Anti-Lock Brake System: Operation

Each wheel speed sensor generates an A.C. signal, the frequency and amplitude of which is directly proportional to the rotational speed of the wheel being sensed. These signals are processed by the ECU when the brakes are applied with the engine running. In addition, the alternator generates a signal indicating engine speed and load. Applying the brake pedal generates a signal through the brake light simulation relay.

As the ECU receives signals from the sensors, it determines needed pressure increases, pressure decreases and holding pressures. These pressure variations are applied through actuating valves within the hydraulic modulator.

Each valve inside the modulator is capable of increasing, decreasing or holding hydraulic pressure according to signals from the ECU. During pressure decrease functions the hydraulic pump is activated by the ECU and remains in operation for 0.6 seconds after the valves close. This returns all excess fluid to the modulator and therefore repressurizes the ABS system.

Signals for hydraulic pump and modulator are obtained through a common overvoltage relay.