On my 85 300 sd the problem was the ovp relay which is located in fuse box engine bay drivers side. Even though the fuse on top was good relay was bad. Not sure how to check relay one way would be to plug it in at a known spot that has current and see if it clicks or not. Some people shake and rattle them and not sure if you hear it rattle that means if it is good or bad. I guess compare it to a known good one.