1995 JEEP YJ service

fied. 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 could be at fault.
BRAKES DO NOT HOLD AFTER DRIVING THROUGH
DEEP WATER PUDDLES
This condition is 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, disassembly and
cleaning will be necessary.
CONTAMINATED BRAKELINING
Brakelining contaminated by water is salvageable.
The lining can either be air dried or dried using heat.
In cases where brakelining is contaminated by oil,
grease, or brake fluid, the lining should be replaced.
Replacement is especially necessary when fluids/lu-
bricants have actually soaked into the lining mate-
rial. However, grease or dirt that gets onto the lining
surface (from handling) during brake repairs, can be
cleaned off. Spray the lining surface clean with Mo-
par brake cleaner.
BRAKE FLUID CONTAMINATION
There are two basic causes of brake fluid contami-
nation. The first involves allowing dirt, debris, or
other materials to enter the cylinder reservoirs when
the cover is off. The second involves adding non-rec-
ommended fluids to the cylinder reservoirs.
Brake fluid contaminated with only dirt, or debris
usually retains a normal appearance. In some cases,
the foreign material will remain suspended in the
fluid and be visible. The fluid and foreign material
can be removed from the reservoir with a suction gun
but only if the brakes have not been applied. If the
brakes are applied after contamination, system flush-
ing will be required. The master cylinder may also
have to be disassembled, cleaned and the piston seals
replaced. Foreign material lodged in the reservoir
compensator/return ports can cause brake drag by re-
stricting fluid return after brake application.
Brake fluid contaminated by a non-recommended
fluid may appear discolored, milky, oily looking, or
foamy. However, remember that brake fluid will
darken in time and occasionally be cloudy in appear-ance. These are normal conditions and should not be
mistaken for contamination.
If some type of oil has been added to the system,
the fluid will separate into distinct layers. To verify
this, drain off a sample with a clean suction gun.
Then pour the sample into a glass container and ob-
serve fluid action. If the fluid separates into distinct
layers, it is definitely contaminated.
The only real correction for contamination by non-
recommended fluid is to flush the entire hydraulic
system and replace all the seals.
BRAKE NOISE
Squeak/Squeal
Factory installed brakelining is made from as-
bestos free materials. These materials have dif-
ferent operating characteristics than previous
lining material. Under certain conditions, as-
bestos free lining may generate some squeak,
groan or chirp noise. This noise is considered
normal and does not indicate a problem. The
only time inspection is necessary, is when noise
becomes constant or when grinding, scraping
noises occur.
Constant brake squeak or squeal may be due to lin-
ings that are wet or contaminated with brake fluid,
grease, or oil. Glazed linings, rotors/drums with hard
spots, and dirt/foreign material embedded in the
brake lining also cause squeak. Loud squeak, squeal,
scraping, or grinding sounds are a sign of severely
worn brake lining. If the lining has worn completely
through in spots, metal-to-metal contact occurs.
Thump/Clunk
Thumping or clunk noises during braking are fre-
quentlynotcaused by brake components. In many
cases, such noises are caused by loose or damaged
steering, suspension, or engine components. However,
calipers that bind on the slide surfaces can generate
a thump or clunk noise. In addition, worn out, im-
properly adjusted, or improperly assembled rear
brakeshoes can also produce a thump noise.
Chatter/Shudder
Brake chatter, or shudder is usually caused by
loose or worn components, or glazed/burnt lining. Ro-
tors with hard spots can also contribute to chatter.
Additional causes of chatter are out of tolerance ro-
tors, brake lining not securely attached to the shoes,
loose wheel bearings and contaminated brake lining.
WHEEL AND TIRE PROBLEMS
Some conditions attributed to brake components
may actually be caused by a wheel or tire problem.
A damaged wheel can cause shudder, vibration and
pull. A worn or damaged tire can also cause pull.
