I didn't drive any of my trucks but I know on my 88 Burb with Dual air it does not work like that. It sounds like it's in Jet engine mode when the flap closes but still can control the fan speed.
HIGH is controled by that relay under the hood on the blower box. Actually it's all controlled there but the High setting is on a different circuit then the rest.
Wishe these were like my Toy with a A/C button and seperate inside out side control. I really hate that GM control. I can't make it work the way I want. I want Def blowing without A/C in the winter.
