how do overdrive transmissions function

Hi soux! Here's a site that explores how this all works. Hope it helps! Good luck! http://auto.howstuffworks.com/automatic-transmission4.htm

I have a 2002 ford ranger how does my overdrive work it and if the light comes on but goes off should I have my transmission checked

My 2003 Ford edge has a O/D button but doesn't seem to do anything,,,had this truck for 7 years,,,,always seems like it needs a forth gear or O/D on highway but the button doesn't do anything and there isn't any light that goes on or off