Anti-lock Brake System (ABS): Features

• The 5.3i type ABS used in the Legacy has a hydraulic control unit, an ABS control module, a valve relay and a motor relay integrated into a single unit (called ABSCM & H/U) for circuit simplicity and reduced weight.
• The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from locking during braking on slippery road surfaces, thereby enabling the driver to maintain the directional control.
• If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking performance as with a conventional brake system. In that case, the warning light comes on to indicate that the ABS is malfunctioning.
• The ABS is a 4-sensor, 4-channel system; the front wheel system is an independent control design, while the rear wheel system is a select-low control design.
  NOTE: Independent control design is a system which controls the front wheel brakes individually.
  NOTE: Select-low control design is a system which applies the same fluid pressure to both the rear wheels if either wheel starts to lock. The pressure is determined based on the lower of the frictional coefficients of both wheels.

Fig 1: Identifying Anti-Lock Brake System Components

Courtesy of SUBARU OF AMERICA, INC.