JEEP CHEROKEE 1994 Service Repair Manual

DRUM BRAKES
INDEX
page page
Brake Drum Refinishing.................... 37
Drum Brake Adjustment................... 35
Drum Brakeshoe Installation................ 34
Drum Brakeshoe Removal (Figs. 1 and 2)...... 34
Support Plate Replacement................. 37Wheel Cylinder Installation................. 37
Wheel Cylinder Overhaul (Figs. 8 and 9)....... 36
Wheel Cylinder Removal................... 36
Wheel Nut Tightening..................... 37
DRUM BRAKESHOE REMOVAL (Figs. 1 and 2)
(1) Raise vehicle and remove rear wheels.
(2) Remove and discard spring nuts securing
drums to wheel studs.
(3) Remove brake drums. If drums prove difficult
to remove, retract brakeshoes. Remove access plug at
the rear of backing plate and back off adjuster screw
with brake tool and screwdriver.
(4) Remove U-clip and washer securing adjuster
cable to parking brake lever.
(5) Remove primary and secondary return springs
from anchor pin with Brake Spring Plier Tool 8078.
(6) Remove holddown springs, retainers and pins
with Retaining Spring Tool C-4070.
(7) Install Spring Clamps C-416 on wheel cylinders
to hold pistons in place.
(8) Remove adjuster lever, adjuster screw and
spring.
(9) Remove adjuster cable and cable guide.
(10) Remove brakeshoes and parking brake strut.
(11) Disconnect cable from parking brake lever
and remove lever.
DRUM BRAKESHOE INSTALLATION
(1) Clean support plate with Mopar brake cleaner.
Replace support plate if worn, or rusted through at
any point. Do not attempt to salvage, or reuse a dam-
aged support plate.
(2) Clean and lubricate anchor pin with light coat
of Mopar multi-mileage grease.
(3) Apply Mopar multi-mileage grease to brake-
shoe contact surfaces of support plate (Figs. 3 and 4).
(4) Lubricate adjuster screw threads and pivot
with Mopar spray lube.
(5) Attach parking brake lever to secondary brake-
shoe. Use new washer and U-clip to secure lever.
(6) Remove wheel cylinder clamps.
(7) Attach parking brake cable to lever.
(8) Install brakeshoes on support plate. Secure
shoes with new holddown springs, pins and retainers.
(9) Install parking brake strut and spring.
(10) Install guide plate and adjuster cable on an-
chor pin.
(11) Install primary and secondary return springs.
(12) Install adjuster cable guide on secondary shoe.
Fig. 1 Nine Inch Drum Brake Components
5 - 34 BRAKESJ

(13) Lubricate and assemble adjuster screw (Fig.
5).
(14) Install adjuster screw, spring and lever and
connect to adjuster cable.
(15) Adjust shoes to drum as described in following
procedure.
(16) Install wheel/tire assemblies and lower vehi-
cle.
(17) Verify firm brake pedal before moving vehicle.
DRUM BRAKE ADJUSTMENT
Rear drum brakes are equipped with a self adjust-
ing mechanism. Under normal circumstances, the
only time adjustment is required is when the shoes
are replaced, removed for access to other parts, or
when one or both drums are replaced.The only tool needed for adjustment is a standard
brake gauge.
Adjustment is performed with the brakeshoes in-
stalled on the support plate. Procedure is as follows:
ADJUSTMENT PROCEDURE
(1) Raise and support vehicle rear end and remove
wheels and brake drums.
Fig. 2 Ten Inch Drum Brake Components
Fig. 3 Shoe Contact Surfaces (9-Inch Support Plate)
Fig. 4 Shoe Contact Surfaces (10-Inch Support Plate)
JBRAKES 5 - 35

(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

The correct tightening sequence is important in
avoiding rotor and drum distortion. The correct se-
quence is in a diagonal crossing pattern (Fig. 12).
Recommended torque range for XJ/YJ wheel lug
nuts is 109-150 Nzm (80-110 ft. lbs.).
Seat the wheel and install the wheel nuts finger
tight. Tighten the nuts in the sequence to half the
required torque. Then repeat the tightening sequence
to final specified torque.
Fig. 11 Typical Location Of Brake Drum Refinish
Limit
Fig. 12 Wheel Nut Tightening Sequence
5 - 38 BRAKESJ

