Anti-Lock Brake System: Operation

Hydraulic system consists of master cylinder, delivery lines, hydraulic modulator, electric solenoid valves, return delivery pump and brake calipers. Solenoid valves are contained within hydraulic modulator. Individual solenoid valves are used for pressure control to left and right front calipers, while rear brake line pressure is controlled by a common, regulated rear solenoid valve.

Hydraulic modulator controls hydraulic pressure to each front brake caliper or both rear calipers independent of pressure produced by brake master cylinder. However, pressure cannot exceed master cylinder pressure.

If wheel lock-up is sensed by speed sensors, ECU sends signal to hydraulic modulator. Hydraulic modulator operates electric return delivery pump to reduce hydraulic pressure at brake caliper(s). This is repeated until lock-up is eliminated.

ABS system has 2 relays (solenoid valve relay and pump motor relay), mounted on top of hydraulic modulator. Battery voltage is supplied to relays through fuse panel. ECU supplies a ground for relays when vehicle is in operation. This ECU-controlled ground supplies voltage signal from solenoid valve relay to front-control solenoid and to rear-control solenoid. Based upon speed sensor signals, ECU controls solenoid operation by supplying ground to energize individual solenoids.

Return delivery pump motor relay closes when ground is provided through control unit. Pressure relieved by solenoid valve operation is returned to master cylinder by return delivery pump motor operation.