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
• detect wheel slip
• control fluid pressure modulation to the brakes
during antilock stop and traction control opera-
tion
• monitor the system for proper operation
• provide communication to the DRBIIIt while in
diagnostic mode
• store diagnostic information in non-volatile
memory
The CAB continously monitors the speed of each
wheel. When a wheel locking tendency is detected,
the CAB will command the appropriate valve to
modulate brake fluid pressure in its hydraulic unit.
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 proper operation. If the CAB
detects a system malfunction, it can disable the
antilock system and turn on the antilock warning
lamp. If the antilock function is disabled, the sys-
tem 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:
• eight valve/solenoid drivers
• pump/motor actuation
• ABS warning indicator actuation (PCI BUS)
• red brake warning indicator actuation (PCI BUS)
• diagnostic communication (PCI BUS)
3.1.5 HYDRAULIC CONTROL UNIT
The hydraulic control unit (HCU) contains the
valve block assembly and the pump/motor assem-
bly.
Valve Block Assembly: The valve block assem-
bly contains inlet valves, outlet valves, and shuttle
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.If a wheel locks, the inlet
valve is closed to prevent any further pressure
increase. Then the outlet valve is opened to release
the pressure to the accumulators until the wheel is
no longer slipping. Once the wheel is no longer
slipping, the outlet valve is closed and the inlet
valve is opened to reapply pressure. If the wheel is
decelerating within its predetermined limits (prop-
er slip ratio), both valves will close to hold the
pressure constant.
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 may turn on themotor
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.
3.1.6 SENSORS
Wheel Speed Sensors and Tone Wheels: One
wheel speed sensor (WSS) is located at each wheel.
The sensor has internal circuitry powered by 12
volts from the controller antilock brake (CAB). The
sensor generates and sends a DC voltage signal
back to the CAB. The signal is toggled in proportion
to the speed of the toothed tone wheel as it passes
the sensor pole. The CAB uses the signal to activate
ABS functions as required.
Because of the internal circuitry, correct
sensor function cannot be determined by a
resistance check across the pins of the sensor.
The front wheel sensors are attached to a boss in
the steering knuckle. The tone wheels are an inte-
gral part of the front axle shaft. The rear speed
sensors are mounted in the caliper adapter plate
and the rear tone wheels are an integral part of the
rear rotor hubs. The wheel speed sensor air gap
is NOT adjustable.
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
3
GENERAL INFORMATION
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