power lock JEEP CHEROKEE 1995 Service Repair Manual

TRAC-LOK DIFFERENTIAL
OPERATION
In a conventional differential, the torque applied to
the ring gear is transmitted to the axle shafts through
the differential gears. During normal operation, the
torque transmitted to each wheel is equal at all times.
However, if one wheel spins, the opposite wheel will
generate only as much torque as the spinning wheel.
In the Trac-Lok differential, part of the ring gear
torque is transmitted through clutch packs. The clutch
packs contain multiple disc. The clutch will have radial
grooves on the plates, and concentric grooves on the
discs or bonded fiber material which is smooth.
In operation, the Trac-Lok clutches are engaged by
two concurrent forces. The first being preload force ex-
erted through Belleville spring washers. The second is
from separating forces generated by the side gears (Fig.
1).
The Trac-Lok design provides the normal differential
action needed for turning corners. It also provides for
the transmission of equal torque to both wheels when
driving straight ahead. When one wheel loses traction,
the clutch packs transfer torque to the wheel having the
most traction. Trac-lok differentials resist wheel spin on
bumpy roads. It also provides more pulling power when
one wheel loses traction. Pulling power is continuous
until both wheels lose traction. If both wheels slip due
to unequal traction, Trac-Lok operation is normal. In ex-
treme cases of differences of traction, the wheel with
the least traction may spin. This occurs after the Trac-
Lok has transferred as much torque as possible to the
non-spinning wheel.
NOISE DIAGNOSIS
If chatter occurs when turning corners, the most
probable cause is incorrect or contaminated lubri-
cant. Before removing the Trac-Lok unit for repair,
drain, flush and refill the axle with the specified lu-
bricant. Refer to Lubricant change in this Group.
A container of Trac-Lok Lubricant (friction modi-
fier) should be added after.
Vehicles with a limited slip differential should be
road tested by making 10 to 12 slow figure-eight
turns. This maneuver will pump the lubricant
through the clutch discs.
Refer to Group 0, Lubrication and Maintenance for
additional information.
DIFFERENTIAL TEST
WARNING: WHEN SERVICING VEHICLES WITH A
LIMITED SLIP DIFFERENTIAL DO NOT USE THE EN-
GINE TO TURN THE AXLE AND WHEELS. BOTH
REAR WHEELS MUST BE RAISED AND THE VEHI-
CLE SUPPORTED. A LIMITED SLIP AXLE CAN EX-
ERT ENOUGH FORCE (IF ONE WHEEL IS IN
CONTACT WITH THE SURFACE) TO CAUSE THE
VEHICLE TO MOVE.
The differential can be tested without removing the
differential case by measuring rotating torque. Make
sure brakes are not dragging during this measure-
ment.
(1) Engine off, transmission in neutral, and park-
ing brake off.
(2) Place blocks in front and rear of both front
wheels.
(3) Jack up one rear wheel until it is completely off
the ground.
(4) Remove wheel and bolt special tool to studs.
Fig. 1 Limited Slip Differential OperationÐBoth
Wheels Driving
JREAR SUSPENSION AND AXLES 3 - 45

BRAKES
CONTENTS
page page
ABS BRAKE DIAGNOSIS................... 3
ABS OPERATION AND SERVICE............ 33
BRAKE FLUIDÐBRAKE BLEEDINGÐ
BRAKELINES AND HOSES............... 10
DISC BRAKES.......................... 45
DRUM BRAKES......................... 55
GENERAL INFORMATION.................. 1MASTER CYLINDERÐCOMBINATION VALVE . . 15
PARKING BRAKES....................... 60
POWER BRAKE BOOSTERÐBRAKE PEDALÐ
BRAKELIGHT SWITCH.................. 22
SERVICE BRAKE DIAGNOSIS............... 4
SPECIFICATIONS........................ 70
GENERAL INFORMATION
INDEX
page page
Antilock Brakes (ABS)....................... 1
Brake Components......................... 1
Brake Fluid/Lubricants/Cleaning Solvents......... 2
Brake Safety Precautions.................... 2Brake Warning Lights....................... 1
Brakelining Material........................ 1
Jeep Body Code Letters..................... 2
BRAKE COMPONENTS
Power assist front disc and rear drum brakes are
standard on Cherokee/Wrangler models. Disc brake
components consist of single piston calipers and ven-
tilated rotors. Rear drum brakes are dual shoe units
with cast brake drums.
The parking brake mechanism is lever and cable
operated. The cables are attached to levers on the
rear drum brake secondary shoes. The parking
brakes are operated by a foot pedal on YJ models and
a hand lever on XJ models.
A 205 mm dual diaphragm vacuum power brake
booster is used for all applications. Two master cylin-
ders are used; 4-cylinder YJ models have a one-piece
master cylinder. All other models have a two-piece
master cylinder with plastic reservoir.
All models are equipped with a combination valve.
The valve contains a pressure differential valve and
switch and a fixed rate rear proportioning valve.
BRAKELINING MATERIAL
Factory brakelining on all models consists of an or-
ganic base material combined with metallic particles.
The lining does not contain asbestos.
BRAKE WARNING LIGHTS
Cherokee/Wrangler models are equipped with one
or two brake warning lights. A red warning light is
standard on all models. An amber light is added on
models with ABS brakes. Both lights are located in
the instrument panel.
The red light is in circuit with the pressure differ-
ential switch (in the combination valve), and with the
parking brake switch. The light alerts the driver
when the parking brakes are applied, or when a
pressure differential exists between the front and
rear hydraulic systems. The light also illuminates for
a few seconds at start up as part of a bulb check.
The ABS warning light is amber in color and is lo-
cated in the same side of the instrument cluster as
the red warning light. The amber light only illumi-
nates when an ABS circuit fault occurs.
ANTILOCK BRAKES (ABS)
An antilock brake system (ABS) is available on
Cherokee/Wrangler models as an option. The system
is an electronically operated, all-wheel brake control
system. The ABS system is designed to prevent
wheel lockup during periods of high wheel slip brak-
ing. Refer to the antilock brake section for operation
and service information.
JBRAKES 5 - 1

