I have a Troy-built lawn tractor (model D609G, G609G, U609H or X609G series) which when the shift lever is engaged to FWD or REV it will not move. Is it possible the belt off the Transmission Idler pulley is loose or the cause of the non-movement? What is it that controls the engagement of the transmission? I looked at the belt and it seems intact but I didn't do a deep inspection of the belt... I thought it was still intact. What would be the most likely cause of this so I can pinpoint the problem?