B2300 Mazda Pick up 1997. Blows hot air at all tmes except when place in max AC. Changed heather control val?

Sounds like your blend door is broken. This is an EXTREMELY common, and well known problem on that model year range of Ford Explorer and Ford Ranger, which happens to be the same truck with different badges. The symptoms are a "hit & miss" when the temperature selection is made. If this is the problem, you'll need to replace the entire heater box embly, since the blend door itself, is not serviced separately, which requires instrument panel removal for access. Hope this helps.