A pressure decrease cycle is initiated when speed
sensor signals indicate high wheel slip at one or
more wheels. At this point, the CAB closes the inlet
then opens the outlet valve, which also opens the
return circuit to the accumulators. Fluid pressure is
allowed to bleed off (decrease) as needed to prevent
wheel lock.
Once the period of high wheel slip has ended, the
CAB closes the outlet valve and begins a pressure
increase or hold cycle as needed.
PRESSURE HOLD
Both solenoid valves are closed in the pressure
hold cycle. Fluid apply pressure in the control chan-
nel is maintained at a constant rate. The CAB main-
tains the hold cycle until sensor inputs indicate a
pressure change is necessary.
PRESSURE INCREASE
The inlet valve is open and the outlet valve is
closed during the pressure increase cycle. The pres-
sure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.
WHEEL SPEED SENSORS AND TONE WHEEL
DESCRIPTION
A wheel speed sensor is used at each wheel. The
front sensors are mounted to the steering knuckles.
The rear sensors are mounted at the outboard end of
the axle. Tone wheels are mounted to the outboard
ends of the front and rear axle shafts. The gear type
tone wheel serves as the trigger mechanism for each
sensor.
OPERATION
The sensors convert wheel speed into a small digi-
tal signal. The CAB sends 12 volts to the sensors.
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 the toothed tone wheel
passes the wheel speed sensor. This digital signal is
sent to the CAB. The CAB measures the voltage and
amperage of the digital signal for each wheel.
G-SWITCH
DESCRIPTION
The G-switch (Fig. 2) is located under the rear
seat. The switch has directional arrow and must be
mounted with the arrow pointing towards the front
of the vehicle.
OPERATION
The switch is monitored by the CAB at all times.
The switch contains three mercury switches which
monitor vehicle deceleration rates (G-force). Sudden
changes in deceleration rates trigger the switch,
sending a signal to the CAB.
ABS WARNING LAMP
DESCRIPTION
The amber ABS warning lamp and red warning
lamp are located in the instrument cluster. The
amber ABS warning lamp illuminates at start-up to
perform a self check. The lamp goes out when the
self check program determines the system is operat-
ing normal. The red brake warning lamp is used to
alert the driver of a hydraulic fault or that the park-
ing brake is applied.
OPERATION
If an ABS component exhibits a fault the CAB will
illuminate the ABS warning lamp and register a
trouble code in the microprocessor. The lamp is con-
trolled by the CAB. The CAB controls the lamp send-
ing a message to the instrument cluster.
If red warning lamp is illuminate with the amber
warning lamp, this may indicate a electronic brake
distribution fault.
The red warning lamp will illuminate if an ABS
component exhibits a fault and the amber lamp is
burned out.
Fig. 2 G-Switch
1 – SWITCH PART NUMBER
2 – ARROW INDICATES FRONT OF SWITCH FOR PROPER
MOUNTING
5 - 40
BRAKES
WJ
DESCRIPTION AND OPERATION (Continued)