Control JEEP GRAND CHEROKEE 2002 WJ / 2.G Owner's Manual

and therefore creates pressure in the pump. The tun-
ing of the front and rear axle orifices and valves
inside the gerotor pump is unique and each system
includes a torque-limiting pressure relief valve to
protect the clutch pack, which also facilitates vehicle
control under extreme side-to-side traction varia-
tions. The resulting pressure is applied to the clutch
pack and the transfer of torque is completed.
Under conditions in which opposite wheels are on
surfaces with widely different friction characteristics,
Vari-loktdelivers far more torque to the wheel on
the higher traction surface than do conventional
Trac-loktsystems. Because conventional Trac-lokt
differentials are initially pre-loaded to assure torque
transfer, normal driving (where inner and outer
wheel speeds differ during cornering, etc.) produces
torque transfer during even slight side-to-side speed
variations. Since these devices rely on friction from
this preload to transfer torque, normal use tends to
cause wear that reduces the ability of the differential
to transfer torque over time. By design, the Vari-lokt
system is less subject to wear, remaining more con-
sistent over time in its ability to transfer torque. The
coupling assembly is serviced as a unit. From a ser-
vice standpoint the coupling also benefits from using
the same lubricant supply as the ring and pinion
gears.
DIAGNOSIS AND TESTING
GEAR NOISE
Axle gear noise can be caused by insufficient lubri-
cant, incorrect backlash, tooth contact, worn/damaged
gears, or the carrier housing not having the proper
offset and squareness.
Gear noise usually happens at a specific speed
range. The noise can also occur during a specific type
of driving condition. These conditions are accelera-
tion, deceleration, coast, or constant load.
When road testing, first warm-up the axle fluid by
driving the vehicle at least 5 miles and then acceler-
ate the vehicle to the speed range where the noise is
the greatest. Shift out-of-gear and coast through the
peak-noise range. If the noise stops or changes
greatly:
²Check for insufficient lubricant.
²Incorrect ring gear backlash.
²Gear damage.
Differential side gears and pinions can be checked
by turning the vehicle. They usually do not cause
noise during straight-ahead driving when the gears
are unloaded. The side gears are loaded during vehi-
cle turns. A worn pinion mate shaft can also cause a
snapping or a knocking noise.
BEARING NOISE
The axle shaft, differential and pinion bearings can
all produce noise when worn or damaged. Bearing
noise can be either a whining, or a growling sound.
Pinion bearings have a constant-pitch noise. This
noise changes only with vehicle speed. Pinion bearing
noise will be higher pitched because it rotates at a
faster rate. Drive the vehicle and load the differen-
tial. If bearing noise occurs, the rear pinion bearing
is the source of the noise. If the bearing noise is
heard during a coast, the front pinion bearing is the
source.
Worn or damaged differential bearings usually pro-
duce a low pitch noise. Differential bearing noise is
similar to pinion bearing noise. The pitch of differen-
tial bearing noise is also constant and varies only
with vehicle speed.
Axle shaft bearings produce noise and vibration
when worn or damaged. The noise generally changes
when the bearings are loaded. Road test the vehicle.
Turn the vehicle sharply to the left and to the right.
This will load the bearings and change the noise
level. Where axle bearing damage is slight, the noise
is usually not noticeable at speeds above 30 mph.
LOW SPEED KNOCK
Low speed knock is generally caused by a worn
U-joint or by worn side±gear thrust washers. A worn
pinion shaft bore will also cause low speed knock.
VIBRATION
Vibration at the rear of the vehicle is usually
caused by a:
²Damaged drive shaft.
²Missing drive shaft balance weight(s).
²Worn or out-of-balance wheels.
²Loose wheel lug nuts.
²Worn U-joint(s).
²Loose/broken springs.
²Damaged axle shaft bearing(s).
²Loose pinion gear nut.
²Excessive pinion yoke run out.
²Bent axle shaft(s).
Check for loose or damaged front-end components
or engine/transmission mounts. These components
can contribute to what appears to be a rearend vibra-
tion. Do not overlook engine accessories, brackets
and drive belts.
All driveline components should be examined
before starting any repair.
(Refer to 22 - TIRES/WHEELS - DIAGNOSIS AND
TESTING)
3 - 92 REAR AXLE - 226RBAWJ
REAR AXLE - 226RBA (Continued)

(7) Press metal retaining ring onto axle shaft with
Installer 7913 and a press (Fig. 30).
(8) Install axle in vehicle.
PINION SEAL
REMOVAL
(1) Raise and support the vehicle.
