See Figures 1 and 2
The Anti-Lock Brake System (ABS) is 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 anti-lock brake systems found on these vehicles hold the wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways while braking.
There are conditions for which ABS 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.
Under normal braking conditions, ABS functions in the same manner as a standard brake system. The system is merely a combination of electrical and hydraulic components, working together, to control the flow of brake fluid to the wheels when necessary.
The ABS control unit is the electronic brain of the system, receiving and interpreting speed signals from the speed sensors. The control unit will enter anti-lock mode when it senses impending wheel lock at any wheel, and immediately control the brake line pressure to the affected wheel.
The actuator assembly is separate from the master cylinder and booster. It contains the wheel circuit valves used to control brake fluid pressure to each wheel circuit.
The wheel speed sensors monitor decelerating wheel speed and provide data to the control module.
The Altima uses a four-channel ABS system with independent wheel speed sensors/controls at each wheel. The 240SX uses a three-channel system with a single wheel speed sensor for the rear wheels (located on the differential) and independent wheel speed sensors/controls for each of the front wheels.