JEEP CHEROKEE 1994 Service Repair Manual

voir. Fluid pressure is allowed to bleed off (decrease)
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. 12). 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. 13).
The pressure increase cycle is used to counteract un-
equal wheel speeds. This cycle controls reapplication
of fluid apply pressure after a pressure decrease cy-
cle.
HCU PUMP AND PEDAL TRAVEL SENSOR
OPERATION
The HCU pump has two functions during antilock
braking. First, the pump supplies the extra volume
of fluid needed. And second, the pump maintains
brake pedal height. The fluid source for the pump is
the master cylinder reservoir. The reservoir and
HCU are interconnected by hoses.
The pump motor is activated by the ECU. How-
ever, the signal to run the pump actually comes from
the pedal travel sensor.
The pedal travel sensor is mounted in the forward
face of the brake booster (Fig. 14). The sensorplunger is actuated by movement of the booster dia-
phragm plate. The sensor has a total of seven pedal
positions, six of which are monitored. The six pedal
positions monitored range from full release to full ap-
ply. Each pedal position (toward full apply), gener-
ates an increasing degree of electrical resistance in
the sensor.
The ECU continuously monitors electrical resis-
tance at the pedal travel sensor. The ECU activates
the pump whenever sensor electrical resistance in-
creases during ABS mode braking.
Fig. 10 Three-Channel ABS Hydraulic Control Circuit
Fig. 11 Solenoid Valves In Pressure Decrease Cycle
5 - 44 ANTILOCK BRAKE SYSTEM OPERATIONJ

At the start of antilock braking, pedal height will
decrease as the volume of fluid in the master cylin-
der is used up. When pedal height drops a predeter-
mined amount, the pedal travel sensor will signal
the ECU to run the pump. At this point, the pump is
activated to supply the extra fluid volume and re-
store pedal height at the same time.
The pump does not run continuously. It cycles on/
off according to signals from the travel sensor and
ECU. The pump is connected directly to the mastercylinder reservoir by hoses. During antilock braking,
the additional volume of fluid needed is drawn by the
pump from the reservoir.
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 slip or 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.
Fig. 12 Solenoid Valves In Pressure Hold Cycle
Fig. 13 Solenoid Valves In Pressure Increase Cycle
Fig. 14 Pedal Travel Sensor Actuation
JANTILOCK BRAKE SYSTEM OPERATION 5 - 45

ECU OPERATION
The antilock ECU controls all phases of antilock
operation. It monitors and processes input signals
from all of 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 thepressure decrease, hold and increase phases to retard
and prevent wheel lock during periods of high wheel
slip.
Solenoid valve operation is selective. The solenoid
valves may not be cycled simultaneously, nor are
they all cycled in the same pressure modulation
phase at the same time. The ECU cycles the valves
in each control channel as needed. For example, sen-
sor inputs may indicate that only the left front wheel
requires modulation during a period of high slip.
5 - 46 ANTILOCK BRAKE SYSTEM OPERATIONJ

ABS COMPONENT SERVICE
INDEX
page page
Acceleration Sensor Installation.............. 52
Acceleration Sensor Removal............... 52
Combination Valve ReplacementÐXJ......... 55
Combination Valve ReplacementÐYJ......... 55
Component Serviceability................... 47
Correct Fluid Level....................... 48
ECU ReplacementÐXJ.................... 53
ECU ReplacementÐYJ.................... 53
Front Wheel Sensor Installation.............. 49
Front Wheel Sensor Removal............... 49
HCU InstallationÐXJ...................... 54
HCU InstallationÐYJ...................... 54HCU RemovalÐXJ....................... 53
HCU RemovalÐYJ....................... 54
Importance of Clean Brake Fluid............. 48
Master Cylinder Installation................. 50
Master Cylinder Removal.................. 50
Pedal Travel Sensor Service................ 51
Power Brake Booster Installation............. 51
Power Brake Booster Removal.............. 51
Rear Wheel Sensor Installation.............. 50
Rear Wheel Sensor Removal............... 49
Recommended Brake Fluid................. 48
Wheel Sensor Air Gap Adjustment........... 49
COMPONENT SERVICEABILITY
The ABS components are serviced as assemblies
(Figs. 1 and 2); they are not repairable. The follow-
ing ABS components can be replaced separately:
²center feed master cylinder
²master cylinder-to-booster seal
²power brake booster (includes matched pedal
travel sensor)
²booster check valve and grommet
²pedal travel sensor and select fit caps²combination valve
²HCU and pump motor assembly
²ECU
²acceleration sensor
²wheel sensors
²system wire harnesses
The axle shaft tone wheels are not serviceable. If a
tone wheel becomes damaged, it will be necessary to
replace the axle shaft, or disc brake rotor and hub
assembly on 2-wheel drive models.
Fig. 1 ABS Hydraulic Component Locations (XJ)
JABS COMPONENT SERVICE 5 - 47