(2) Remove wheel and tire assemblies.
(3) Remove rear brake calipers and rotors.
(4) Mark propeller shaft and pinion yoke for
installation reference.
(5) Remove the propeller shaft from the yoke.
(6) Rotate pinion gear three or four times and ver-
ify that pinion rotates smoothly.
(7) Record torque necessary to rotate the pinion
gear with a inch pound dial-type torque wrench.
(8) Using a short piece of pipe and Spanner
Wrench 6958 to hold the pinion yoke and remove pin-
ion nut and washer (Fig. 31).
(9) Remove pinion companion flange with Remover
C-452 and Flange Wrench C-3281. (Fig. 32)
Fig. 30 BEARING RETAINING RING
1 - PRESS
2 - AXLE
3 - AXLE BEARING
4 - INSTALLER
5 - METAL RETAINING RING
Fig. 31 Pinion Yoke Holder
1 - PIPE
2 - PINION YOKE
3 - SPANNER WRENCH
4 - LOWER CONTROL ARM
Fig. 32 Pinion Yoke Remover
1 - FLANGE WRENCH
2 - YOKE
3 - YOKE PULLER
WJREAR AXLE - 226RBA 3 - 111
AXLE BEARINGS/SEALS (Continued)

(5) If rotating torque is low, use Wrench 6958 to
hold the pinion yoke (Fig. 37) and tighten the pinion
shaft nut in 6.8 N´m (5 ft. lbs.) increments until
rotating torque is achieved.
CAUTION: If the maximum tightening torque is
reached prior to reaching the required rotating
torque, the collapsible spacer may have been dam-
aged. Replace the collapsible spacer.
(6) Install propeller shaft with reference marks
aligned.
(7) Fill differential with gear lubricant.
(8) Install the brake rotors and calipers.
(9) Install wheel and tire assemblies.
(10) Lower the vehicle.
COLLAPSIBLE SPACER
REMOVAL
(1) Raise and support the vehicle.
(2) Remove wheel and tire assemblies.
(3) Remove rear brake calipers and rotors.
(4) Mark propeller shaft and pinion yoke for
installation reference and remove propeller shaft.
(5) Rotate pinion gear a minimum of ten times and
verify pinion rotates smoothly.
(6) Record rotate torque of the pinion gear, with an
inch pound torque wrench.
(7) Hold pinion yoke with Spanner Wrench 6958
and remove pinion nut and washer (Fig. 38).
(8) Remove pinion yoke with Remover C-452 and
Wrench C-3281 (Fig. 39).
Fig. 37 PINION SHAFT NUT
1 - SPANNER WRENCH
2 - PIPE
3 - TORQUE WRENCH
Fig. 38 PINION YOKE HOLDER
1 - 1 in. PIPE
2 - PINION YOKE
3 - SPANNER WRENCH
4 - LOWER CONTROL ARM
Fig. 39 PINION YOKE PULLER
1 - WRENCH
2 - PINION YOKE
3 - PULLER
WJREAR AXLE - 226RBA 3 - 113
PINION SEAL (Continued)

CAUTION: Vari-lokTplenum must be seated against
the differential case prior to installing the ring gear
side differential bearing.
(1) Install differential side bearings with Installer
C-4340 and Handle C-4171 (Fig. 70).
(2) Install differential case into the housing.
(3) Remove support and lower vehicle.
PINION GEAR/RING GEAR
REMOVAL
NOTE: The ring and pinion gears are serviced as a
matched set. Never replace one gear without the
other matched gear.
(1) Raise and support vehicle.
(2) Mark pinion yoke and propeller shaft for
installation reference.
(3) Disconnect propeller shaft from pinion yoke
and tie shaft to underbody.
(4) Remove differential from axle housing.
(5) Place differential case in a vise with soft metal
jaw.
(6) Remove bolts holding ring gear to differential
case.(7) Drive ring gear from differential case with a
rawhide hammer (Fig. 71).
(8) Hold the pinion yoke with Spanner Wrench
6958 and remove the pinion yoke nut and washer
(Fig. 72).
