Originally Posted by Mjay
Not sure on your coach, but a lot of coach's and semi trucks the A/C compressor will engage when the defrost is selected.
This helped the defrost mode by removing moisture out of the cabin air.
Almost all vehicles work this way(AC on when on defrost)
regarding OP post if your experiencing this AC on no matter what position then its possible wired wrong or its not switching positions.
On a side note on my Allegro Bus there is a button with snow flakes on it that I have to push to disengage the AC compressor