Hydraulic Control Assembly - Antilock Brakes: Description and Operation

MODES OF OPERATION
The ABS modulator consists of the inlet solenoid valve, outlet solenoid valve, reservoir, pump, pump motor and the damping chamber. The modulator reduces the caliper fluid pressure directly, and it is also referred to as a circulating type because the brake fluid circulates through the caliper, reservoir and the master cylinder. The hydraulic control has three modes: pressure reduction, pressure retaining and pressure intensifying. The hydraulic circuit is the independent four channel type, one channel for each wheel.

ABS Modulator Schematic:

Pressure Intensifying Mode - Inlet valve open, outlet valve closed Master cylinder fluid is pumped out to the caliper.

Pressure Retaining Mode - Inlet valve closed, outlet valve closed Caliper fluid is retained by the inlet valve and outlet valve.

Pressure Reduction Mode - Inlet valve closed, outlet valve open Caliper fluid flows through the outlet valve to the reservoir.

Motor Operation Mode - When starting the pressure reduction mode, the pump motor is ON. When stopping ABS operation, the pump motor is OFF. The reservoir fluid is pumped out by the pump, through the damping chamber, to the master cylinder.

Wheel Speed And Modulator Control Schematic:

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