Fig. 70 Install Differential Side Bearings
1 - HANDLE
2 - DIFFERENTIAL
3 - BEARING
4 - INSTALLER
Fig. 71 RING GEAR
1 - DIFFERENTIAL CASE
2 - RING GEAR
3 - RAWHIDE HAMMER
Fig. 72 Pinion Yoke Holder
1 - PIPE
2 - PINION YOKE
3 - SPANNER WRENCH
4 - LOWER CONTROL ARM
3 - 124 REAR AXLE - 226RBAWJ
DIFFERENTIAL CASE BEARINGS (Continued)

INSTALLATION.........................23
MASTER CYLINDER
DESCRIPTION.........................23
OPERATION...........................24
DIAGNOSIS AND TESTING - MASTER
CYLINDER/POWER BOOSTER...........24
STANDARD PROCEDURE - MASTER
CYLINDER BLEEDING PROCEDURE......25
REMOVAL.............................25
INSTALLATION.........................25
PEDAL
DESCRIPTION
DESCRIPTION - STANDARD PEDAL.......25
DESCRIPTION - ADJUSTABLE PEDALS....25
OPERATION...........................26
REMOVAL
REMOVAL - NON-ADJUSTABLE PEDAL....26
REMOVAL - ADJUSTABLE PEDALS........27
INSTALLATION
INSTALLATION - NON-ADJUSTABLE PEDAL . 28
INSTALLATION - ADJUSTABLE PEDALS....28
PEDAL MOTOR
REMOVAL.............................28
INSTALLATION.........................28
POWER BRAKE BOOSTER
DESCRIPTION.........................29
OPERATION...........................29
REMOVAL.............................31
INSTALLATION.........................31
ROTORS
DIAGNOSIS AND TESTING
DIAGNOSIS AND TESTING - FRONT DISC
BRAKE ROTOR.......................31DIAGNOSIS AND TESTING - REAR DISC
BRAKE ROTOR.......................32
STANDARD PROCEDURE - DISC ROTOR
MACHINING..........................33
REMOVAL
REMOVAL - FRONT DISC BRAKE ROTOR . . 33
REMOVAL - REAR DISC BRAKE ROTOR . . . 33
INSTALLATION
INSTALLATION - FRONT DISC BRAKE
ROTOR .............................34
INSTALLATION - REAR DISC BRAKE
ROTOR .............................34
PARKING BRAKE
OPERATION...........................34
DIAGNOSIS AND TESTING - PARKING BRAKE . 34
CABLES
REMOVAL
REMOVAL - FRONT PARKING BRAKE
CABLE..............................35
REMOVAL - REAR PARKING BRAKE
CABLES............................36
INSTALLATION
INSTALLATION - FRONT PARKING BRAKE
CABLE..............................37
INSTALLATION - REAR PARKING BRAKE
CABLES............................37
LEVER
REMOVAL.............................38
INSTALLATION.........................39
SHOES
REMOVAL.............................39
INSTALLATION.........................39
ADJUSTMENTS - PARKING BRAKE SHOE....40
BRAKES - BASE
DESCRIPTION
Dual piston disc brake calipers are used on the
front. Single piston disc brake calipers are used on
the rear. Ventilated disc brake rotors are used on the
front and solid rotors are used on the rear.
Power brake assist is supplied by a vacuum oper-
ated, dual diaphragm power brake booster. The mas-
ter cylinder used for all applications has an
aluminum body and nylon reservoir with single filler
cap. A fluid level indicator is mounted to the side of
the reservoir.
The braking force of the rear wheels is controlled
by electronic brake distribution (EBD). The EBD
functions like a rear proportioning valve. The EBD
system uses the ABS system to control the slip of the
rear wheels in partial braking range. The braking
force of the rear wheels is controlled electronically by
using the inlet and outlet valves located in the HCU.
Factory installed brake linings on all models con-
sists of organic base material combined with metallic
particles.
DIAGNOSIS AND TESTING - BASE BRAKE
SYSTEM
Base brake components consist of the brake shoes,
calipers, rear park brake drums/rotors, front brake
rotors, brake lines, master cylinder, booster, HCU
and parking brake shoes.
Brake diagnosis involves determining if the prob-
lem is related to a mechanical, hydraulic, electrical
or vacuum operated component.
The first diagnosis step is the preliminary check.
PRELIMINARY BRAKE CHECK
(1) Check condition of tires and wheels. Damaged
wheels and worn, damaged, or underinflated tires
can cause pull, shudder, vibration, and a condition
similar to grab.
5 - 2 BRAKES - BASEWJ

BRAKE PADS / SHOES
DESCRIPTION
DESCRIPTION - FRONT DISC BRAKE SHOES
The calipers are twin piston type. The calipers are
free to slide laterally on the anchor, this allows con-
tinuous compensation for lining wear.
