Welcome Visitor! Please take a few seconds and No, in 1991 the computer is not involved with AC operation directly
Computer does get the 12v from the pressure switch so computer can increase the idle level of engine when compressor is on, so it is "aware" that AC is on
And computer will activate WOT relay on startup and at wide open throttle to cut 12v to AC Clutch
As for the purple wire, not sure
Ford DID use a purple wire in later years as the 12v sent to activate AC, my 1994 uses a purple wire
I have diagrams for 1991, but not 1992/3 so maybe they changed it mid-1991 to purple wire
But in any case up to 1997 that wire from the AC dash switch would run directly to the Pressure switch in engine bay, nowhere else, for sure
1998 and up the computer DID activate AC system, direct control
Look at the two wires on the pressure switch, the colors, one is from the dash AC switch, the other goes to WOT relay
Computer does get the 12v from the pressure switch so computer can increase the idle level of engine when compressor is on, so it is "aware" that AC is on
And computer will activate WOT relay on startup and at wide open throttle to cut 12v to AC Clutch
As for the purple wire, not sure
Ford DID use a purple wire in later years as the 12v sent to activate AC, my 1994 uses a purple wire
I have diagrams for 1991, but not 1992/3 so maybe they changed it mid-1991 to purple wire
But in any case up to 1997 that wire from the AC dash switch would run directly to the Pressure switch in engine bay, nowhere else, for sure
1998 and up the computer DID activate AC system, direct control
Look at the two wires on the pressure switch, the colors, one is from the dash AC switch, the other goes to WOT relay
