The Subaru Legacy anti-lock brake system uses a 4-sensor, 4-channel system. A speed signal for each wheel is generated by a speed sensor at the wheel. The hydraulic actuator contains 4 control solenoids, one for each wheel brake line. The system is capable of controlling brake line fluid pressure to any or all of the wheels as the situation demands.
When the ECU receives signals showing one or more wheels about to lock, it sends an electrical signal to the solenoid valve(s) within the actuator to release the brake pressure in the line. The solenoid moves to a position which holds the present line pressure without allowing it to increase. If wheel deceleration is still outside the pre-programmed values, the solenoid is momentarily moved to a position which releases pressure from the line. As the wheel unlocks or rolls faster, the ECU senses the increase and signals the solenoid to open, allowing the brake pedal to increase line pressure.
This cycling occurs several times per second when ABS is engaged. In this fashion, the wheels are kept just below the point of lock–up and control is maintained. When the hard braking ends, the ECU resets the solenoids to its normal or build mode. Brake line fluid pressures are then increased or modulated directly by pressure on the brake pedal. Fluid released to the ABS reservoirs is returned to the master cylinder by the pump and motor within the actuator.
The front and rear wheels are controlled individually, although the logic system in the ECU reacts only to the lowest rear wheel speed signal. This method is called Select Low and serves to prevent the rear wheels from getting greatly dissimilar signals which could upset directional stability.
The operator may hear a popping or clicking sound as the pump and/or control valves cycle on and off during normal 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. If ABS is engaged, the operator may notice some pulsation in the body of the vehicle during a hard stop; this is generally due to suspension shudder as the brake pressures are altered rapidly and the forces transfer to the vehicle.
Although the ABS 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 ABS operation. The sound should not be interpreted as lock–up, but rather than as an indication of the system holding the wheel(s) just outside the point of lock–up. Additionally, the final few feet of an ABS–engaged stop may be completed with the wheels locked; the electronic controls do not operate below about 3 mph (5 km/h).