Need Help with OverDrive situation

Got a friend that's been going through alot of hard times lately and I've been wanting to help him as much as I can so i need some help from you guys. He has a 87 dodge d50, same thing as a mighty max but the overdrive is not engaging! It's an automatic and I know he's checked vaccum lines and changed the tranny fluid but no luck. Is there anything I could look for to see if it could be the cause. Tried searching but hadn't found anything yet. I've got pretty good knowledge of cars, especially honda's, but nothing of mitsubishis. Just need some advice!

sounds like a bad temp sensor