where to start to look for the repair for transmission overdrive

I would say bring it to a tranny shop could be alot of different things going bad. Instead of throughing money after money trying to find out what the problem is

Is the tranny fluid up to the correct level? And what color is it? I think that is a lock up converter in that. The converter could be going bad witch would mean you should rebuild the trans. If the fluid smells burned and is black in color that means the trans has over heated and has flashed the fluid. Some times it will work to change the fluid and the filter. But if it was flashed changeing the fluid you could lose all gears

Check your overdrive wire going from button on shifter to plug in. My wire on my ranger was plum into replaced and workes fine now !