1994 JEEP CHEROKEE lock

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

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

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

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