Modulator Fault (Latching)

MODULATOR FAULT (LATCHING)

WARNING LIGHT
ANTILOCK

DETECTION
This fault is detected during the drive-oft cycle, and anytime thereafter until the next ignition reset.

DESCRIPTION
The modulator is a group often solenoid valves controlled by the CAB to modulate brake pressure during antilock control. There are:

- four isolation valves, one for each wheel.
- three build valves, one for each front wheel and one for the rear wheels together.
- three decay valves, one for each front wheel and one for the rear wheels together.

During normal braking (non-antilock control), the solenoids are functionally invisible to the brake system.

Modulator power comes from the solenoid drivers in the CAB, which are activated by command of the CAB for antilock control. The solenoid drivers are powered through the system relay when energized. ABS can be disabled by de-energizing the system relay.

A modulator fault is detected when an open or short circuit is detected in any of the ten solenoids. The valves are tested about every half second. Upon detection, the ANTI LOCK warning light is illuminated and the MODULATOR FAULT is set. Antilock function is disabled and the lamp is illuminated until the next ignition reset.