder itself cannot be
disassembled and is serviced only as an assembly.
POWER BRAKE BOOSTER
A dual diaphragm, vacuum operated power brake
booster is used with the ABS master cylinder (Fig.
Fig. 3 AntiLock Hydraulic Control Unit (HCU)
Fig. 4 ABS Power Brake Booster/Master Cylinder Assembly
5 - 40 BRAKESJ

4). The engine intake manifold serves as the vacuum
source for booster operation.
The booster is mounted on the engine compartment
side of the dash panel. The master cylinder is
mounted on attaching studs at the front of the
booster. The master cylinder central valves are di-
rectly actuated by the booster push rod.
The pedal travel sensor is mounted in the forward
face of the booster shell. The sensor plunger is actu-
ated by the booster diaphragm plate.
PEDAL TRAVEL SENSOR
The pedal travel sensor signals brake pedal posi-
tion to the antilock ECU. The sensor signal is based
on changes in electrical resistance. The resistance
changes occur in steps that are generated by changes
in brake pedal position. A resistance signal gener-
ated by changing brake pedal position, will cause the
ECU to run the antilock pump when necessary.
The sensor is a plunger-type, electrical switch
mounted in the forward housing of the power brake
booster (Fig. 5). The sensor plunger is actuated by
movement of the booster diaphragm plate.
The tip on the sensor plunger is color coded. The
tip must be matched to the color dot on the face of
the brake booster front shell (Fig. 5).
WHEEL SPEED SENSORS
A sensor is used at each wheel. The sensors convert
wheel speed into an electrical signal. This signal is trans-
mitted to the antilock electronic control unit (ECU).
A gear-type tone ring serves as the trigger mecha-
nism for each sensor. The tone rings are mounted at
the outboard ends of the front and rear axle shafts.
Different sensors are used at the front and rear
wheels (Fig. 6). The front/rear sensors have the same
electrical values but are not interchangeable.
ELECTRONIC CONTROL UNIT (ECU)
A separate electronic control unit (ECU) monitors,
operates and controls the antilock system (Fig. 7).
The ECU contains dual microprocessors. The logic
block in each microprocessor receives identical sensor
signals. These signals are processed and compared si-
multaneously (Fig. 8).
The ECU is located under the instrument panel. It
is located at the right side of the steering column.
The power up voltage source for the ECU is through
the ignition switch in the On and Run positions.
The antilock ECU is separate from the other vehi-
cle electronic control units. It contains a self check
program that illuminates the amber warning light
when a system fault is detected. Faults are stored in
a diagnostic program memory and are accessible
with the DRB II scan tool.
ABS faults remain in memory until cleared, or until af-
ter the vehicle is started approximately 50 times. Stored
faults arenoterased if the battery is disconnected.
ACCELERATION SWITCH
An acceleration switch (Fig. 9), provides an addi-
tional vehicle deceleration reference during 4-wheel
drive operation. The switch is monitored by the anti-
lock ECU at all times.
The switch reference signal is utilized by the ECU
when all wheels are decelerating at the same speed.
Equal wheel speeds occur during braking in undiffer-
entiated 4-wheel ranges.
Fig. 5 Pedal Travel Sensor Location
Fig. 6 Wheel Speed Sensors
JBRAKES 5 - 41