See Figures 1 and 2
Anti-lock braking 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 these vehicles holds the individual wheels just below the point of locking, thereby allowing some steering response and preventing the rear of the vehicle from sliding sideways.
Electrical signals are sent from the wheel speed sensors to the ABS control unit; when the system detects impending lock-up at any wheel, solenoid valves within the hydraulic unit cycle to control the line pressure as needed. The systems employ normal master cylinder and vacuum booster arrangements; no hydraulic accumulator is used, nor is any high pressure fluid stored within the system. The system employs a conventional master cylinder and vacuum booster arrangements; no hydraulic accumulator is used, nor is any high pressure fluid stored within the system.
The Front Wheel Drive (FWD) vehicle family uses a 3-channel anti-lock system. The 3-channel system uses 3 solenoids in the hydraulic unit to control brake pressure in the left front, right front and rear circuits. A proportioning valve within the rear circuit equalizes pressure to each rear wheel.
The All Wheel Drive (AWD) vehicle family use a 2 channel system. The left front and right rear wheels share a control solenoid as do the right front and left rear wheels. The system contains a select-low valve which reacts to reduced pressure in one circuit and balances the pressure to the opposite rear wheel. In this fashion, the anti-lock function is provided at 3 wheels, rather than just the one originating the lock-up signal.