What could cause a 1991 Lincoln Town Car to stop running when you turn on the AC?

A dirty or defective idle air control valve (IAC). When you initialize the a/c, it sends a signal over to the Power Control Module(PCM)(Computer) and it energizes both the a/c clutch, and the IAC to raise the idle. If not, then the compressor is bad. The IAC valve(MOTOR)just needs 2 b cleaned (after it's off) with a Good Carberator cleaner-clean it until the cleaner runs clear & the pintel valve -inside-moves freely.