hand lever. Also note if vehicle was being operated
with parking brake partially applied (this will cause
red light to remain on).
(7) Check brake pedal operation. Verify that pedal
does not bind and has adequate free play. If pedal
lacks free play, check pedal and power booster for be-
ing loose or for bind condition. Do not road test until
condition is corrected.
(8) If components inspected look OK, road test ve-
hicle.
ROAD TESTING
(1) If amber warning light is on, problem is with
antilock system component. Refer to antilock diagno-
sis section.
(2) If red warning light is not on, proceed to step
(4).
(3) If red warning light is on, proceed as follows:
(a) See if parking brakes are applied. If brakes
are applied, release them and proceed to step (4).
(b) Note if brake pedal is abnormally low. If
pedal is definitely low and red light is still on,
check front/rear hydraulic circuits for leak.Do not
road test. Inspect and repair as needed.
(4) Check brake pedal response with transmission
in Neutral and engine running. Pedal should remain
firm under steady foot pressure. If pedal falls away,
do not road test as problem is in master cylinder, or
HCU on ABS models. If pedal holds firm, proceed to
next step.
(5) During road test, make normal and firm brake
stops in 25-35 mph range. Note faulty brake opera-
tion such as hard pedal, pull, grab, drag, noise, fade,
etc.
(6) Return to shop and inspect brake components.
Refer to inspection and diagnosis information.
COMPONENT INSPECTION
Fluid leak points and dragging brake units can
usually be located without removing any components.
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 boost-
ers. Refer to the wheel brake service procedures in
this group for more information.
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
²when parking brakes are applied
²low pedal caused by leak in front/rear brake hy-
draulic circuit
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 plus
low pedal indicates the pressure differential switch
and valve have been actuated due to a system leak.
On models with ABS brakes, the amber warning
light only illuminates when an ABS malfunction has
occurred. The ABS light operates independently of
the red warning light.
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 at the
end of 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 the red warn-
ing light is on, the likely cause is a leak in the hy-
draulic system.
A decrease in master cylinder fluid level may only
be the result of normal lining wear. Fluid level will
drop somewhat as lining wear occurs. It is a result of
the outward movement of caliper and wheel cylinder
pistons to compensate for normal wear.
SPONGY PEDAL
Air in the system is the usual cause of a spongy
pedal. Brake drums machined way beyond allowable
limits (too thin), or substandard brake lines and
hoses can also cause a condition similar to a spongy
JSERVICE BRAKE DIAGNOSIS 5 - 5

pedal. The proper course of action is to bleed the sys-
tem, 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 lin-
ing 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 described
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, ro-
tors and drums. A drag condition also worsens as
temperature of the brake parts increases.
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 bolts or slide surfaces
²wrong length caliper mounting bolts (too long)
²loose caliper mounting bracket
²distorted rotor, brake drum, or shoes
²brakeshoes 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 compensatorport or faulty power booster (binds-does not release).
The condition will worsen as brake temperature in-
creases.
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 (NON-ABS BRAKES ONLY)
Pedal pulsation is caused by parts that are loose,
or beyond tolerance limits. This type of pulsation is
constant and will occur every time the brakes are ap-
plied.
Disc brake rotors with excessive lateral runout or
thickness variation, or out of round brake drums are
the primary causes of pulsation.
On vehicles with ABS brakes, remember that pedal
pulsation is normal during antilock mode brake
stops. If pulsation occurs during light to moderate
brake stops, a standard brake part is either loose, or
worn beyond tolerance.
BRAKE 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 com-
ponent are further causes of pull. A damaged front
tire (bruised, ply separation) can also cause pull.
Wrong caliper bolts (too long) will cause a partial ap-
ply condition and pull if only one caliper is involved.
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 re-
duced that fade occurs. If the opposite brake unit is
still functioning normally, its braking effect is magni-
5 - 6 SERVICE BRAKE DIAGNOSISJ