DESCRIPTION - REAR DISC BRAKE SHOES
The rear disc brakes consist of single piston float-
ing-type calipers and solid rotors. The rear caliper is
mounted on an anchor attached to an adapter
attached the rear axle tube flange. The anchors are
secured to the adapters with mounting bolts. The
disc brake rotor splash shield is part of the adaptor.
The disc brake rotor has a built in brake drum used
for the parking brakes (Fig. 6). The parking brake
shoes are mounted to the adaptor.
OPERATION
OPERATION - FRONT DISC BRAKE SHOES
When the brakes are applied fluid pressure is
exerted against the caliper pistons. The fluid pres-
sure is exerted equally and in all directions. This
means pressure exerted against the caliper pistons
and within the caliper bores will be equal (Fig. 7).
Fluid pressure applied to the pistons is transmit-
ted directly to the inboard brake shoe. This forces the
shoe lining against the inner surface of the disc
brake rotor. At the same time, fluid pressure within
the piston bores forces the caliper to slide inward on
the slide pins. This action brings the outboard brake
shoe lining into contact with the outer surface of the
disc brake rotor.
Fluid pressure acting simultaneously on the pis-
tons and caliper to produces a strong clamping
action. When sufficient force is applied, friction will
stop the rotors from turning and bring the vehicle to
a stop.Application and release of the brake pedal gener-
ates only a very slight movement of the caliper and
pistons. Upon release of the pedal, the caliper and
pistons return to a rest position. The brake shoes do
not retract an appreciable distance from the rotor. In
fact, clearance is usually at, or close to zero. The rea-
sons for this are to keep road debris from getting
between the rotor and lining and in wiping the rotor
surface clear each revolution.
The caliper piston seals control the amount of pis-
ton extension needed to compensate for normal lining
wear.
During brake application, the seals are deflected
outward by fluid pressure and piston movement (Fig.
8). When the brakes (and fluid pressure) are
released, the seals relax and retract the pistons.
The front outboard brake shoes have wear indica-
tors.
OPERATION - REAR DISC BRAKE SHOES
When the brakes are applied fluid pressure is
exerted against the caliper pistons. The fluid pres-
sure is exerted equally and in all directions. This
means pressure exerted against the caliper pistons
and within the caliper bores will be equal (Fig. 7).
Fluid pressure applied to the pistons is transmit-
ted directly to the inboard brake shoe. This forces the
Fig. 6 Rear Disc Brake Rotor
1 - PARKING BRAKE DRUM SURFACE
2 - REAR DISC BRAKE ROTOR
Fig. 7 Brake Caliper Operation
1 - CALIPER
2 - PISTON
3 - PISTON BORE
4 - SEAL
5 - INBOARD SHOE
6 - OUTBOARD SHOE
5 - 10 BRAKES - BASEWJ

shoe lining against the inner surface of the disc
brake rotor. At the same time, fluid pressure within
the piston bores forces the caliper to slide inward on
the slide pins. This action brings the outboard brake
shoe lining into contact with the outer surface of the
disc brake rotor.
Fluid pressure acting simultaneously on the pis-
tons and caliper to produces a strong clamping
action. When sufficient force is applied, friction will
stop the rotors from turning and bring the vehicle to
a stop.
Application and release of the brake pedal gener-
ates only a very slight movement of the caliper and
pistons. Upon release of the pedal, the caliper and
pistons return to a rest position. The brake shoes do
not retract an appreciable distance from the rotor. In
fact, clearance is usually at, or close to zero. The rea-
sons for this are to keep road debris from getting
between the rotor and lining and in wiping the rotor
surface clear each revolution.
The caliper piston seals control the amount of pis-
ton extension needed to compensate for normal lining
wear.
During brake application, the seals are deflected
outward by fluid pressure and piston movement (Fig.
8). When the brakes (and fluid pressure) are
released, the seals relax and retract the pistons.
The front outboard brake shoes have wear indica-
tors.
REMOVAL
REMOVAL- FRONT DISC BRAKE SHOES
(1) Raise and support vehicle.
(2) Remove wheel and tire assembly.(3) Drain small amount of fluid from master cylin-
der brake reservoir withcleansuction gun.
(4) Bottom caliper pistons into the caliper by pry-
ing the caliper over (Fig. 9).
(5) Remove the caliper support spring by prying
the spring out of the caliper (Fig. 10).
(6) Remove the caliper slide pin bushing caps and
remove the slide pins (Fig. 11).
(7) Remove caliper from the anchor.