Severely worn tires with very little tread left can
JSERVICE BRAKE DIAGNOSIS 5 - 7

produce a condition similar to grab as the tire loses
and recovers traction.
Flat-spotted tires can cause vibration and wheel
tramp and generate shudder during brake operation.
A tire with internal damage such as a severe bruise
or ply separation can cause vibration and pull. The
pull will be magnified when braking.
DIAGNOSING PARKING BRAKE MALFUNCTIONS
Adjustment Mechanism
Parking brake adjustment is controlled by a ca-
ble tensioner mechanism. The cable tensioner,
once adjusted at the factory, will not need further
attention under normal circumstances. There are
only two instances when adjustment is required.
The first is when a new tensioner, or cables have
been installed. And the second, is when the ten-
sioner and cables are disconnected for access to
other brake components.
Parking Brake Switch And Warning Light Illumination
The parking brake switch on the lever, or foot
pedal, is in circuit with the red warning light. The
switch will illuminate the red light only when the
parking brakes are applied. If the light remains on
after parking brake release, the switch or wires are
faulty, or cable tensioner adjustment is incorrect.
If the red light comes on while the vehicle is in mo-
tion and brake pedal height decreases, a fault has oc-
curred in the front or rear brake hydraulic system.
Parking Brake problem Causes
In most cases, the actual cause of an improperly
functioning parking brake (too loose/too tight/wont
hold), can be traced to a drum brake component.
The leading cause of improper parking brake
operation, is excessive clearance between the
brakeshoes and the drum surface. Excessive
clearance is a result of: lining and/or drum
wear; oversize drums; or inoperative shoe ad-
juster components.
Excessive parking brake lever travel (sometimes de-
scribed as a loose lever or too loose condition), is the re-
sult of worn brakeshoes/drums, improper brakeshoe
adjustment, or incorrectly assembled brake parts.
A ``too loose'' condition can also be caused by inop-
erative brakeshoe adjusters. If the adjusters are mis-
assembled, they will not function. In addition, since
the adjuster mechanism only works during reverse
stops, it is important that complete stops be made.
The adjuster mechanism does not operate when roll-
ing stops are made in reverse. The vehicle must be
brought to a complete halt before the adjuster lever
will turn the adjuster screw.
A condition where the parking brakes do not hold, will
most probably be due to a wheel brake component.
Items to look for when diagnosing a parking brake
problem, are:
²rear brakeshoe wear or adjuster problem
²rear brake drum wear
²brake drums machined beyond allowable diameter
(oversize)
²parking brake front cable not secured to lever
²parking brake rear cable seized
²parking brake strut reversed
²parking brake strut not seated in both shoes
²parking brake lever not seated in secondary shoe
²parking brake lever or brakeshoe bind on support
plate
²brakeshoes reversed
²adjuster screws seized
²adjuster screws reversed
²holddown or return springs misassembled or lack
tension
²wheel cylinder pistons seized
Brake drums that are machined oversize are diffi-
cult to identify without inspection. If oversize drums
are suspected, diameter of the braking surface will
have to be checked with an accurate drum gauge.
Oversize drums will cause low brake pedal and lack
of parking brake holding ability.
Improper parking brake strut and lever installation
will result in unsatisfactory parking brake operation.
Intermixing the adjuster screws will cause drag, bind
and pull along with poor parking brake operation.
Parking brake adjustment and parts replacement pro-
cedures are described in the Parking Brake section.
MASTER CYLINDER/POWER BOOSTER TEST
(1) Start engine and check booster vacuum hose
connections. Hissing noise indicates vacuum leak.
Correct any vacuum leak before proceeding.
(2) Stop engine and shift transmission into Neu-
tral.
(3) Pump brake pedal until all vacuum reserve in
booster is depleted.
(4) Press and hold brake pedal under light foot
pressure.
(a) If pedal holds firm, proceed to step (5).
(b) If pedal does not hold firm and falls away,
master cylinder is faulty due to internal leakage.
