See Figure 1
The Anti-Lock Brake (ALB/ABS) system is an answer to the real-world problem of panic stops and braking on slippery or uneven pavement. The need for brakes that can use what traction is available to stop the car without locking-up becomes apparent during hazardous braking situations. These situations can cause the front, rear or all four wheels to skid or lock on a conventional brake system. If the rear wheels skid, stability is lost or compromised and if the front wheels skid, it is maneuverability that is affected. Beyond theory, what often happens when braking under such hazardous conditions is simply an out-of-control four-wheel lock-up.
Honda's ALB/ABS is a computer-controlled system that augments otherwise conventional hydraulic brakes by monitoring each wheel's rotational speed as compared to vehicle road speed. When during braking it detects one or more wheels are rotating slower than the rate of travel, it compensates by modulating the brakes, allowing the car to slow but preventing the impending lock-up. In many cases, the goal to maximize stability and maneuverability under adverse braking conditions is met. The system became available on certain models in 1992.
There are conditions, however, for which the ALB/ABS system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. And the same is possible on snow, ice and loose terrain. This renders the vehicle totally uncontrollable until road contact is regained. Extreme steering maneuvers at high speed or cornering beyond the limits of tire adhesion can result in skidding which is independent of vehicle braking. For this reason, the system is named "anti-lock" rather than anti-skid.
Under normal conditions, the ALB/ABS system functions in the same manner as a conventional brake system and is not able to be detected by the operator. The system is a combination of electrical, mechanical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary.
The ALB/ABS control unit is the electronic brain of the system, receiving and interpreting signals from the wheel sensors which read and send a magnetic signal generated by the gear pulsers, toothed rings that rotate with the wheels. The unit will enter anti-lock mode when the wheel sensors detect impending wheel lock at any wheel, and will immediately control the brake line pressures to the affected wheel(s) by issuing output signals to the Modulator Unit.
The modulator unit contains solenoid valves which react to the signals from the control unit. The solenoids controls brake fluid pressure to the wheels. They work by allowing brake line pressure to build according to brake pedal pressure, hold (by isolating the system from the pedal and maintaining current pressure) or decrease (by isolating the pedal circuit and bleeding some fluid from the line).
The decisions regarding these functions are made very rapidly and each solenoid valve has a response time of five milliseconds or less. Honda provides three solenoid valves; the front wheels are controlled separately, while the rear wheels are controlled together.
Additionally the ALB/ABS system utilizes a pressure switch which sends the pressure reading to the control unit; an ALB/ABS pump assembly; a fail-safe relay which operates when a malfunction is detected by electronically disconnecting the solenoid valve ground circuit and illuminating the indicator light. The indicator light has a dual purpose of alerting the driver of system malfunction, and when switched to a different mode, it is used in system diagnosis by relaying the problem "codes" the control unit has detected with any of the components of the ALB/ABS system.
Although the ALB/ABS system prevents wheel lock-up under hard braking, as brake pressure increases, wheel slip is allowed to increase as well. This slip will result in some tire chirp during ALB/ABS operation. The sound should not be interpreted as lock-up but rather as an indication of the system holding the wheel(s) just outside the locking point. The final few feet of an ALB/ABS-engaged stop may be completed with the wheels locked, as the system is inoperative below 6 mph (10 kph).
When the ignition is ON and vehicle speed is over 6 mph (10 kph), the control unit monitors the function of the system. Should a fault be noted, such as loss of signal from a sensor, the ALB/ABS system is immediately disabled by the control unit and the ALB/ABS warning lamp illuminates to inform the operator. When the ALB/ABS system is disabled, the vehicle retains normal braking capacity without the benefits of anti-lock.