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

SYSTEM RELAYS
The ABS system has two relays, which are the
main and motor pump relays. The motor pump relay
is used for the motor pump only. The main relay is
used for the solenoid valves and remaining system
components. The main relay is connected to the ECU
at the power control relay terminal.
The pump motor relay starts/stops the pump motor
when signaled by the ECU. The start/stop signal tothe ECU is generated by the pedal travel sensor. Re-
fer to the ABS schematic at the end of this section
for circuit details.
IGNITION SWITCH
The antilock ECU and warning light are in
standby mode with the ignition switch in Off or Ac-
cessory position. No operating voltage is supplied to
the system components.
A 12 volt power feed is supplied to the ECU, re-
lays, solenoid valves, and warning light when the ig-
nition switch is in the ON, Start and Run positions.
Refer to the ABS system schematic at the end of this
section for details.
SYSTEM WARNING LIGHTS
Two warning lights are used. The standard brake
system light is red. The antilock system light is am-
ber. Both lights are in the instrument cluster. The
amber ABS light is in circuit with the ECU and op-
erates independently of the red brake light.
The amber light indicates antilock system condi-
tion. It is in circuit with the valve body solenoids and
main relay. The light illuminates (flashes) at
start-up for the self check. The light then goes out
when the self check program determines system op-
eration is normal.
If an ABS fault occurs either during the start-up
self check, or during normal operation, the amber
light remains on until the fault is corrected.
COMBINATION VALVE
A combination valve is used with the ABS system
(Fig. 2). The valve contains a front/rear brake pres-
sure switch and proportioning valve. The valve is
connected between the master cylinder and hydraulic
control unit (HCU).
Fig. 7 Anti-Lock ECU
Fig. 8 ECU Dual Microprocessor Schematic
Fig. 9 Acceleration Switch
5 - 42 BRAKESJ

ANTILOCK BRAKE SYSTEM OPERATION
INDEX
page page
ABS Operation in Antilock Braking Mode....... 43
ABS Operation in Normal Braking Mode....... 43
Acceleration Switch Operation............... 45
ECY Operation.......................... 46HCU Pump and Pedal Travel Sensor Operation . 44
HCU Solenoid Valve Operation.............. 43
System Power-Up and Initialization........... 43
Wheel Speed Sensor Operation............. 45
SYSTEM POWER-UP AND INITIALIZATION
The antilock system is in standby mode with the
ignition switch in Off or Accessory position. The an-
tilock electrical components are not operational.
Turning the ignition switch to On or Run position
allows battery voltage to flow through the switch to
the ECU ignition terminal.
The ABS system is activated when battery voltage
is supplied to the ECU. The ECU performs a system
initialization procedure at this point. Initialization
consists of a static and dynamic self check of system
electrical components.
The static check occurs immediately after the igni-
tion switch is turned to the On position. The dynamic
check occurs when vehicle road speed reaches ap-
proximately 10 kph (6 mph). During the dynamic
check, the ECU briefly cycles the pump to verify op-
eration. The HCU solenoids are checked continu-
ously.
If an ABS component exhibits a fault during ini-
tialization, the ECU illuminates the amber warning
light and registers a fault code in the microprocessor
memory.
ABS OPERATION IN NORMAL BRAKING MODE
The ECU monitors wheel speed sensor inputs con-
tinuously while the vehicle is in motion. However,
the ECU will not activate any ABS components as
long as sensor inputs and the acceleration switch in-
dicate normal braking.
During normal braking, the master cylinder, power
booster and wheel brake units all function as they
would in a vehicle without ABS. The HCU compo-
nents are not activated.
ABS OPERATION IN ANTILOCK BRAKING MODE
The purpose of the antilock system is to prevent
wheel lockup during periods of high wheel slip. Pre-
venting lockup helps maintain vehicle braking action
and steering control.
The antilock ECU activates the system whenever
sensor signals indicate periods of high wheel slip.
High wheel slip can be described as the point where
wheel rotation begins approaching zero (or lockup)
during braking. Periods of high wheel slip occur
when brake stops involve high pedal pressure and
rate of vehicle deceleration.The antilock system retards lockup during high
slip conditions by modulating fluid apply pressure to
the wheel brake units.
Brake fluid apply pressure is modulated according
to wheel speed, degree of slip and rate of decelera-
tion. A sensor at each wheel converts wheel speed
into electrical signals. These signals are transmitted
to the ECU for processing and determination of
wheel slip and deceleration rate.
The Jeep ABS system has three fluid pressure con-
trol channels. The front brakes are controlled sepa-
rately and the rear brakes in tandem (Fig. 10). A
speed sensor input signal indicating high slip condi-
tions activates the ECU antilock program.
Two solenoid valves are used in each antilock con-
trol channel (Fig. 11). The valves are all located
within the HCU valve body and work in pairs to ei-
ther increase, hold, or decrease apply pressure as
needed in the individual control channels.
The solenoid valves are not static during antilock
braking. They are cycled continuously to modulate
pressure. Solenoid cycle time in antilock mode can be
measured in milliseconds.
HCU SOLENOID VALVE OPERATION
Normal Braking
During normal braking, the HCU solenoid valves
and pump are not activated. The master cylinder and
power booster operate the same as a vehicle without
an ABS brake system.
Antilock Pressure Modulation
Solenoid valve pressure modulation occurs in three
stages which are: pressure increase, pressure hold,
and pressure decrease. The valves are all contained
in the valve body portion of the HCU.
Pressure Decrease
The outlet valve is opened and the inlet valve is
closed during the pressure decrease cycle (Fig. 11).
A pressure decrease cycle is initiated when speed
sensor signals indicate high wheel slip at one or
more wheels. At this point, the ECU opens the outlet
valve. Opening the outlet valve also opens the hy-
draulic return circuit to the master cylinder reser-
JANTILOCK BRAKE SYSTEM OPERATION 5 - 43