Overhaul or replace cylinder.
(5) Start engine and note pedal action.
(a) If pedal falls away slightly under light foot
pressure then holds firm, proceed to step (6).
(b) If no pedal action is discernible, or hard pedal
is noted, power booster or vacuum check valve is
faulty. Install known good check valve and repeat
steps (2) through (5).
(6) Rebuild booster vacuum reserve as follows: Re-
lease brake pedal. Increase engine speed to 1500
rpm, close throttle and immediately turn off ignition.
5 - 8 SERVICE BRAKE DIAGNOSISJ

(7) Wait a minimum of 90 seconds and try brake
action again. Booster should provide two or more vac-
uum assisted pedal applications. If vacuum assist is
not provided, perform booster and check valve vac-
uum tests.
POWER BOOSTER CHECK VALVE TEST
(1) Disconnect vacuum hose from check valve.
(2) Remove check valve and seal from booster (Fig.
1).
(3) Hand operated vacuum pump can be used for
test (Fig. 2).
(4) Apply 15-20 inches vacuum at large end of
check valve (Fig. 1).
(5) Vacuum should hold steady. If gauge on pump
indicates any vacuum loss, valve is faulty and must
be replaced.
POWER BOOSTER VACUUM TEST
(1) Connect a vacuum gauge to the booster check
valve with a short length of hose and tee fitting (Fig.
3).(2) Start and run engine at idle speed for one
minute.
(3) Pinch hose shut between vacuum source and
check valve (Fig. 3).
(4) Stop engine and observe vacuum gauge.
(5) If vacuum drops more than one inch HG (33
millibars) within 15 seconds, booster diaphragm or
check valve is faulty.
Fig. 1 Typical Vacuum Check Valve And Seal
Fig. 2 Typical Hand Operated Vacuum Pump
Fig. 3 Booster Vacuum Test Connections
JSERVICE BRAKE DIAGNOSIS 5 - 9

bubbles that are distributed throughout the hydrau-
lic system. This will make extra bleeding operations
necessary.
²Bleed only one wheel brake unit at a time and use
a bleed hose to bleed each wheel brake unit (Fig. 3).
²Attach one end of bleed hose to the bleed screw
and insert the opposite hose end in a glass container
partially filled with brake fluid (Fig. 3). A glass con-
tainer makes it easier to see air bubbles as they exit
the bleed hose. Be sure the end of the bleed hose is
immersed in fluid; this prevents air from being
drawn back into cylinder and brakeline.
BRAKE BLEEDING (WITH STANDARD BRAKES)
(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. Then fill cylinder reservoir with
Mopar brake fluid.
(3) Open all caliper and wheel cylinder bleed
screws. Close bleed screws after fluid begins flowing
from each bleed screw.
(4) Top off master cylinder reservoir again.
(5) Bleed master cylinder and combination valve at
brakeline fittings. Have helper operate brake pedal
while bleeding cylinder and valve.
(6) Bleed wheel brakes in recommended sequence
which is: right rear; left rear; right front; left front.
Bleed procedure is as follows:
(a) Open caliper or wheel cylinder bleed fitting
1/2 to 3/4 turn.
(b) Have helper depress and hold brake pedal to
floorpan.
(c) Tighten bleed fitting and have helper release
brake pedal. Continue bleeding operation until
fluid entering bleed container is clear and free of
bubbles.
(d) Repeat bleeding operation at remaining
wheel brake units.
(7) Top off master cylinder fluid level and verify
proper brake operation before moving vehicle.
BRAKE BLEEDING (WITH ABS BRAKES)
ABS system bleeding requires conventional bleed-
ing methods plus use of the DRB scan tool. The pro-
cedure involves performing a conventional bleed,
followed by use of the scan tool to cycle and bleed the
HCU pump and solenoids. A second conventional
bleed procedure is then required remove any air re-
maining in the system.
