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Anti-lock Braking System
The Anti-lock braking system works
by measuring how fast the wheels
are turning during braking and
comparing their speed. If any wheel
is rotating much slower than the
others (on the verge of locking up
and skidding), the system reduces
hydraulic pressure to that wheel's
brake caliper. When that wheel's
speed matches the other wheels, the
system applies normal hydraulic
pressure. This can take place several
times per second at each wheel. You
feel the Anti-lock brake system
working as rapid pulsations in the
brake pedal under very hard braking.
Each wheel has a wheel speed sensor
assembly. As the wheel rotates, the
sensor sends electrical pulses to the
Anti-lock control unit. The pulse
frequency varies with the wheel
speed.
Technical Information
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The electrical output of the Anti-lock
control unit is connected to the
Modulator/solenoid unit. During
braking, the Anti-lock control unit
monitors the pulse frequencies from
the four wheels. When the control
unit detects a wheel locking up, it
energizes the appropriate solenoid in
the modulator/solenoid unit. There
are three solenoids: one for each
front wheel, and one for the rear
wheels. The energized solenoid
reduces hydraulic pressure to one
side of a modulator valve. This, in
turn, reduces hydraulic pressure in
the brake line going to the affected
wheel. When that wheel speeds up
because of the reduced braking
effort, the control unit de-energizes
the solenoid. This builds hydraulic
pressure on the modulator valve. The
pressure increases in the hydraulic
line to the wheel.
For the system to react quickly, the
modulator/solenoid unit must have
brake fluid under high pressure.
This is supplied by a diaphragm-type
accumulator that is pressurized by an
electric pump. A pressure-sensing
switch on the accumulator controls
this pump.
The control unit also contains error
detection circuitry. The control unit
monitors the operation of the wheel
sensors, solenoids, pump, and
electronics. If the control unit
detects any faults, it shuts off power
to the pump motor and solenoids.
The ANTILOCK light on the instru-
ment panel comes on. The brakes
then work like a conventional system
without anti-lock capabilities.
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