From the Allison Site
Q. Automatic Grade Braking - What happens on ice if it downshifts and slips?
A. Grade Braking monitors the front and rear wheel speeds/slips and can determine if the vehicle is slipping. If this occurs, grade braking will be exited and the transmission will up-shift to the normal range depending on throttle position and output speed.
Could be an axle sensor issue? Just a thought.
Added on Edit:
Allison FAQ Site.
http://www.allisontransmission.com/s...tegoryID=11#85