Honda 1995 Civic Owner's Manual page 194

Anti-lock Brake System
Only on cars equipped with ABS
The 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 pulsations in the
brake pedal.
Technical Information
Each wheel has a wheel speed
sensor assembly. As the wheel
rotates, the sensor sends electrical
pulses to the ABS control unit. The
pulse frequency 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
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 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 the pump motor
and solenoids. The light on the
instrument panel comes on. The
brakes then work like a conventional
system without anti-lock capabilities.