1990 Dodge Dynasty won't shift into over drive

Usually when your vehicle's computer system detects a problem with the transmission, it will go into protective mode which is no overdrive. This tells you that there is something wrong with the transmission or a component for the transmission. In most cases, it is simply a sensor (i.e. speed control sensor)on the transmission . It could be more serious such as a pressure problem though. Your local transmission shop can tell you if it is any of the sensors. Prolonged driving in this condition can harm the transmission severly so get it diagnosed as soon as possible.