(1) If a new master cylinder is to be installed,
bleed cylinder on bench before installing it in vehicle.
Refer to procedure in section covering master cylin-
der service.(2) Wipe master cylinder reservoir and cap clean
before removing cap. This avoids having dirt fall into
fluid. Then fill reservoir with Mopar brake fluid.
(3) Perform conventional brake bleed as described
in steps (4) and (5).
(4) Bleed master cylinder and combination valve at
brakeline fittings. Have helper depress and release
brake pedal while bleeding cylinder and valve.
(5) Bleed wheel brakes in recommended sequence
which is: right rear; left rear; right front; left front.
Bleed procedure is as follows:
(a) Attach bleed hose to caliper bleed screw. Im-
merse end of hose in glass container partially filled
with brake fluid. Be sure hose end is submerged in
fluid (Fig. 3).
(b) Have helper depress and hold brake pedal to
floorpan.
(c) Open bleed screw 1/2 turn. Close bleed screw
when brake pedal contacts floorpan.Do not pump
brake pedal at any time while bleeding. This
compresses air into small bubbles which are
distributed throughout system. Additional
bleeding operations will then be necessary to
remove all trapped air from the system.
(d) Repeat bleeding operation at each wheel
brake unit fluid entering glass container is free of
air bubbles. Check reservoir fluid level frequently
and add fluid if necessary.
(6) Perform HCU bleed procedure with DRB scan
tool as follows:
(a) Connect scan tool to ABS diagnostic connec-
tor. Connector is under carpet at front of console,
just under instrument panel center bezel.
(b) Select CHASSIS SYSTEM, followed by
TEVES ABS BRAKES, then BLEED BRAKES.
When scan tool displays TEST COMPLETE, dis-
Fig. 3 Typical Bleed Hose And Fluid Container
JBRAKE FLUIDÐBRAKE BLEEDINGÐBRAKELINES AND HOSES 5 - 11

MASTER CYLINDERÐCOMBINATION VALVE
INDEX
page page
Combination Valve Replacement (Non-ABS)..... 16
General Service Information................. 15
Master Cylinder and Combination Valve Installation
(With ABS)............................. 20
Master Cylinder and Combination Valve Removal
(With ABS)............................. 17Master Cylinder Bench Bleeding.............. 21
Master Cylinder Installation (Non-ABS)......... 16
Master Cylinder Overhaul (4-Cylinder Models).... 16
Master Cylinder Removal (Non-ABS)........... 15
Reservoir Replacement (2-Piece Master Cylinder) . 19
GENERAL SERVICE INFORMATION
Master Cylinder
Two different master cylinders are used. A one-piece
cast aluminum cylinder is used on 4-cylinder YJ models
(Fig. 1). All other models have a two-piece master cylin-
der with removable nylon reservoir (Fig. 2).
The two master cylinders are serviced differently.
The reservoir and grommets are the only replaceable
parts on the two-piece master cylinder. The one-piece
master cylinder can be overhauled when necessary.
Combination Valve
A combination valve is used in all models. The
valve contains a pressure differential valve and
switch and a rear brake proportioning valve. The
valve is not repairable. It must be replaced if diagno-
sis indicates this is necessary.
The pressure differential switch is connected to the
brake warning light. The switch is actuated by move-
ment of the switch valve. The switch monitors fluid
pressure in the separate front/rear brake hydraulic cir-
cuits.
A decrease or loss of fluid pressure in either hydraulic
circuit will cause the switch valve to shuttle to the low
pressure side. Movement of the valve pushes the switch
plunger upward. This action closes the switch internal
contacts completing the electrical circuit to the red
warning light. The switch valve will remain in an actu-
ated position until repairs are made.
The rear proportioning valve is used to balance front-
rear brake action. The valve allows normal fluid flow
during moderate effort brake stops. The valve only con-
trols (meters) fluid flow during high effort brake stops.
MASTER CYLINDER REMOVAL (NON-ABS)
(1) Remove air cleaner hose, cover and housing.