The wheel brake components such as the calipers,
brakeshoes, wheel cylinders, rotors and drums are all
serviced the same as standard brake system compo-
nents.
RECOMMENDED BRAKE FLUID
Recommended brake fluid for the Jeep ABS system
is Mopar DOT 3 brake fluid. If Mopar fluid is not
readily available, a top quality fluid meeting SAE
J1703 and DOT 3 standards can be used.
Brake fluid used in the ABS system must meet the
SAE and DOT quality standards and be exception-
ally clean.Never use substandard fluid, fluid not
meeting the SAE and DOT standards, reclaimed
fluid, or fluid from open containers.
CORRECT FLUID LEVEL
Correct brake fluid level is marked on the driver
side of the master cylinder reservoir (Fig. 3).
Preferred fluid level is to the MAX indicator mark.
Acceptable fluid level is between the MAX and MIN
marks.
If fluid level is at or below the MIN mark, the
brake hydraulic system should be checked for leaks.
CAUTION: Clean the reservoir caps and exterior
thoroughly before checking fluid level. Do not allow
any dirt or foreign material to enter the reservoirwhile checking fluid level. Such materials can inter-
fere with solenoid valve operation causing an ABS
malfunction.
IMPORTANCE OF CLEAN BRAKE FLUID
The antilock system brake fluid must be kept clean
and uncontaminated. Foreign material in the fluid,
or non-recommended fluids will cause system mal-
functions.
Fig. 2 ABS Hydraulic Component Locations (YJ)
Fig. 3 Reservoir Fluid Level Indicators
5 - 48 ABS COMPONENT SERVICEJ

Clean the reservoir and caps thoroughly before
checking level or adding fluid. Cap open lines and
hoses during service to prevent dirt entry.
Dirt or foreign material entering the ABS hydrau-
lic system through the reservoir opening will circu-
late within the system. The result will be poor brake
performance and possible component failure. Use
clean, fresh fluid only to top off, or refill the system.
WHEEL SENSOR AIR GAP ADJUSTMENT
Only rear sensor air gap is adjustable. The front
sensors are fixed and cannot be adjusted.
A rear sensor air gap adjustment is only
needed when reinstalling an original sensor. Re-
placement sensors have an air gap spacer at-
tached to the sensor pickup face. The spacer
establishes correct air gap when pressed against
the tone ring during installation. As the tone
ring rotates, it peels the spacer off the sensor to
create the required air gap.
Preferred rear sensor air gap is 1.1 mm (0.043 in.).
Acceptable air gap range is 0.92 to 1.275 mm (0.036
to 0.050 in.).
Front sensor air gap is not adjustable. The front
sensors are fixed in position and cannot be adjusted.
Front sensor air gap can only be checked. Air gap
should be 0.040 to 1.3 mm (0.0157 to 0.051 in.). If
front sensor air gap is incorrect, the sensor is either
loose, or damaged.
FRONT WHEEL SENSOR REMOVAL
(1) Raise vehicle and turn wheel outward for eas-
ier access to sensor.
(2) Remove sensor wire from mounting brackets.
(3) Clean sensor and surrounding area before removal.
(4) Remove bolt attaching sensor to steering
knuckle and remove sensor.
(5) Unseat grommet retaining sensor wire in wheel
house panel.
(6) In engine compartment, disconnect sensor wire con-
nector at harness plug. Then remove sensor and wire.
FRONT WHEEL SENSOR INSTALLATION
(1) Apply Mopar Lock N' Seal or Loctite 242 to
bolt that attaches sensor to steering knuckle. Use
new sensor bolt if original bolt is worn or damaged.
(2) Position sensor on steering knuckle. Seat sen-
sor locating tab in hole in knuckle and install sensor
attaching bolt finger tight.
(3) Tighten sensor bolt to 14 NIm (11 ft. lbs.) torque.
(4) Attach sensor wire to steering knuckle bracket
with grommets on sensor wire.
(5) Route sensor wire forward and behind shock
absorber. Then attach sensor wire to spring seat
bracket with grommets on sensor wire.
(6) Route sensor wire to outer sill bracket. Remove
all twists or kinks from wire.(7) Attach sensor wire to sill bracket with grom-
met. Be sure wire is free of twists and kinks.
(8) Verify sensor wire routing. Wire should loop
forward and above sill bracket. Loose end of wire
should be below sill bracket and towards brake hose.
(9) Seat sensor wire grommet in body panel and
clip wire to brake line at grommet location.
(10) Connect sensor wire to harness in engine com-
partment.
REAR WHEEL SENSOR REMOVAL
(1) On XJ models, if separate connectors are not
used to attach sensor harness to each sensor wire,
proceed as follows:
(a) Raise and fold rear seat forward for access to
rear sensor connectors (Figs. 4 and 5).
(b) Disconnect sensors at rear harness connectors.
(c) Push sensor grommets and sensor wires
through floorpan.
Fig. 4 Acceleration Switch And Rear Sensor
Connections (XJ)
Fig. 5 Rear Sensor Connections (XJ)
JABS COMPONENT SERVICE 5 - 49

