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Anti-lock Brake System
The Anti-lock Brake System (ABS)
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 ABS working as rapid pulsa-
tions in the brake pedal.
Technical Information
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Each wheel has a wheel speed sensor
assembly. As the wheel rotates, the
sensor sends electrical pulses to the
ABS control unit. The pulse fre-
quency varies with the wheel speed.
The electrical output of the ABS
control unit is connected to the
modulator/solenoid unit. During
braking, the ABS control unit moni-
tors the pulse frequencies from the
four wheels. When the control unit
detects a wheel locking up, it ener-
gizes the appropriate solenoid in the
modulator/solenoid unit. There are
four solenoids: one for each wheel.
The energized solenoid reduces
hydraulic pressure to one side of a
modulator valve. This, in turn, re-
duces 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 h i g h pressure.
This is supplied by an 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. It monitors the
operation of the wheel sensors,
solenoids, pump, and electronics. If
the control unit detects any faults, it
shuts off power to t h e pump motor
and solenoids. The l i g h t on the
instrument panel comes on.
The brakes then w o r k like a conven-
tional system without anti-lock
capabilities.
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