1995 JEEP YJ wheel

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

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

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

WHEEL SPEED SENSOR OPERATION
Wheel speed input signals are generated by a sen-
sor and tone ring at each wheel. The sensors, which
are connected directly to the ECU, are mounted on
brackets attached to the front steering knuckles and
rear brake support plates.
The sensor triggering devices are the tone rings
which are similar in appearance to gears. The tone
rings are located on the outboard end of each front/rear axle shaft. The speed sensors generate a signal
whenever a tone ring tooth rotates past the sensor
pickup face.
The wheel speed sensors provide the input signal
to the ECU. If input signals indicate ABS mode brak-
ing, the ECU causes the HCU solenoids to decrease,
hold, or increase fluid apply pressure as needed.
The HCU solenoid valves are activated only when
wheel speed input signals indicate that a wheel is
approaching a high slip, or lockup condition. At this
point, the ECU will cycle the appropriate wheel con-
trol channel solenoid valves to prevent lockup.
The wheel sensors provide speed signals whenever
the vehicle wheels are rotating. The ECU examines
these signals for degree of deceleration and wheel
slip. If signals indicate normal braking, the solenoid
valves are not activated. However, when incoming
signals indicate the approach of wheel slip, or lockup,
the ECU cycles the solenoid valves as needed.
ACCELERATION SWITCH OPERATION
The ECU monitors the acceleration switch at all
times. The switch assembly contains three mercury
switches that monitor vehicle ride height and decel-
eration rates (G-force). Sudden, rapid changes in ve-
hicle and wheel deceleration rate, triggers the switch
sending a signal to the ECU. The switch assembly
provides three deceleration rates; two for forward
braking and one for rearward braking.
ECU OPERATION
The antilock ECU controls all phases of antilock
operation. It monitors and processes input signals
from the system sensors.
It is the ECU that activates the solenoid valves to
modulate apply pressure during antilock braking.
The ECU program is able to determine which wheel
control channel requires modulation and which fluid
pressure modulation cycle to use. The ECU cycles the
solenoid valves through the pressure decrease, hold
and increase phases.
ABS COMPONENT SERVICEABILITY
The ECU, acceleration sensor, wheel sensors, and
wire harnesses are serviced as assemblies only. The
axle shaft tone wheels are also not serviceable. If a
tone wheel becomes damaged, it will be necessary to
replace the axle shaft, or disc brake rotor and hub
assembly.
SPEED SENSOR AIR GAP
Front sensor air gap is fixed and not adjustable.
Only rear sensor air gap is adjustable.
Although front air gap is not adjustable, it can be
checked if diagnosis indicates this is necessary. Front
Fig. 7 Pressure Hold Cycle
Fig. 8 Pressure Increase Cycle
JABS OPERATION AND SERVICE 5 - 37