I have a 1989 Dodge D100. It quit shifting into overdrive today. It shifts smoothly into 1,2and 3 but will not go into o.d.. I am thinking it is the od unit but I have no idea where it is. Any help would be appreciated. It gets poor enough mileage without losing the overdrive.

Could be lots of things bad solenoid, blown fuse, bad wires, bad TPS. Does the OD switch work?