See Figures 1 and 2
Anti-Lock Braking (ALB) systems are designed to prevent locked-wheel skidding during hard braking or during braking on slippery surfaces. The front wheels of a vehicle cannot apply steering force if they are locked and sliding; the vehicle will continue in its previous direction of travel. The four wheel anti-lock brake systems found on Honda automobiles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways.
There are conditions for which the ALB system provides no benefit. Hydroplaning is possible when the tires ride on a film of water, losing contact with the paved surface. 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. Wheel spin during acceleration on slippery surfaces may also fool the system into entering its fail-safe mode.
Under normal braking conditions, the ALB system functions in the same manner as a standard brake system. The system is a combination of electrical and hydraulic components, working together to control the flow of brake fluid to the wheels when necessary.
The ALB Control Unit is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The unit will enter anti-lock mode when it senses impending wheel lock-up at any wheel and immediately controls the brake line pressure(s) to the affected wheel(s). The modulator assembly is separate from the master cylinder and booster. It contains the wheel circuit solenoid valves used to control the brake fluid pressure to each wheel circuit.
During anti-lock braking, line pressures are controlled or modulated by the rapid cycling of electronic valves within the modulator. These valves can allow pressures within the system to increase, remain constant or decrease depending on the needs of the moment as registered by the control unit. The front wheels are controlled individually. The rear wheel circuits receive the same pressure control, based on the rear wheel with the greatest locking tendency.
The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during anti-lock operation. The sounds are due to normal operation and are not indicative of a system problem. Under most conditions, the sounds are only faintly audible. When ALB is engaged, the operator may notice some pulsation in the brake pedal and/or body of the vehicle during a hard stop; this is also normal operation but can be surprising to a driver who engages ALB for the first time.
Although the ALB 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 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 point of lock-up. Additionally, the final few feet of an ALB-engaged stop may be completed with the wheels locked; the electronic controls do not operate at very low speeds.
When the ignition is ON and the vehicle's speed is over 6 mph, the control unit monitors the function of the system. Should a fault be noted, such as loss of signal from a speed sensor or excessive pump running time, the ALB system is immediately de-energized. Normal brake function remains but the anti-lock function is disabled.