Pedal Feel/Vehicles Characteristics; System Components; Abs And Red Brake Warning Indicator; Controller Antilock Brake (Cab) - Jeep KJ Liberty Cherokee 2003 Service Manual

GENERAL INFORMATION
wheel is locked. The antilock system maintains an
average of approximately 20% wheel slip.
It is important to remember that the antilock
brake system does not shorten the vehicle stopping
distance under all driving conditions, but provides
improved control of the vehicle while stopping.
Vehicle stopping distance is still dependent on ve-
hicle speed, weight, tires, road surface, and other
factors.
3.1.1 PEDAL FEEL/VEHICLES
CHARACTERISTICS
There are several pedal feel/vehicle characteris-
tics that are considered normal for antilock braking
that may require further explanation.
When stopping conditions activate the antilock
brakes, the driver may feel some vibrations/
pulsations in the brake pedal and may hear the
solenoid valves clicking and the pump motor run-
ning. The vibrations/pulsations are caused by the
isolating, building and decaying of brake fluid pres-
sure within the brake lines. The ABS prevents
complete wheel lock-up, but some wheel slip is
required for the best braking performance. This slip
may result in some tire chirping, depending on the
road surface. The chirping should not be inter-
preted as total wheel lock-up. Total wheel lock-up
leaves black tire marks on dry pavement, antilock
braking may leave some light marks.
At the end of an ABS stop, the ABS may function
all the way down to near 0 km/h (0 mph). There may
be a slight brake pedal drop anytime the ABS is
deactivated.
In case of braking on a bumpy surface, the ABS
module may activate the ABS function when it
detects wheel locking tendencies due to wheel hop.
In that event, the brake pedal may pulsate with a
perceived loss of deceleration. ABS braking may
also be activated at times while on dry pavement
with sand, gravel, or other loose debris on the road.
It should be noted that the pulsating pedal feel
characteristic will not illuminate the brake warning
lamps or set a trouble code that is stored in the
Controller Antilock Brake (CAB). When investigat-
ing a hard pedal feel, inspect the sensor and tone
wheel teeth for chips/broken teeth, damaged sensor
pole tips, excessive runout of the tone wheel, or
excessive air gap.

3.1.2 SYSTEM COMPONENTS

ANTILOCK BRAKE SYSTEM
•

controller antilock brake (CAB)

• vacuum booster
• master cylinder (w/center valves)
•
hydraulic control unit (HCU)
• valve block assembly: 6 valve solenoids (3 inlet
•
pump/motor assembly:
1 motor
2 pumps
• internal G (acceleration) sensor
•
1 internal proportioning valve
• 3 wheel speed sensor/tone wheel assemblies
• ABS warning indicator
•
fuses and wiring harness
3.1.3 ABS AND RED BRAKE WARNING
INDICATOR
The amber ABS warning indicator is located in
the instrument cluster. It is used to inform the
driver that the antilock function has been turned off
due to a system malfunction. On the KJ, the CAB
controls the lamp indirectly. The CAB monitors its
own functions. If the CAB determines that the ABS
indicator should be on, the CAB sends a message
via the PCI BUS to the instrument cluster and the
cluster turns on the indicator. The instrument clus-
ter sends an "are you there" message over the PCI
BUS, if the CAB does not respond the instrument
cluster will illuminate the ABS indicator.
The red brake warning indicator is located in the
instrument cluster. It can be activated by applica-
tion of the parking brake, a leak in the front or rear
wheel brake hydraulic circuit which causes the
master cylinder reservoir to be low on fluid, or by
turning the ignition switch to the start position.
The red brake warning indicator can also be turned
on if the brake fluid level switch circuit becomes
open or shorted to ground.
3.1.4 CONTROLLER ANTILOCK BRAKE
(CAB)
The antilock brake controller (CAB) is a
microprocessor-based device that monitors wheel
speeds and controls the antilock functions.
The primary functions of the CAB are:
• monitor wheel speeds
• detect wheel locking tendencies
• control fluid pressure modulations to the brakes
during antilock stop operation
• control the ABS warning indicator
• monitor the system for proper operation
•
provide communication to the DRBIII while in
diagnostic mode
• store diagnostic information in non-volatile mem-
ory
2
valves, 3 outlet valves)
3 accumulators