The idea behind this mod is to be able to control the A/C Compressor with a single switch. This duplicates the blue “A/C” switch that is found in many automobiles. You can then leave the A/C control in the MAX position, and re-circulate chilled air without running the compressor. Basically, this mod will cut circuit 348 (A/C demand signal) which tells the PCM (Power-train Control Module) that the A/C control panel is in a selection that uses the A/C compressor. This mod does not interfere with the PCM, it just interrupts the signal send from the A/C panel.
This procedure was researched using the Ford service CD and has been verified by technicians who work at Ford dealerships