A/C compressor runs when A/C is selected in cab or when Defrost is selected, A/C is used to dry the air before it is blown onto the windshield, this drier air makes defrosting work faster.
Front part of the compressor is called it's clutch, it has two wires connected, one is a ground the other runs from A/C control in the cab, when you select A/C or Defrost that wire will have 12volts and that is what activates the clutch and engages the compressor.
That wire passes thru 2 switches on the A/C refrigerant lines, one is a Cycle switch the other a pressure switch, but both monitor pressure, if both these switches are closed the the 12v passes to the clutch, if either opens 12volts is cut and the clutch disengages, this could be from temporary high or low pressure which is normal since pressure IN the system is effected by outside temperature.
The Clutch has an "air gap" that needs to be in a specific range or clutch will engage and disengage randomly, to test for this simply test voltage at the 2 wires on the clutch while engine is running and A/C on, if 12volts is present and clutch still disengages then "air gap" is the issue, not uncommon on Fords, if 12volts goes off when clutch does then "air gap" is good, the "system" is cycling the clutch off and on.
On the front of the radiator is the A/C condenser it will often have an electric fan, this fan is needed to keep the temp of refrigerant down, it is usually only on when you are stopped and there is no air flow, if this fan isn't working or the fins of condenser are getting clogged up, limiting air flow, then one of the switches may detect high pressure and shut off clutch until pressure drops.
Last edited by RonD; 04-24-2015 at 11:26 AM.