What is wrong with my car's air conditioning?
 

I have a 1991 Toyota MR2 (SW21) and the AC is behaving very oddly. The AC works perfectly, so long as it's turned OFF. When I push the button labelled AC the button lights up green the engine idles up an extra 1k RPM and the AC stops working; cold air stops an hot air replaces it. When the AC is turned off the air coming out the vents is significantly colder than the outside air, so I know I'm not imagining it. I have yet to check if the air coming out the vents when the ford vcm 2 is on is hotter than the outside air, but it could be, it was hot.

I'm really curious why this is happening.