lockup. The basic parts of an antilock brake system are
ABS COMPUTERa microcomputer that
functions as the "brain" of the ABS system. The
computer receives wheel-end performance data from
each wheel speed sensor. When the wheels try to lock,
the computer delivers commands to operate the
hydraulic actuator to control brake pressure. The
computer also monitors brake pedal position, detects
and prevents potential wheel lockup conditions while
maintaining optimum braking performance, stores and
displays diagnostic codes, and alerts the operator of a
system malfunction by turning on the system lamp.
HYDRAULIC ACTUATORan electric-
hydraulic valve that modulates the amount of braking
pressure (psi) going to a specific wheel circuit.
TRIGGER WHEELSa toothed ring that is
mounted on each wheel spindle or hub.
WHEEL SPEED SENSORSa magnetic
sensor that uses trigger wheel rotation to produce a
weak alternating current.
The operation of an antilock brake system is as
A wheel speed sensor is mounted at each wheel
to measure trigger wheel rotation in rpms. The sensor
sends alternating or pulsing current signals to the ABS
If one or more wheels decelerate at a rate above
an acceptable perimeter, the sensor signals reduce
frequency and the ABS computer activates the
hydraulic actuators. The actuator then cycles ON and
OFF as much as 15 times per second to reduce braking
pressure to the brake assembly for that wheel. This
action prevents the vehicle from skidding.
The ABS computer will continue to modulate
brake pressure until the operator releases the brake
pedal, the wheel speed sensor no longer detects a lockup
condition, or the vehicle stops.
Tips on using antilock braking systems are as
Always "brake and steer" when using antilock
brakes. Most operators were taught to pump the brakes
and turn hard to the right or left to compensate for
skidding. With antilock brakes, all a operator needs to
do is "brake and steer." With four-wheel antilock
brakes, push the brake pedal hard while steering
normally and keep your foot firmly on the brake pedal
until the vehicle comes to a complete stop. Operators
Figure 7-20.Combination valve.
Figure 7-21.Basic antilock brake system.