When the EBCM detects that a wheel lock condition will occur, an ABS event begins. During an ABS event, hydraulic pressure in individual wheel circuits is controlled in order to prevent wheel lock from occurring. A separate hydraulic line and 2 solenoids are provided for each brake circuit. The ABS can hold, decrease or increase hydraulic pressure in each brake circuit. However, the ABS cannot increase the hydraulic pressure in a brake circuit above the amount which is applied to the brake pedal during braking.

An ABS event may occur at speeds equal to or greater than 13 km/h (8 mph). During an ABS event, a series of rapid pulsations will be felt in the brake pedal. These pulsations are caused by changes in the positions of the solenoid valves as the EBCM responds to input from the wheel speed sensor(s). During an ABS event on dry pavement, the tires may make intermittent chirping sounds as 10-20% wheel slip occurs. The ABS solenoids and pump motor will also make an audible sound when operating. These sounds and pedal pulsations are to be considered normal during antilock braking.

The base brake system operation during normal braking should be no different than that of previous systems. Maintaining a constant pressure on the brake pedal without causing wheel lock will provide the shortest stopping distance while still maintaining vehicle stability.

When the EBCM detects wheel slip, the EBCM closes the isolation valve in order to prevent additional hydraulic pressure from entering the brake circuit of the wheel(s). Pressure in the brake circuit then remains constant.

If wheel slip continues after the isolation valve is closed, the EBCM opens the dump valve in order to decrease hydraulic pressure in the brake circuit. The fluid which is released from the brake circuit is stored in the accumulator and is later returned to the master cylinder or the fluid reservoir by the pump.

If the EBCM detects that the speed of the wheel(s) is too great during the pressure hold or pressure decrease mode, the EBCM increases the hydraulic pressure to the brake circuit by ensuring that the dump valve is closed and then opening the isolation valve. The increased pressure comes from the master cylinder and is directly related to the pressure applied to the brake pedal.

The EBCM commands the BRAKE indicator On when the ABS bulb check is performed. However, the EBCM does not illuminate the BRAKE indicator at any other time. For a more detailed description of BRAKE indicator function, refer to Brake Warning Lamp Description in Hydraulic Brakes.

The ABS indicator is located in the instrument panel cluster and illuminates within the following conditions:

When the ABS indicator is commanded on, loss of ABS function is indicated. The base brake system may function normally.

The export vehicle is equipped with an electronic brake control lamp relay which acts as a safeguard indication whenever the ignition is ON and the 10-way EBCM harness connector is seated improperly or disconnected. A constant ground is normally supplied to the relay through the EBCM. If this ground is lost for any reason, the ABS indicator illuminates whenever the ignition is ON. The ABS system may or may not remain functional if this occurs.