I have an 89 bronco. A/C works but goes to defrost at 30+ mph then goes almost off after that till I slow way down then comes back on. Any help would be awesome. Also it has no overdrive switch in it. Is this normal?

 

I would check for a vacuum leak.

 

In 1989, the only Ford 4 speed automatic was the AOD, which was mechanically shifted, not electrically. No overdrive ON/OFF switch. To select non-overdrive operation, i.e., only gears 1-2-3, pick the proper DRIVE shift quadrant position for it. Same for allowing O/D (4th gear) operation. The "allow O/D operation" position usually is a "D" with an "O" around it. The non-overdrive position is just a "D".

Another possibility is that you have a 3-speed automatic, the C6. In which case the shift quadrant only has one "D" position.