•
ABS system malfunction
The red brake warning indicator is controlled by
the vehicle’s “smart” cluster. There is a parallel
path to chassis ground that will illuminate the red
brake warning indicator:
•
through the parking brake switch contacts
•
through the combination valve brake warning
indicator switch
•
through the CAB via PCI Bus
The CAB can control the operation of both the
yellow ABS warning indicators and the red brake
warning indicator
•
If the CAB is disconnected, the yellow indicator
will be illuminated.
•
If the axle type or tire size are not programmed
properly, the yellow indicator will blink.
•
If a system DTC is set, both the yellow and red
indicator will be illuminated.
3.2.4
BRAKE LAMP SWITCH CIRCUIT
An input from this switch prepares the CAB for a
possible antilock stop.
The antilock system uses an input signal from the
brake lamp switch. When the driver applies the
brake pedal, the circuit voltage is 12 volts. A re-
leased brake pedal will close the switch circuit and
provide 0 volts. This signal tells the CAB that the
pedal is depressed.
3.2.5
HYDRAULIC CONTROL UNIT
The HCU on the EBC 2 has only a separate rear
valve body.
The HCU on the EBC 325 has an integral valve
body for controlling the front and rear brakes.
Within the HCU are solenoids, valves, check
valves, and a reset switch necessary to apply and
release brake pressure as required to avoid wheel
lockup, keep the wheels rolling, and maintain opti-
mum deceleration.
The isolation valve(s) is/are normally open, allow-
ing unrestricted flow from the master cylinder to
the wheels. When the CAB determines antilock
intervention is required, the valve closes to isolate
the master cylinder hydraulic circuit. Fluid is
trapped in the circuit and then prevented from
reaching the wheels.
The dump valve(s) is/are pulsed on and off by the
CAB. This valve cycles only if the isolation valve is
closed. When dump is on, it allows fluid to the low
pressure accumulator for temporary storage. This
causes the pressure to the wheel to decrease. When
the valve is off, fluid is allowed to the wheel.
The brake return check valve allows the HCU to
drain faster after the ABS stop, when the brake is
released.
The Reset Switch is used on the EBC 2 system
only. The switch is positioned in the HCU to moni-
tor the master cylinder (input), rear brake (output)
and accumulator pressure. During normal braking,
pressure is equal for input and output and the
switch remains open. During an Antilock stop the
switch will close as the isolation valve is cycled and
pressure becomes unequal between input and out-
put. At the end of an ABS stop the isolation valve
will open as master cylinder pressure and rear
pressure equalize. The switch is used to monitor
correct operation of the HCU and to set a diagnostic
trouble code if incorrect pressure is detected.
When the brakes are applied, fluid is forced from
the master cylinder outlet ports to the HCU inlet
ports. This pressure is transmitted through three
normally open isolation valves inside the HCU,
then through the outlet ports of the HCU to the
wheels. If the CAB senses that a wheel is about to
lock, based on wheel speed sensor data, it pulls the
normally open isolation valve closed for that circuit.
This prevents any more fluid from entering that
circuit. The CAB continues to look at the sensor
signal to determine if the wheel is still decelerating.
If deceleration is still taking place, the normally
closed dump valve for that circuit is opened. This
dumps any pressure that is trapped between the
normally open valve and the brake back into an
accumulator. Once the affected wheel comes back
up to speed, the CAB returns the valves to their
normal condition allowing the affected brake to be
reapplied. On the EBC 325 system, the pump and
accumulators are used to provide rapid response
during the reapply sequence and to minimize pedal
feedback.
3.2.6
WHEEL SPEED SENSORS
The EBC 325 ABS system uses one wheel speed
sensor on each front wheel, and one speed sensor
mounted in the rear axle for the rear wheels.
The EBC 2 used only one speed sensor mounted
in the rear axle for the rear wheels.
The sensor measures the wheel speed by moni-
toring a rotating tone wheel. The signal generated
by the sensor and tone wheel is transmitted to the
CAB.
Each sensor has:
•
a magnetic/coil pick-up (speed sensor) that is
mounted to a fixed component
•
an air gap between the tone wheel and the speed
sensor assembly
As the teeth of the tone wheel move through the
magnetic field of the sensor, an AC voltage is
generated. This signal frequency increases or de-
creases proportionally to the speed of the wheel.
3
GENERAL INFORMATION