POWER BRAKE BOOSTER REMOVAL
(XJ WITH ABS)
(1) Disconnect vacuum and vent hoses at air
cleaner cover.
(2) Loosen clamp securing air cleaner hose to in-
take manifold. Use screwdriver to tap clamp loose.
(3) Remove air cleaner cover and hose. Then re-
move air filter from air cleaner housing (Fig. 4).
(4) Remove two bolts and one nut that secure air
cleaner housing to body (Fig. 4).
(5) Remove air cleaner housing from engine com-
partment (Fig. 4).
(6) Disconnect wire at combination valve pressure
differential switch (Fig. 5). Do not pull on wires to
disconnect. Unsnap lock tabs on connecter to remove
wires.
(7) Disconnect canister vacuum line at manifold
fitting (Fig. 6).(8) Disconnect brake booster vacuum hose at in-
take manifold fitting (Fig. 7). Move hose aside for
working clearance.
(9) Unseat small S-clip that secures brakelines
(Fig. 8).
(10) Remove front brakeline that connects master
cylinder front port to combination valve front port
(Fig. 8).
Fig. 3 Power Brake Booster Internal Components
5 - 24 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

MASTER CYLINDER/POWER BRAKE BOOSTER
A 25 mm bore master cylinder and 205 mm (8.07
in.) dual diaphragm power brake booster are used for
all ABS applications (Fig. 2).
The master cylinder has a removable plastic reser-
voir which is the only serviceable component. The
cylinder body and pistons are not repairable and are
serviced as an assembly. The check valve and grom-
met are the only serviceable parts on the booster.
The booster itself is only serviced as an assembly.
COMBINATION VALVE
A combination valve is used with the ABS system
(Fig. 2). The valve contains a front/rear brake pres-
sure differential switch and rear brake proportioning
valve. The combination valve is connected between
the master cylinder and HCU.
The pressure differential switch is connected to the
red brake warning light. The switch is actuated by
movement of the switch valve. The switch monitors
fluid pressure in the separate front/rear brake hy-
draulic circuits.
A decrease or loss of fluid pressure in either hy-
draulic circuit will cause the switch valve to shuttle
forward or rearward in response to the pressure dif-
ferential. Movement of the switch valve will push the
switch plunger upward. This closes the switch inter-
nal contacts completing the electrical circuit to the
red warning light. The switch valve remains in an
actuated position until the fault is repaired.
The rear proportioning valve is used to balance front-
rear brake action.
ELECTRONIC CONTROL UNIT (ECU)
A separate electronic control unit (ECU) operates
the ABS system (Fig. 3). The ECU is separate from
other vehicle electrical circuits. ECU voltage source
is through the ignition switch in the Run position.The ECU is located under the instrument panel in
the passenger compartment. On YJ models, it is just
above the heater plenum in line with the glove box.
In left hand drive XJ models, it at the right side of
the steering column. In right hand drive models, it is
near the cowl panel
The ECU contains dual microprocessors. A logic
block in each microprocessor receives identical sensor
signals. These signals are processed and compared si-
multaneously.
The ECU contains a self check program that illu-
minates the ABS warning light when a system fault
is detected. Faults are stored in a diagnostic program
memory and are accessible with the DRB scan tool.
ABS faults remain in memory until cleared, or un-
til after the vehicle is started approximately 50
times. Stored faults arenoterased if the battery is
disconnected.
WHEEL SPEED SENSORS
A speed sensor is used at each wheel. The sensors
convert wheel speed into an electrical signal. This
signal is transmitted to the antilock 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. 4). The front/rear sensors have the same
electrical values but are not interchangeable.
Fig. 2 ABS Master Cylinder-Booster-Combination
Valve-HCU
Fig. 3 Antilock ECU
5 - 34 ABS OPERATION AND SERVICEJ

