The AC compressor drags the engine down and there is a sensor (or several) that tell the computer to increase the idle speed when the AC is turned on. If it wasn't connected the computer wouldn't know to tell the engine to increase RPMs.
On the Zetec ('95 at least) there is also a sensor on the power steering line that runs over the timing belt cover. It tells the computer that the pressure is building which makes the computer tell the engine to increase the RPMs a bit to compensate for the drag caused by the power steering pump. My power steering sensor is broke due to [censored] wires that couldn't stand the heat! The wire leading to it broke when I was replacing my timing belt. I need to buy the plug and not the sensor.
DB