(2) Raise vehicle.
(3) Disconnect sensor wires at rear axle connectors.
(4) Remove wheel and tire assembly.
(5) Remove brake drum.
(6) Remove clips securing sensor wires to brake
lines or rear axle and rear brake hose.
(7) Unseat sensor support plate grommet.
(8) Remove bolt attaching sensor to bracket and
remove sensor.
REAR WHEEL SENSOR INSTALLATION
(1) Insert sensor wire through support plate hole
and seat sensor grommet in support plate.
(2) Apply Mopar Lock N' Seal or Loctite 242 to
original sensor bolt. Use new bolt if original is worn
or damaged.
(3) Install sensor bolt finger tight only at this
time.
(4) Set sensor air gap as follows:
(a) Iforiginal sensoris being installed, remove
any remaining pieces of cardboard spacer from sen-
sor pickup face. Then adjust air gap to preferred
setting of 1.1 mm (0.043 in.) with brass feeler
gauge (Fig. 6). Tighten sensor bolt to 11 Nzm (11 ft.
lbs.) torque.
(b) Ifnew sensoris being installed, push card-
board spacer on sensor face (Fig. 7) against tone
ring. Then tighten sensor bolt to 8 Nzm (6 ft. lbs.)
torque. Correct air gap will be established as tone
ring rotates and peels spacer off sensor face.
(c) Verify sensor air gap adjustment. If adjust-
ment changed after tightening bolt, readjust sensor
air gap as needed.
(5) On YJ, connect rear sensor wires to connectors
at axle. On XJ, route sensor wires to rear seat area.
(6) Feed sensor wires through floorpan access hole
and seat sensor grommets in floorpan.
(7) Verify that rear sensor wire are secured to rear
brake hose and axle with clips. Verify that wire is
clear of rotating components.
(8) Install brake drum and wheel.
(9) Lower vehicle.
(10) On XJ, connect sensor wire to harness connec-
tor. Then reposition carpet and fold rear seat down.
MASTER CYLINDER REMOVAL
(1) Disconnect pedal travel sensor wires.
(2) Remove air cleaner and hoses on XJ models.
(3) Remove clamps that secure reservoir hoses to
HCU pipes.
(4) Position small drain container under master
cylinder reservoir. Remove reservoir hoses from HCU
pipes and allow fluid to drain into container before
removing reservoir. Discard fluid drained from reser-
voir.
(5) Pump brake pedal to exhaust all vacuum from
power brake booster.
(6) Disconnect necessary brakelines at master cyl-
inder and combination valve. Also remove combina-
tion valve bracket bolt.
(7) Remove nuts attaching master cylinder to
booster mounting studs.
(8) Remove master cylinder. Pull cylinder forward
and off studs. Then work cylinder past combination
valve, brakelines, pedal travel sensor and out of en-
gine compartment.
MASTER CYLINDER INSTALLATION
(1) If new master cylinder is being installed, bleed
cylinder on bench before installing it in vehicle.
(2) Work master cylinder into position and install
it in booster. Be sure cylinder is properly seated on
booster studs. Also be sure booster-to-cylinder seal is
not displaced during installation.
(3) Connect reservoir hoses to HCU pipes.
(4) Verify that master cylinder and booster are
properly connected.
(5) Install and tighten master cylinder attaching
nuts to 34 Nzm (25 ft. lbs.) torque.
(6) Connect brakelines to master cylinder.
(7) Install combination valve, if removed and in-
stall bolt that secures valve bracket to master cylin-
der.
(8) Connect sensor wires.
Fig. 6 Setting Air Gap On Original Rear Sensor
Fig. 7 New Rear Sensor With Air Gap Spacer
5 - 50 ABS COMPONENT SERVICEJ

