Compressor cycling on and off

[dpujara]

My A/C was blowing semi cold air so I thought the refrigerent may be low. I bought myself a can and a guage from Auto zone.
Before re-filling the refrigerent, I started the car and the A/C and hooked the guage to the refrigerent line. That's when I noticed that the indicator on the guage started at "green" when the compressor was off. When the compressor turned on, the pointer moved to the "blue", but as soon as it moved to "blue", the compressor turned off and the pointer moved back to "green" and this kept going on and on.
Please help with suggestions as to what might be wrong.

[DeltaP]

I dont know diddly about them cheap ass guages. I only work with real tools and equipment. But from what you seem to be saying it seems the refrigerant charge is low. Be careful not to overcharge it. If its got a small leak it can also be low on refrigerant oil. Now that can do some $$ damage!

[12Ounce]

Even tho I have two sets of service gauges, I also use the simple single gauge for a situation such as yours. I also agree that it sounds as though you are low on refrigerant. As suggested, a refrigerant leak will also leak lubricant ... especially if the leak is "low" in the system where lubricant is likely to be pooled.

Assuming your system is in fairly good shape ... that is, the leak is small ... most of the lubricant is still in the system ... and there is little air or moisture is in the system. If all of this is true .... its OK to add some refrigerant. (Don't ask me how one would determine if the system is "in fairly good shape" ... its more trouble than its worth. Most folks don't do the correct thing ... which is: Fix the leak!)

You are reading the gauges correctly ... its the "when" to read that is a bit confusing. All those colored areas are to be used only when the compressor is running .... and at the lowest reading. This means, for your system now ... just before the compressor clutch drops out.

Just keep adding refrigerant until the compressor stays in .... with cabin fans on "hi" ... and doors and windows open.

[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.