Fig. 8 Lining Wear Compensation By Piston Seal
1 - PISTON
2 - CYLINDER BORE
3 - PISTON SEAL BRAKE PRESSURE OFF
4 - CALIPER HOUSING
5 - DUST BOOT
6 - PISTON SEAL BRAKE PRESSURE ON
Fig. 9 Bottoming Caliper Piston
1 - ROTOR
2 - CALIPER
Fig. 10 Caliper Support Spring
1 - SUPPORT SPRING
2 - CALIPER
WJBRAKES - BASE 5 - 11
BRAKE PADS / SHOES (Continued)

tions also allows the driver to set steering wheel tilt
and seat position to the most comfortable position.
The position of the brake and accelerator pedals can
be adjusted without compromising safety or comfort
in actuating the pedals. Repositioning the pedals
does not change the effort required for actuation.
Change of pedal position is accomplished by means
of a motor driven screw. Operating the adjustable
pedal switch activates the pedal drive motor. The
pedal drive motor turns a screw that changes the
position of the brake and accelerator pedals. The
pedal can be moved rearward (closer to the driver) or
forward (away from driver). The brake pedal is
moved on its drive screw to a position where the
driver feels most comfortable (Fig. 52).
The accelerator pedal is moved at the same time
and the same distance as the brake pedal. The accel-
erator pedal adjustment screw is turned by a flexible
shaft slaved off the brake adjustment screw.
Neither the pedal drive motor nor drive mecha-
nism are subject to the mechanical stress of brake or
accelerator application.
²SYSTEM FEATURES:
²Range of Adjustment: The pedals may be
adjusted up to 3 in. (75 mm)
²Pedal Adjustment Speed: 0.5 in./sec (12.5
mm/sec)
²Pedal Adjustment Inhibitors: Pedal adjust-
ment is inhibited when the vehicle is in reverse or
when cruise control is activated.
²Memory: An optional memory feature is avail-
able. This allows storing of one or two preferred
pedal positions in the Adjustable Pedal Module
(APM). A preferred position can be stored and
recalled using the door-mounted switches. A stored
pedal position can be recalled (but not stored)
using the Remote Keyless Entry (RKE).
²
Adjustable Pedal Feedback Message: The Elec-
tronic Vehicle Information Center (EVIC) will display
a message when the APS is disabled. ie:9Adjustable
Pedal Disabled - Cruise Control Engaged9or9Adjust-
able Pedal Disabled - Vehicle in Reverse9.
²Damage Prevention: Foot pressure or debris
can stall pedal adjustment. In order to avoid dam-
age to system components during pedal adjust-
ment, the APM will monitor pedal position sensor
voltage. If the APM does not detect expected volt-
age change within 1.5 seconds, it will cut power to
the adjustable pedal motor.
OPERATION
The brake pedal is attached to the booster push
rod. When the pedal is depressed, the primary
booster push rod is depressed which moves the
booster secondary rod. The booster secondary rod
depresses the master cylinder piston.
REMOVAL
REMOVAL - NON-ADJUSTABLE PEDAL
(1) Remove retainer clip that holds booster to
pedal pin (Fig. 53).
Fig. 52 ADJUSTABLE PEDALS ASSEMBLY
1 - HARNESS
2 - ADJUSTABLE PEDAL BRACKET
3 - CABLE
4 - ACCELERATOR PEDAL
5 - BRAKE PEDAL
6 - ADJUSTABLE PEDAL MOTOR
7 - BRAKE LIGHT SWITCH
8 - ADJUSTABLE PEDALS MODULE
Fig. 53 Push Rod Retainer Clip
1 - RETAINER CLIP
2 - PUSH ROD
3 - PEDAL PIN
5 - 26 BRAKES - BASEWJ
PEDAL (Continued)

INSTALLATION
INSTALLATION - FRONT DISC BRAKE ROTOR
NOTE: If a new rotor is installed it must be match
mounted to the hub/bearing.
(1) Install rotor on hub studs in its original loca-
tion.
(2) Install the caliper anchor assembly on the
knuckle. Install anchor bolts and tighten to 90-115
N´m (66-85 ft. lbs.).
(3) Install wheel and tire assembly.
(4) Remove support and lower the vehicle.
(5) Pump brake pedal to seat caliper pistons and
brake shoes. Do not move vehicle until firm brake
pedal is obtained.
INSTALLATION - REAR DISC BRAKE ROTOR
(1) Install rotor on axle studs.
(2) Install the caliper anchor assembly.
(3) Install anchor bolts and tighten to 90-115 N´m
(66-85 ft. lbs.).
(4) Install wheel and tire assembly.
