Anti-Lock Brake System: Operation: Modulator

ABS modulator consists of inlet solenoid valve, outlet solenoid valve, reservoir pump, pump motor and damping chamber. ABS modulator hydraulic circuit is an independent 4 channel-type, with one channel for each wheel.

ABS modulator reduces caliper fluid pressure directly. It is a circulating-type modulator. It circulates brake fluid through caliper, reservoir and master cylinder.

ABS modulator hydraulic control has 3 modes:

• Pressure reduction mode: Inlet valve closed and outlet valve open. Caliper fluid flows through outlet valve to reservoir.
• Pressure retaining mode: Inlet valve closed and outlet valve closed. Caliper fluid is retained by inlet valve and outlet valve.
• Pressure intensifying mode: Inlet valve open and outlet valve closed.
• Motor operation mode: When starting pressure reduction mode, pump motor is on. When stopping ABS operation, pump motor is off. Reservoir fluid is pumped out by pump, through damping chamber, to master cylinder.

When wheel speed drops sharply below vehicle speed, inlet valve closes to retain caliper fluid pressure. When wheel speed drops further, outlet valve opens momentarily to reduce caliper fluid pressure; pump motor starts at this time. As wheel speed is restored, inlet valve opens momentarily to increase caliper fluid pressure.