A CRJ900 brake temperature sensor had previously been MEL'ed but the faulty sensor was not disconnected so it continued repeated false over temperature warnings.

Narrative

On taxi out to the runway; a brake overheat warning started for the left inboard brake. We stopped and proceeded to complete the Brake Overheat Checklist on the ground. Shortly after completing the checklist the warning cleared. We reviewed the dispatch release and noted that the brake temperature sensor for the same left inboard brake was deferred. Since the MEL gave no procedure relating to the sensor; we concluded it was a faulty indication related to the deferral. We departed and had a several repeats of the brake overheat warning which would subsequently clear themselves. Lowering the landing gear gave an indication that the brake had cooled. The brake overheat warning system then reset with reset button. The brake overheat warnings were due to a faulty brake temperature sensor which has previously failed the day before and was displaying no readings. That led to the deferral of the of the monitor system. The sensor was not disconnected under that deferral. Once the sensor was written up again on arrival maintenance examined the brake and disconnected the sensor from the system under a different sub-category of the same MEL. The aircraft was released my maintenance and was taken by a different crew. Maintenance via Operations should provide more guidance in the MEL for when situations like this arise.