ware will enable the solenoids to perform the same
brake fluid management control as the proportion-
ing valves.
During an ABS stop, the system still uses the
diagonal hydraulic split; however, the brake system
pressure is further split into four control channels.
During ABS operation, all wheels are controlled
independently and are on four separate control
channels.
During an antilock stop, “wheel lock-up” does not
necessarily mean that the wheel has locked, it
means only that the wheel is turning slower than
the vehicle speed. This is called “wheel slip” and is
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 system pre-
vents 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. This 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
system may detect wheel locking tendencies due to
wheel hop and cycle the ABS. 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: 8 valve solenoids (4 inlet
valves, 4 outlet valves)
• 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. The CAB controls the
lamp indirectly. The CAB monitors its own func-
tions. If the CAB determines that the ABS warning
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 indicator will
remain lit during every key cycle until the circuit or
component fault is repaired and the CAB no longer
detects the fault. After repair of a sensor signal
fault or a pump motor fault, the CAB must sense all
four wheels at 25 km/h (15 mph) before it will
extinguish the ABS and Indicators.
The Instrument Cluster will illuminate the ABS
Warning Indicator if it loses communication with
the CAB.
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 CAB indicates an Electronic Brake Distri-
bution (EBD) failure.
2
GENERAL INFORMATION
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