allison man We have a 2002 Mountain Aire on a Workhorse chassis with the Vortec 8.1 engine and Allison 1000 trans. After driving a distance we sometimes cannot back-up. The trans seems to come out of reverse and locks into third when I shift into drive; each shift is accompanied by a clunking noise. If the ignition is turned off then I am able to back up alright. We have had the coach into a Workhorse service center twice and a Allison service center twice with no success. Allison did send codes to the factory and told us that the problem was a signal coming to the transmission control module from an out side source. The coach is currently at our dealership where they have checked the converters, backup monitor and back up lights with no success. Any help would be appreciated.