Anti-lock Brake System (ABS): Feature

• The 5.3i type ABS used in the Impreza 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*¹ , while the rear wheel system is a select-low control design*² .
• Vehicles with Sports ABS have an additional function which independently controls*¹ the rear wheels according to the value sent from the lateral G sensor. (STi models)

*1: A system which controls the left and right wheel brakes individually.

*2: A system which applies the same fluid pressure to both wheels if either of the left or right wheels 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 Component

Courtesy of SUBARU OF AMERICA, INC.