I have always wondered about the fresh air control knob. The control shuts off the air to the fan when pushed all the way in, at half pull it let's air bypass the heater core, and fully pulled will bringnthe air thru the heater core. The adjacent control will control the fluid flow from the cylinder head for heat and we also have a defrost control. By adding a baffle and cutting a little, I was able to make a cabin air recirculating function instead of allowing air to bypass the heater core. While I had it apart I also replaced the blower motor with a 15amp motor. Now be patient, is gonna take a few posts. I'm still trying to figure out the forum and pics via a cellphone.