[tomj76]

The Windstar A/C system uses a dual action pressure switch to regulate the system and avoid "slugging" (where liquid refrigerant enters the compressor). Without this regulation the system would continue removing heat from the refrigerant until it nearly all would be in liquid form.

Other manufactures (i.e. GM) use compressors that vary the displacement with demand.

The dual action pressure switch performs in the following manner: When the system is off, the pressure will be above a certain minimum pressure. The switch will be "on" to allow the compressor to operate. This prevents the system from damage due to lack of refrigerant. As the compressor operates, the low side pressure (where the switch is located) drops. When it pressure is below the 'cutout' point, the switch opens, shutting down the compressor. Refrigerant continues to flow through the system as it returns to equilibrium. When the pressure is once again above the minimum pressure, the switch resets and turns the compressor back on.

The way you can tell that the system has a low charge is that the switch cycles quickly, since the low charge results leads to very little refrigerant in the liquid state, leading to fast changes in the pressure. The Ford service manual gives guidelines for the cycling time at a given outside air temperature.

FYI, the chemical used for the refrigerant in R-134A is also used for "canned air". While A/C users are strongly advised to contain any leaks, there are uses for the same chemical that intentionally vent it directly into the air.