lockup. The basic parts of an antilock brake system are as follows:

ABS COMPUTER - a 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 ACTUATOR - an electric- hydraulic valve that modulates the amount of braking pressure (psi) going to a specific wheel circuit.

TRIGGER WHEELS - a toothed ring that is mounted on each wheel spindle or hub.

WHEEL SPEED SENSORS - a magnetic sensor that uses trigger wheel rotation to produce a weak alternating current.

The operation of an antilock brake system is as follows:

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 computer.

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 follows:

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.