Fig. 1 Master Cylinder And Combination Valve
(4-Cyl. YJ Models)
Fig. 2 Master Cylinder And Combination Valve (All
Except 4-Cyl. YJ Models)
JMASTER CYLINDERÐCOMBINATION VALVE 5 - 15

(2) Disconnect brake lines at master cylinder and
combination valve.
(3) Remove nuts attaching master cylinder to
booster studs.
(4) Remove master cylinder.
(5) Remove cylinder cover and drain fluid.
(6) If two-piece master cylinder reservoir requires
service, refer to reservoir replacement procedure in
this section.
MASTER CYLINDER INSTALLATION (NON-ABS)
(1) Bleed master cylinder on bench before installa-
tion. Refer to procedure in this section.
(2) If new two-piece master cylinder is being in-
stalled, remove plastic protective sleeve from primary
piston shank. Also check condition of seal at rear of
cylinder body. Reposition seal if dislodged. Replace
seal if cut, or torn.
(3) Clean cylinder mounting surface of brake booster.
Use shop towel wetted with brake cleaner for this pur-
pose. Dirt, grease, or similar materials will prevent
proper cylinder seating and could result in vacuum leak.
(4) Slide master cylinder onto brake booster studs.
(5) Install nuts attaching master cylinder to booster
studs. Tighten nuts to 25 Nzm (220 in. lbs.) torque.
(6) Connect brakelines to master cylinder and com-
bination valve (Figs. 1 and 2).
(7) Fill and bleed brake system.
COMBINATION VALVE REPLACEMENT (NON-ABS)
The combination valve is not a repairable compo-
nent. The valve is serviced as an assembly whenever
diagnosis indicates replacement is necessary.
(1) Remove air cleaner cover and hose for access to
valve, if necessary.
(2) Disconnect differential pressure switch wire at
combination valve. Do not pull switch wire to discon-
nect. Unsnap connecter lock tabs to remove.
(3) Disconnect brakelines at combination valve and
remove valve.
(4) Connect brakelines to replacement valve. Start
line fittings by hand to avoid cross threading.
Tighten fittings snug but not to required torque at
this time.
(5) Connect wire to pressure differential switch.
(6) Bleed brakes.
(7) Tighten brakeline fittings to 18-24 Nzm
(160-210 in. lbs.) torque after bleeding.
MASTER CYLINDER OVERHAUL (4-CYLINDER
MODELS)
CYLINDER DISASSEMBLY
(1) Examine cylinder cover seal. Discard seal if
torn or distorted.
(2) Clamp cylinder in vise (Fig. 3).(3) Remove piston retaining snap ring. Press and
hold primary piston inward with wood dowel or sim-
ilar tool. Then remove snap ring (Fig. 4).
(4) Remove and discard primary piston (Fig. 5).
Piston is serviced only as assembly.
(5) Remove secondary piston (Fig. 6). 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.
(6) Discard secondary piston. Do not disassemble
piston as components are only serviced as assembly.
MASTER CYLINDER 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
Fig. 3 Mounting Cylinder In Vise
Fig. 4 Removing/Installing Piston Snap Ring
5 - 16 MASTER CYLINDERÐCOMBINATION VALVEJ

POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCH
INDEX
page page
Brake Pedal Installation..................... 31
Brake Pedal Removal...................... 31
Brakelight Switch Adjustment................. 32
Brakelight Switch Installation................. 32
Brakelight Switch Removal.................. 32
General Information....................... 22
Power Brake Booster Installation (XJ with ABS) . . . 27
Power Brake Booster Installation (XJ Without ABS).. 30
Power Brake Booster Installation (YJ).......... 30
Power Brake Booster Operation.............. 23
Power Brake Booster Removal (XJ with ABS).... 24
Power Brake Booster Removal (XJ Without ABS) . . 29
Power Brake Booster Removal (YJ)............ 30
GENERAL INFORMATION
A 205 mm (8.07 in.) dual diaphragm power brake
booster is used for all applications (Figs. 1 and 2).
