General Description

The anti-lock brake system prevents locking of the wheels under heavy braking. A car with locked wheels cannot be steered. The anti-lock brake system controls the application of the brakes, allowing the driver to maintain control of the steering.


Under anti-lock conditions, the anti-lock brake system modulates the pressure of the brake fluid. The system has three channels: the pressure at each front caliper is controlled independently, while the rear calipers are controlled jointly. Whenever a wheel is likely to lock during braking, the anti-lock brake system modulates the brake pressure in that circuit. The modulation of the brake pressure keeps the wheels from locking. When the brake pressure is reduced to the point where there is no longer any possibility of wheel locking, the system returns to the conventional braking system mode of operation.


Battery voltage is applied at all times from fuses ABS B1 and ABS B2 to the ABS control unit. When the ignition is in RUN, battery voltage is supplied to the control unit from fuse 17. The control unit is case grounded. Inputs are received from the brake light switch, alternator, the individual speed sensors located at each wheel, and from the ABS pressure switch. With the charging and brake systems functioning correctly, the control unit can activate the tail-safe relays, modulator solenoids, and ABS motor on demand. The anti-lock brake control unit uses the inputs from the brake switch and wheel speed sensors to control the modulator solenoid unit. The solenoids adjust the hydraulic pressure applied to each front caliper, and both rear calipers.


The ABS control unit has a self-diagnostic function. When the self-diagnostic function detects a fault, the control unit turns on the "anti-lock" indicator and disables the anti-lock brake system. If the fault is not in the conventional braking system, the brakes will continue to operate normally but without the anti-lock feature.