The ABS indicator Lamp Relay is a normally closed relay. The ABS indicator is always on unless the EBCM energizes the relay coil and the relay contacts are opened. This logic ensures that the ABS indicator lamp will always be illuminated if there is a malfunction with the EBCM.

The Air ABS provides individual wheel braking control by using an electronic brake control module (EBCM), four wheel speed sensors and four air brake modulators. The EBCM monitors the rate of deceleration during braking and subsequently adjusts the brake application pressure at each wheel. The ABS will provide improved steerability and vehicle stability.

The rear axle brakes are controlled completely independent of each other. The brake application pressure at each individual wheel is adjusted solely on the basis of its behavior on the road surface on which it is traveling.

While each steering axle brake is under the control of an individual modulator, the EBCM does not treat these brakes totally independent of each other. The EBCM will control the braking force differences between the two brakes in order to minimize steering wheel pull, in the event each wheel is traveling on a different road surface.

In the case of vehicles equipped with tandem rear axles (6×2 or 6×4), the wheel speed sensors are installed at the wheels on the axle that is most likely to lose traction first. A single modulator controls both curb side brakes on the tandem, and another modulator controls both brakes on the drivers side of the tandem.

Since the brakes will likely be on the same road surface, both brakes on one side of the tandem are treated as one with this arrangement.

During normal braking, air pressure from the brake valve enters the control port of the service relay valve (either an antilock or standard service relay valve). The service relay valve delivers air through the ABS modulator located near the brake wheel and into the brake actuator.

If a service brake application is made and the wheel speed sensors detect an impending lockup, the EBCM will immediately begin modification of the brake application using the antilock modulator(s) at the affected wheel(s).

Solenoid valves contained in the modulator are energized and de-energized by the EBCM in order to modify the brake application. When a solenoid coil is energized, the solenoid valve will either open or close, causing exhaust or re-application of air pressure to the brake actuator. The solenoids in each modulator are controlled independently by the EBCM.

The EBCM will perform a self test which continuously checks for proper operation of the entire ABS. In order to alert the driver of a malfunction with the ABS, the EBCM will illuminate the ABS indicator lamp (located in the instrument panel).

In order to assist the technician locate a malfunction with a specific ABS component, the EBCM will illuminate any diagnostic status lamps associated with the malfunction (located on the side of the EBCM housing).