The only serviceable parts on the power brake
booster (Figs. 1 and 2) are the check valve, and vac-
uum hose. The booster itself is not serviceable. Re-
place the booster as an assembly whenever diagnosis
indicates a malfunction has occurred.
Brake Pedal And Brakelight Switch
A suspended-type brake pedal is used on all mod-
els. The pedal pivots on a shaft mounted in the pedalsupport bracket. The bracket is attached to the dash
and instrument panels on all models.
A plunger-type, adjustable brakelight switch is
used on all models. The switch is attached to a flange
on the pedal support bracket.
The brake pedal is a serviceable component. The
pedal, pivot pin, sleeve, pedal bushings and spacers/
washers are all replaceable parts. The pedal bracket
can also be replaced when necessary.
Fig. 1 Brake Booster/Master Cylinder Assembly (XJ)
5 - 22 POWER BRAKE BOOSTERÐBRAKE PEDALÐBRAKELIGHT SWITCHJ

ABS OPERATION AND SERVICE
INDEX
page page
ABS Component Serviceability............... 37
ABS Diagnostic Connector.................. 35
ABS Operation in Antilock Braking Mode........ 36
ABS Operation in Normal Braking Mode........ 35
ABS System Power-Up and Initialization........ 35
Acceleration Switch........................ 35
Acceleration Switch Installation............... 39
Acceleration Switch Operation................ 37
Acceleration Switch Removal................ 39
Combination Valve........................ 34
ECU Installation (XJ Models)................. 40
ECU Operation........................... 37
ECU Removal (XJ Models).................. 40
ECU Removal/Installation (YJ Models).......... 41
Electronic Control Unit (ECU)................ 34
Front Wheel Sensor Installation............... 38
Front Wheel Sensor Removal................ 38HCU Installation (XJ)....................... 43
HCU Installation (YJ)....................... 44
HCU Operation........................... 36
HCU Removal (XJ)........................ 41
HCU Removal (YJ)........................ 44
Hydraulic Control Unit (HCU)................. 33
Ignition Switch........................... 35
Master Cylinder/Power Brake Booster.......... 34
Rear Wheel Sensor Installation and Adjustment . . . 38
Rear Wheel Sensor Removal................ 38
Speed Sensor Air Gap..................... 37
System Description........................ 33
System Relays........................... 35
System Warning Light...................... 35
Wheel Speed Sensor Operation.............. 37
Wheel Speed Sensors..................... 34
SYSTEM DESCRIPTION
The Jeep antilock brake system (ABS) is an elec-
tronically operated, all wheel brake control system.
The system is designed to prevent wheel lockup
and maintain steering control during periods of high
wheel slip when braking. Preventing lockup is accom-
plished by modulating fluid pressure to the wheel
brake units.
The hydraulic system is a three channel design.
The front wheel brakes are controlled individually
and the rear wheel brakes in tandem (Fig. 1). The
ABS electrical system is separate from other electri-
cal circuits in the vehicle. A specially programmed
electronic control unit (ECU) operates the system
components.
ABS system major components include:
²hydraulic control unit (HCU)
²electronic control unit (ECU)
²wheel speed sensors and axle shaft tone rings
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, pump body, accumulators, pump motor,
and wire harnesses (Fig. 2).
The pump, motor, and accumulators are combined
into an assembly attached to the valve body. The ac-
cumulators store the extra fluid released to the sys-
tem for ABS mode operation. The pump provides the
fluid volume needed and is operated by a DC type
motor. The motor is controlled by the ECU.The valve body contains the solenoid valves. The
valves modulate brake pressure during antilock brak-
ing and are controlled by the ECU.
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.
Fig. 1 Jeep ABS System
JABS OPERATION AND SERVICE 5 - 33