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Brake pedal position is maintained during an an-
tilock stop by being a closed system. The CAB
continues to control pressure in individual hydrau-
lic 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 indicator.
If the antilock function is disabled, the system will
revert to standard base brake system operation.
The CAB inputs include the following:
•
three power feeds: valve, pump, and ignition
switch.
•
four wheel speed sensor signals.
•
brake lamp switch
•
battery voltage
•
diagnostic communication (PCI BUS)
The CAB outputs include the following:
•
six valve/solenoid drivers
•
four wheel speed sensors supply voltage
•
pump/motor actuation
•
diagnostic communication (PCI BUS)
•
Vehicle Speed Signal (VSS)
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 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
GENERAL INFORMATION
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.
3.1.6 SENSORS
Wheel Speed Sensors and Tone Wheels: One
wheel speed sensor (WSS) is located at each wheel
and sends a small DC voltage signal to the CAB.
The CAB sends 12 volts down to the sensor. The
sensor has an internal magneto resistance 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 volt-
age and amperage of the DC signals for each wheel.
The front wheel speed sensors are attached to a
boss in the steering knuckle. The tone wheel is an
integral part of the front bearing hub. The rear
speed sensor is mounted to the rear knuckle and the
rear tone wheel is an integral part of the rear
bearing hub. The wheel speed sensor air gap is
NOT adjustable. Because of internal circuitry,
a resistance check of wheel speed sensors will
not determine correct or incorrect 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 correct mini-
spare. When driven over rough road surfaces, the
wheel speed sensor signals may be erratic and
cause a false trouble code.
3.2 DIAGNOSTIC TROUBLE CODES
The Controller Antilock Brake (CAB) may report
any of several diagnostic trouble codes (DTC)s. The
possible DTC's for the ABS are listed in the table of
contents.
Diagnostic trouble codes are retained in memory
until erased using the DRB, or automatically erased
after 255 key cycles or 3,500 miles.
3
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