ABS DIAGNOSTIC CONNECTOR
The ABS diagnostic connector is inside the vehicle.
The connector is the access point for the DRB scan
tool.
On XJ models, the connector is located under the
instrument panel to the right of the steering column.
On some models, the connecter may be tucked under
the carpeting on the transmission tunnel. The con-
necter is a black, 6-way type.
On YJ models, the connector is under the instru-
ment panel by the the driver side kick panel. The
connecter is a black, 6 or 8-way type.
The DRB scan tool kit contains adapter cords for
both types of connecter. Use the appropriate cord for
test hookup.
ACCELERATION SWITCH
An acceleration switch (Fig. 5), provides an addi-
tional vehicle deceleration reference during 4-wheel
drive operation. The switch is monitored by the an-
tilock ECU at all times. The switch reference signal
is utilized by the ECU when all wheels are deceler-
ating at the same speed.
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 ECU. The main re-
lay is connected to the ECU at the power control re-
lay terminal. The pump motor relay starts/stops the
pump motor when signaled by the ECU.
IGNITION SWITCH
The antilock ECU and warning light are in standby
mode with the ignition switch in Off or Accessory po-
sition. No operating voltage is supplied to the system
components.A 12 volt power feed is supplied to the ECU and
warning light when the ignition switch is in the Run
position.
SYSTEM WARNING LIGHT
The amber ABS warning light is in circuit with the
ECU and operates independently of the red brake
warning light.
The ABS light indicates antilock system condition.
The light illuminates (flashes) at start-up for the self
check. The light goes out when the self check pro-
gram determines system operation is normal.
ABS SYSTEM POWER-UP AND INITIALIZATION
battery voltage is supplied to the ECU ignition ter-
minal when the ignition switch is turned to Run po-
sition. 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 after the ignition switch is
turned to Run position. The dynamic check occurs
when vehicle road speed reaches approximately 10
kph (6 mph). During the dynamic check, the ECU
briefly cycles the pump and solenoids to verify oper-
ation.
If an ABS component exhibits a fault during initial-
ization, 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.
Fig. 4 Wheel Speed SensorsFig. 5 Acceleration Switch
JABS OPERATION AND SERVICE 5 - 35

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 may occur
when brake stops involve high pedal pressure and
rate of deceleration.
The antilock system prevents 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 ABS system has three fluid pressure control
channels. The front brakes are controlled separately
and the rear brakes in tandem (Fig. 1). A speed sen-
sor input signal indicating a high slip condition acti-
vates the ECU antilock program.
Two solenoid valves are used in each antilock con-
trol channel. The valves are all located within the
HCU valve body and work in pairs to either increase,
hold, or decrease apply pressure as needed in the in-
dividual 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 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. 6).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, which also opens the return circuit to the ac-
cumulators. Fluid pressure is allowed to bleed off (de-
crease) as needed to prevent wheel lock.
Once the period of high wheel slip has ended, the
ECU closes the outlet valve and begins a pressure in-
crease or hold cycle as needed.
Pressure Hold
Both solenoid valves are closed in the pressure hold
cycle (Fig. 7). Fluid apply pressure in the control
channel is maintained at a constant rate. The ECU
maintains the hold cycle until sensor inputs indicate
a pressure change is necessary.
Pressure Increase
The inlet valve is open and the outlet valve is
closed during the pressure increase cycle (Fig. 8). The
pressure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.
Fig. 6 Pressure Decrease Cycle
5 - 36 ABS OPERATION AND SERVICEJ

The engine accessory drive belt must be removed
prior to removing the fan (if installed at pump) or
fan pulley.
(4) Remove engine drive belt as follows:
(a) Loosen two rear power steering pump mount-
ing bolts A (Fig. 5).
(b) Loosen upper pump pivot bolt B and lower
lock nut C (Figs. 6 or 7).
(c) Loosen pump adjusting bolt D (Fig. 5) until
belt can be removed.(d) Remove belt.
(5) Check condition of all pulleys.
(6) The power steering pump must be removed
from its cast mounting bracket to gain access to bolt
E. Bracket mounting bolt E is located behind the
power steering pump (Fig. 7).
(7) Remove two bolts A (Fig. 5).
(8) Remove locknut C and belt adjustment bolt D
(Figs. 6 or 7).
(9) Remove bolt B (Fig. 6). Position power steering
pump to the side. Hold pump in position with wire.
Do not disconnect hydraulic lines from pump.
(10) Remove bolts E, F and G (Fig. 7) and remove
pump mounting bracket.
Fig. 4 Fan Mounting Nuts
Fig. 5 P.S. Pump Rear Mounting BoltsÐTypical
Fig. 6 P.S. Pump Front Mounting Bolt/LocknutÐ
Typical
Fig. 7 Bracket Mounting BoltsÐTypical
JCOOLING SYSTEM SERVICE PROCEDURES 7 - 15