indicated as a percentage. 0% slip means that the
wheel is rolling free and 100% slip means that the
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 detect wheel locking tendencies due to
wheel hop and cycle ABS. In that event the brake
pedal may pulsate with a perceived loss of deceler-
ation. 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 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
valves, 3 outlet valves)
4 accumulators
•
brake fluid level switch (part of the fluid reser-
voir)
•
pump/motor assembly:
1 motor
2 pumps
•
4 wheel speed sensor/tone wheel assemblies
•
ABS warning indicator
•
fuses and wiring harness
•
fluid reservoir (integral part of master cylinder
assembly)
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 WJ, 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 cluster sends a message over the
PCI BUS at regular intervals, 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
2
GENERAL INFORMATION
