ABS modulator valve

The clicking noise from the wheel end on the Freightliner truck in the shop is the ABS modulator valve running a self check on its self to check the operation of the relay. This is a Bendix part and the part number is 550414 and its a M-21 modulator. This modulator works by getting a signal form the wheel speed sensor, when the wheel speed sensor senses the wheel is about to lock up, and it exhaust the air pressure going to the air can on the wheel that is going to lock up.

During normal braking application both the exhaust and supply solinoids are deenergized. when the brakes are applied air pressure, going to the service brake chamber, seats the exhaust diaphram on the exhaust port along with the aid of spring pressure. When wheel lock up is detected the antilock controller simoltaniously sends power to the exhaust and supply solinoids. Power is sent to the exhaust solinoid to open the exhaust diaphram and give the air from the brake chamber a path to escape to the atmosphere, so that there will be no braking force on the wheel that is about to lock up. Power is sent to the supply solinoid to close the supply diaphram so that air wont be allowed to flow to the brake can.