(9) Fill reservoir and bleed brakes. Refer to proce-
dure in this section.
(10) Install air cleaner and hoses.
PEDAL TRAVEL SENSOR SERVICE
CAUTION: The pedal travel sensor and booster
must form a matched set. The cap on the sensor
plunger and booster shell are color coded for iden-
tification, and to ensure they are used as matched
sets. Be sure the color of the sensor cap and the
color dot on the booster shell are the same before
installation. Refer to the Sensor Replacement infor-
mation before installing a new or original sensor.
PEDAL TRAVEL SENSOR REMOVAL
(1) Disconnect wires at sensor.
(2) Pump brake pedal to exhaust all vacuum from
booster.
(3) Unseat sensor retaining ring.
(4) Remove sensor from booster (Fig. 8).
PEDAL TRAVEL SENSOR REPLACEMENT
A new pedal travel sensor is supplied with four dif-
ferent color caps. The caps are color coded to ease
matching them with the color code dot on the booster
shell.
Compare the color of the new sensor cap and the
color dot on the booster shell. If the colors match,
proceed with sensor installation. However, if the col-
orsdo not match, select and install the correct color
cap on the sensor plunger before proceeding.
PEDAL TRAVEL SENSOR INSTALLATION
(1) Check color dot on face of brake booster. Then
check color of cap on sensor plunger. If colors match,
proceed with installation. If colors do not match, in-
stall correct color cap on end of plunger.
(2) Install O-ring on sensor.
(3) Install sensor retaining ring on booster flange.
(4) Insert sensor in retaining ring and booster.
(5) Verify that retaining ring is properly engaged
in sensor and that sensor is seated in booster.
(6) Connect wires to sensor.
(7) Check sensor operation with DRB II scan tool.
POWER BRAKE BOOSTER REMOVAL
(1) Pump brake pedal until all vacuum is ex-
hausted from power brake booster.
(2) Disconnect pedal travel sensor.
(3) Remove air cleaner and hoses.
(4) Remove clamps that secure reservoir hoses to
HCU pipes. Then remove hoses from pipes.
(5) Disconnect brakelines at master cylinder.
(6) Remove combination valve bracket bolt if nec-
essary.
(7) Remove nuts attaching master cylinder to
booster mounting studs.(8) Remove master cylinder. Pull cylinder forward
and off studs. Then work cylinder past combination
valve, brakelines, pedal travel sensor and out of en-
gine compartment.
(9) Disconnect vacuum hose at booster check valve.
(10) Disconnect booster push rod from brake pedal.
(11) Remove nuts attaching booster to passenger
compartment side of dash panel.
(12) Slide booster forward and work it out of en-
gine compartment.
POWER BRAKE BOOSTER INSTALLATION
(1) Position booster on dash panel. Align booster
mounting studs with holes in panel and seat booster
(Fig. 9).
(2) In passenger compartment, install booster at-
taching nuts on mounting studs. Tighten attaching
nuts to 41 Nzm (30 ft. lbs.) torque.
(3) Install seal on master cylinder. Seal is slight
interference fit to help hold it in place.
Fig. 8 Pedal Travel Sensor Mounting
Fig. 9 Booster Attachment At Dash PanelÐYJ
Shown
JABS COMPONENT SERVICE 5 - 51

