The brake actuator consists of the pressure holding solenoid valve, pressure reduction solenoid valve, pump, motor, and reservoir. When the brake actuator receives a signal from the Skid Control ECU, the solenoid is turned on or off and the hydraulic pressure of the wheel cylinder is increased, decreased, or held to optimize the slip rate for each wheel. In addition, the hydraulic circuit is changed to meet the requirements of each type of control.
The hydraulic circuit in the ABS for FF vehicles is divided into front right wheel and rear left wheel system and the front left wheel and rear right wheel system as shown in the diagram. The explanation hereafter is only given for the operation of one of these systems, but the other systems operate in the same manner.
(1) During normal braking (When the system does not operate)
During normal braking, the control signal from the Skid Control ECU is not input. For this reason, the pressure holding and reduction solenoid valves are off, port (a) on the pressure holding solenoid side is opened, and port (b) on the pressure reduction solenoid side is closed. When the brake pedal is depressed, the brake fluid from the master cylinder flows through port (a) on the holding solenoid side and is directly transmitted to the wheel cylinder. At this time, the operation of check valve (2) prevents the brake fluid from being transmitted to the pump side.
(2) During emergency braking (when the ABS operates)
<1> Pressure reduction mode
The control signal from the Skid Control ECU turns on the pressure holding and reduction solenoids by closing port (a) on the pressure holding solenoid side and opening port (b) on the pressure reduction solenoid side. This passes brake fluid through the port (b) to the reservoir to decrease the hydraulic pressure on the wheel cylinder. At this time, port (e) is closed by the decent of the reservoir. The pump continues to run while the ABS is operating, so the brake fluid that enters the reservoir is drawn by the pump and returned to the master cylinder.
<2> Holding mode
The control signal from the Skid Control ECU turns on the pressure holding solenoid and turns off the pressure reduction solenoid by shutting off the port (a) and port (b). This shuts off the wheel cylinder hydraulic pressure from both the master cylinder and reservoir sides to hold the wheel cylinder hydraulic pressure constant.
<3> Pressure increasing mode
The control signal from the Skid Control ECU turns off the pressure holding and reduction solenoids by opening port (a) on the pressure holding solenoid side and closing port (b) on the pressure reduction solenoid side the same as during normal braking. This cause the hydraulic pressure from the master cylinder to work on the wheel cylinder to cause the wheel cylinder hydraulic pressure to increase.
HINT
The brake assist changeover solenoid valve is only used in vehicles equipped with BA.
Inspection method