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.

with rear-wheel antilock brakes should step firmly with care. and if they feel the wheel locking, they should release some pressure.

Expect noise and vibration in the brake pedal when antitock brakes are in use. The mechanical noise or pulsation of antilock brakes when in use might catch an operator by surprise; however, these sensations tell you that the system is working.

Remember that you can steer while braking with a four-wheel antilock brake system. Steering is not always instinctive in an emergency. But steer out of danger while braking with antilock brakes. And remember that while you have steering capability, your vehicle may not turn as quickly white braking on a slippery road, as it would on dry pavement.

The rear-wheel antitock brakes typically found on light-duty trucks provide vehicle stability but do not give you the steering capability of four-wheel antilock brakes.

Anti lock brakes can often stop more quickly than conventional brakes but they can't overcome the law of physics. Antilock brakes function well on wet-paved surfaces and icy or packed snow-covered roads. Stopping times will be longer on gravel or fresh snow, although operators won't experience the dangerous lockup of wheels usually associated with conventional brakes.

Drive safely because antilock brakes are only as good as the operators using them. Antilock brakes cannot compensate for driving too fast, too aggressively or failing to maintain a safe distance between vehicles. They cannot guarantee recovery from a spin or skid before braking. Also avoid extreme steering maneuvers while antilock brakes are engaged.

Your antilock braking system instrument panel tight will go on for a few seconds after starting the ignition. The tight goes on so the system can conduct the normal system test. If the tight does not go on during ignition or if the tight goes on during normal driving, this means that a problem has been detected and the antilock braking system has been shut off. Conventional braking will continue. Consult the manufacturer's service manual if this problem occurs.

Since exact antitock brake systems vary, consult the vehicle manufacturer's service and repair manuals for more details of system operation.