I'm converting my '93 chevy no option work truck into a Silverado. Swapped in a '05 LS 4.8L and added cruise. Now I'm adding air. I bought a '89 Silverado for a donor truck for the heater box and the controls. I'm using a Sanden A/C pump. My question is how do I get the recirculating door to close. I'm assuming it should be closed during A/C. From the wiring diagram I have it looks like it activates from the air inlet pressure switch that's in the back of the original R4 pump through the A/C heater contoller to the inlet air valve motor. If I'm correct and I'm not sure that I am, using the Sanden A/C pump, do I need to install a pressure switch somewhere in the system and wire it into the A/C heater controller or am I way off ? Thanks for any help.