AC only blows cool air when driving?

I have an 01 Ford Explorer Sport Trac. The past few years I have had to add freon to my vehicle before each summer in order for the AC to work. However this year, it only works when I'm driving. I checked the gauge that comes with the can of freon to see if it was charged, and it was (between 35-45 psi). The compressor clicks every 2 or 3 seconds, and only blows warm air. I have asked people at automotive stores what could be wrong, and they all said it sounds like it's low on freon. When it's undercharged, the compressor doesn't kick on. But when it's between 35-45 psi, it engages and disengages too often. Also, the radiator and dryer have been replaced within that past couple of years. Any ideas?

For years and years, I've tried not to answer any question relate to AC system repair due to the knowledge requires as well as the complexity involve in the mechanical aspects of the system like electrical/ electronic, wiring diagram reading and understand it is a must skill that you have to have in automotive repair field. Etc.

The tooling is another one, it's not like with a few hand tools and you can do it; specially in the modern vehicle with all gadgets, bells and whistles and the last thing is the safety involve in the working with any AC system.

Keep all of these things in mind and do not try to work on anything if you're not sure how to do it or don't know how the system work or lack of equipment and proper tools to do the repair. SAFETY is job ONE. Ask any question if in doubt and I'll give you the right answer, if I can't answer the question, other members will chime in. Alright?

Now if the AC system recycling too often like you've said, that's telling me one of the following could be the cause for the problem your car has:

1- Low on freon due to a leak: This of course the leak (or leaks) has to be located and repair before the system can be put under vacuum to remove air/ moisture, recheck for leak before recharging to full level.
The following videos are for your viewing to understand how to work with, diagnoses and have a basic understanding the AC system:

How to read and troubleshoot with AC pressures and more:
http://www.aircondition.com/tech/questions/82/

This video will show you how to find and repair AC leak(s):

How to recharge AC system:

The basics of an AC system:

AC performance test:

2- AC system has high head pressure (As show on the High side of the manifold gauge): Poor or no air flow through the condenser, restriction in the system, too much freon (overcharging), cooling fans, fan clutch has failed, compressor has failed to build up pressure. Etc.

3- Pressure switches has failed (Low and High). Failed ambient temperature/ interior temperature sensors, wiring problem, evaporator freeze up (Evaporator temperature sensor, thermister, thermal bulb are other names) clutch relay has failed (Getting hot and cycling between on and off) Etc.

That is just a few things could go wrong, you'll need proper tools: Hand tools, special tools, a DVOM, scan tool to read for code(s), live data and actuate the components, switches, relays and the right wiring diagrams for the vehicle. Parts now a day are expensive and most electrical or electronic are non- refund or non return.

Hope I have give you a few ideas for you to see what you're looking at. If you're not really mechanically inclined and not familiar with the Explorer AC system or lack of tooling. I urge you to seek for professional help. HTH

Maybe it's time for a new compressor.

Maybe it's the thermostat?

I also want to know.