joyful_sorrow

My Mustang all of a sudden will not go into overdrive. The O/D off light has been flashing for a long while but it recently started acting insane. It has been shifting really hard lately but now wont go into overdrive. When you drive it at first it'll shift normal then when its time to go to overdrive it just revs up high. We recently flushed the transmission and changed the filter and the fluid was not burnt. It looked and smelled normal. We have not taken it to have codes ran on it yet. A friend told us it could be the modulator valve but I cant find anything about a modulator valve on a 2001 mustang online. I dont know if it means anything but I have an exhaust leak at the manifold. Any help would be greatly appreciated.