CAUTION: The pedal travel sensor and booster
must form a matched set. The cap on the sensor
plunger and the color dot on the booster shell are
color coded for identification, and to ensure they
are used as matched sets. Be sure the color on the
sensor cap and booster shell are the same before
installation. Refer to the Sensor Replacement infor-
mation before installing a new or original sensor.
(4) If new pedal travel sensor is being installed in
original booster, compare color dot on booster shell with
color of cap on sensor (Fig. 10). If both are same color,
proceed with installation. However, if colorsdo not
match, select correct color cap from kit supplied with
new sensor and install it on end of sensor plunger.
(5) Install O-ring on pedal travel sensor.
(6) Install sensor retaining ring on booster flange.
Be sure retaining ring is firmly seated.
(7) Insert sensor into booster. Be sure sensor is
fully seated and engaged in retaining ring.
(8) Connect booster push rod to brake pedal.
(9) Attach vacuum hose to booster check valve.
(10) Install master cylinder on booster. Tighten
cylinder attaching nuts to 25-30 Nzm (220-267 in.
lbs.) torque.
(11) Connect brakelines to master cylinder.
(12) Install combination valve bracket bolt if re-
moved.
(13) Connect reservoir hoses to HCU pipes.
(14) Connect sensor wires.
(15) Bleed brakes. Refer to procedure in this section.
(16) Install air cleaner and hoses.
ACCELERATION SENSOR REMOVAL
(1) On XJ models, tilt rear seat assembly forward
for access to sensor (Fig. 11).
(2) On YJ models, move driver seat forward or
rearward for access to sensor and mounting bracket
(Fig. 11).
(3) Disconnect sensor harness (Fig. 10).
(4) On XJ models, remove screws attaching sensor
to bracket. Then remove sensor.
(5) On YJ models, remove screws attaching sensor
bracket to floorpan. Then remove sensor from bracket.
ACCELERATION SENSOR INSTALLATION
(1) Note position of locating arrow on sensor. Sen-
sor must be positioned so arrow faces forward.
CAUTION: The sensor mercury switch will not func-
tion properly if the sensor is mispositioned. Verify
that the sensor locating arrow is pointing to the
front of the vehicle.
(2) Position sensor in mounting bracket (Fig. 10).
(3) Install and tighten sensor attaching screws to
2-4 Nzm (17-32 in. lbs.) torque.
(4) Connect harness to sensor. Be sure harness
connecter is firmly seated.
(5) Move seat back to normal position.
Fig. 10 Booster Push Rod And Pedal Travel Sensor
Fig. 11 Acceleration Sensor Mounting
5 - 52 ABS COMPONENT SERVICEJ

ECU REPLACEMENTÐXJ
ECU Removal
(1) Turn ignition key to Off position.
(2) Remove screws attaching ECU to mounting
bracket (Fig. 12).
(3) Disconnect ECU wiring harness.
(4) Remove ECU.
ECU Installation
(1) If new ECU is being installed, transfer mount-
ing bracket to new ECU.
(2) Tighten ECU-to-mounting bracket screws to
8-13 Nzm (75-115 in. lbs.) torque.
(3) Connect wire harness to ECU.
(4) Position and install ECU.
(5) Tighten ECU attaching nuts to 10-14 Nzm (85-
125 in. lbs.) torque.
ECU REPLACEMENTÐYJ
The antilock electronic control unit (ECU) is attached
to the dash panel inside the passenger compartment. It
is positioned just above the heater/air conditioning ple-
num housing, in line with the glove box (Fig. 13).
The ECU is attached to the dash panel by bolts
and nuts that are accessible from the engine com-
partment. The fasteners are located just to the right
of the battery.
On models with air conditioning, it will be neces-
sary to remove the air conditioning fascia panel and
ducts for access to the ECU and harness connecter.
HCU REMOVALÐXJ
(1) Remove air cleaner.
(2) Remove clamp that secures air cleaner hose
and pipe to fender apron (Fig. 14).(3) Position suitable size fluid drain container un-
der master cylinder reservoir hoses. Disconnect reser-
voir hoses from HCU and drain fluid into container.
Discard old fluid and remove reservoir.
(4) Disconnect pump motor and solenoid harness
wires at HCU (Fig. 15).
Fig. 12 Antilock ECU Mounting (XJ)
Fig. 13 ECU Location (YJ)
Fig. 14 Air Cleaner And Harness Connector Location (XJ)
Fig. 15 HCU Wire Harness Connections
JABS COMPONENT SERVICE 5 - 53