•
monitor the system for proper operation
•
provide communication to the DRBIII
t
while in
diagnostic mode
•
store diagnostic information in non-volatile mem-
ory
The CAB continuously monitors the speed of each
wheel. When a wheel locking tendency is detected,
the CAB will command the appropriate HCU valve
to modulate brake fluid pressure to that wheel.
Brake pedal position is maintained during an an-
tilock stop by being a closed system with the use of
3 accumulators. The CAB continues to control pres-
sure in individual hydraulic circuits until a wheel
locking tendency is no longer present. The CAB
turns on the pump/motor during an antilock stop.
The antilock brake system is constantly moni-
tored by the CAB for the proper operation. If the
CAB detects a system malfunction, it can disable
the antilock system and activate the ABS warning
indicator. If the antilock function is disabled, the
system will revert to standard base brake system
operation.
The CAB inputs include the following:
•
four wheel speed sensors
•
brake lamp switch
•
ignition switch
•
battery voltage
•
diagnostic communication (PCI BUS)
The CAB outputs include the following:
•
six valve/solenoid drivers
•
pump/motor actuation
•
ABS warning indicator actuation
•
red brake warning indicator actuation
•
diagnostic communication (PCI BUS)
3.1.5
HYDRAULIC CONTROL UNIT
The hydraulic control unit (HCU) contains the
valve block assembly, four accumulators, and the
pump/motor assembly.
Valve Block Assembly: The valve block assem-
bly contains 6 valves with three inlet valves and
three outlet valves. The inlet valves are spring-
loaded in the open position and the outlet valves are
spring loaded in the closed position. During an
antilock stop, these valves are cycled to maintain
the proper slip ratio for each channel. The CAB
monitors wheel speeds. If the CAB detects a wheel
deceleration that is disproportionate to the other
wheels, it will close the inlet valve to that wheel.
This prevents any increase in fluid pressure. If the
wheel continues to decelerate disporportionately,
the CAB opens the outlet valve for that wheel to
release fluid pressure from that channel. The re-
leased fluid is routed to the accumulators. When the
wheel speed is no longer disproportionate to the
other wheels, the inlet valve will return to its
normally open position and the outlet valve will
return to the normally closed position.
Pump/Motor Assembly: The pump/motor as-
sembly provides the extra amount of fluid needed
during antilock braking. The pump is supplied fluid
that is released to the accumulators when the outlet
valve is opened during an antilock stop. The pump
is also used to drain the accumulator circuits after
the antilock stop is complete. The pump is operated
by an integral electric motor. This motor is con-
trolled by the CAB. The CAB turns on the motor
when an antilock stop is detected. The pump con-
tinues to run during the antilock stop and is turned
off approximately 3-5 seconds after the stop is
complete. The CAB monitors the pump/motor oper-
ation internally.
Accumulators: The accumulators provide tem-
porary fluid storage during an antilock stop and are
drained by the pump/motor.
3.1.6
SENSORS
Wheel Speed Sensors and Tone Wheels: One
wheel speed sensor (WSS) is located at each wheel
and sends a small digital signal to the control
module (CAB). The CAB sends 12 volts down to the
sensor. The sensor has an internal magneto resis-
tance bridge that alters the voltage and amperage
of the signal circuit. This voltage and amperage is
changed by magnetic induction when a toothed
sensor ring (tone wheel) passes by a stationary
magnetic sensor (wheel speed sensor). The CAB
measures the voltage and amperage of the digital
signals for each wheel.
The front wheel sensor is attached to a boss in the
steering knuckle. The tone wheel is an itegral part
of the front axle shaft. The rear speed sensor is
mounted in the caliper adapter plate (rear disc only)
and the rear tone wheel is an integral part of the
rear rotor hub. The wheel speed sensor air gap
is NOT adjustable. Because of internal cir-
cuitry, a resistance check of WJ wheel speed
sensors will not determine correct or incor-
rect function.
Correct antilock system operation is dependent
on wheel speed signals from the wheel speed sen-
sors. The vehicle’s wheels and tires should all be the
same size and type to generate accurate signals. In
addition, the tires should be inflated to the recom-
mended pressures for optimum system operation.
Variations in wheel and tire size or significant
variations in inflation pressure can produce inaccu-
rate wheel speed signals; however, the system will
continue to function when using the mini-spare.
When driven over rough road surfaces, the rear
wheel speed sensor signals may be erratic and
cause a false trouble code (drum brakes only).
3
GENERAL INFORMATION
