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 can control hydraulic pressure to each front brake caliper or both rear calipers independent of pressure produced by brake master cylinder. However, a higher pressure than that of brake master cylinder is not possible.

If lock-up conditions occur, ECU sends a signal to hydraulic modulator and an electric return delivery pump reduces hydraulic pressure by pumping brake fluid from brake caliper(s) to master cylinder. This procedures is repeated at either front or both rear calipers until lock-up conditions is eliminated.

The ABS system has 2 relays (solenoid valve relay and pump motor relay). Both relays are mounted on top of hydraulic modulator. Current is supplied to both relays from battery through fuse panel. ECU supplies a ground for relays when vehicle is in operation. This supplies a 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 a ground to energize individual solenoids.

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