Honda Civic Sedan 1993 Owner's Manual page 187

Anti-lock Brake System
The Anti-lock Brake System (ABS)
is standard equipment on the Civic
EX sold in the U.S.. It is optional on
the Canadian EX-V model. It is not
available on any other models.
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 pres-
sure to one side of a modulator
valve. This, in turn, reduces hydrau-
lic pressure in the brake line going
to the affected wheel. When that
wheel speeds up because of the re-
duced braking effort, the control
unit de-energizes the solenoid. This
builds hydraulic pressure on the
modulator valve. The pressure in-
creases 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.