(5) Remove support and lower the vehicle.
(6) Pump brake pedal until caliper pistons and
brake shoes are seated.
PARKING BRAKE
OPERATION
The parking brakes operated by a automatic ten-
sioner mechanism built into the hand lever and cable
system. The front cable is connected to the hand
lever and the equalizer. The rear cables attached to
the equalizer and the parking brake shoe actuator.
A set of drum type brake shoes are used for park-
ing brakes. The shoes are mounted to the rear disc
brake adaptor. The parking brake drum is integrated
into the rear disc brake rotor.
Parking brake cable adjustment is controlled by an
automatic tensioner mechanism. The only adjust-
ment if necessary is to the park brake shoes if the
linings are worn.
DIAGNOSIS AND TESTING - PARKING BRAKE
NOTE: Parking brake adjustment is controlled by an
automatic cable tensioner and does not require
adjustment. The only adjustment that may be nec-
essary would be to the park brake shoes if they are
worn.
The parking brake switch is in circuit with the red
warning lamp in the dash. The switch will cause the
lamp to illuminate only when the parking brakes are
applied. If the lamp remains on after parking brake
release, the switch or wires are faulty.
If the red lamp comes on a fault has occurred in
the front or rear brake hydraulic system.
If the red warning lamp and yellow warning lamp
come on, the electronic brake distribution may be at
fault.
In most cases, the actual cause of an improperly
functioning parking brake (too loose/too tight/won't
hold), can be traced to a parking brake component.
NOTE: The leading cause of improper parking brake
operation, is excessive clearance between the park-
ing brake shoes and the shoe braking surface.
Excessive clearance is a result of lining and/or
drum wear, drum surface machined oversize.
Excessive parking brake lever travel (sometimes
described as a loose lever or too loose condition), is
the result of worn brake shoes, improper brake shoe
adjustment, or improperly assembled brake parts.
A too loose condition can also be caused by inoper-
ative or improperly assembled parking brake shoe
parts.
A condition where the parking brakes do not hold,
will most probably be due to a wheel brake compo-
nent.
Items to look for when diagnosing a parking brake
problem, are:
²Brake shoe wear
²Drum surface (in rear rotor) machined oversize
²Front cable not secured to lever
²Rear cable not attached to actuator
²Rear cable seized
²Parking brake lever not seated
²Parking brake lever bind
5 - 34 BRAKES - BASEWJ
ROTORS (Continued)

BRAKES - ABS
TABLE OF CONTENTS
page page
BRAKES - ABS
DESCRIPTION.........................41
OPERATION...........................41
DIAGNOSIS AND TESTING - ANTILOCK
BRAKES............................42
STANDARD PROCEDURE - BLEEDING ABS
BRAKE SYSTEM......................42
SPECIFICATIONS
TORQUE CHART......................42
ELECTRIC BRAKE
DESCRIPTION.........................43
OPERATION...........................43
FRONT WHEEL SPEED SENSOR
DESCRIPTION.........................43
OPERATION...........................43
REMOVAL.............................43INSTALLATION.........................43
G-SWITCH
DESCRIPTION.........................44
OPERATION...........................44
REMOVAL.............................44
INSTALLATION.........................44
REAR WHEEL SPEED SENSOR
DESCRIPTION.........................45
OPERATION...........................45
REMOVAL.............................45
INSTALLATION.........................46
HCU (HYDRAULIC CONTROL UNIT)
DESCRIPTION.........................46
OPERATION...........................46
REMOVAL.............................47
INSTALLATION.........................47
BRAKES - ABS
DESCRIPTION
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 hydraulic system is a three channel design.
The front brakes are controlled individually and the
rear brakes in tandem.
The ABS electrical system is separate from other
vehicle electrical circuits. A separate controller oper-
ates the system.
OPERATION
The antilock CAB 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 20 to 30 percent of
actual vehicle speed during braking. Periods of high
wheel slip occur when brake stops involve high pedal
pressure and rate of vehicle deceleration.
Battery voltage is supplied to the CAB ignition ter-
minal when the ignition switch is turned to Run posi-
tion. The CAB 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 30kph (18 mph). During the dynamic check, the CAB
briefly cycles the pump and solenoids to verify oper-
ation.
If an ABS component exhibits a fault during ini-
tialization, the CAB illuminates the amber warning
light and registers a fault code in the microprocessor
memory.
ANTILOCK BRAKING
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 CAB 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. A speed sensor input
signal indicating a high slip condition activates the
CAB 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
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.
WJBRAKES - ABS 5 - 41