no air ... checked pressure in system ...low when no compressor, jumps up when comp clicks on ...stays on for about 4 seconds, turn off right after that , presure drops down...

wht could it be...?

 

What's the two pressures (High/Low)?

 

I used the gauge on the freeon applicator bottle...it was low when the compressor was off (almost nothing) ... but went up to the edge of the "safe" area when the compressor kicked on ....

 

So you are using just a low pressure gauge? If you are using a consumer type "recharge" product that comes with a tiny pressure gauge then the gauge is going to be as reliable as a shady political administration.

Depending on the temperature outside the pressure should be within a certain range (something less then 100psi). It really helps to have a gauge to monitor the high pressure side as this reading is more constant even if the compressor is running.

Normally when you check the AC you need to idle engine for a few minutes with the AC controls on maximum. In this mode the compressor shouldn't cycle unless something is wrong. Typically it is low refrigerant. If you are going to be serious in diagnosing your AC it would be best to try to borrow some professional style gauges to get a better idea of your issue.

 

I am not telling you that you dont need a set of guages but I was going through the same issue. compressor off= low guage reading, Compressor on= high guage reading. My compressor would kick on and off less than every 5 seconds. So I just hooked up my can of r134a and opened it up until the compressor kicked on and stayed on- engine was as idle and full operating temp. At that point the cheap little guage was reading 20 psi and the air in the cab was getting cooler. I left the can on and basicly emptied it to get to 30 psi. I believe I need more freon but its cold for now. A good set of guages is necessary and I wont tell you not to get a set, you can buy them at autozone also. Or just take it to a mechanic and let him tell you that it is low and needs to be charged if you are worried, then thank him and take it home and refill it yourself.