AC cold to warm in an hour

I have a 2003 V6 xcab with auto transmission with 114k.
My operating temp will drop to almost 30 degrees and hold for 30 minutes or so.
With in an hour it will be 60 degrees. Also the volume or speed of the air out of the vents seems to drop. My mechanic fixed .25 lb low on freon and a leaking schrader valve.
But it is still doing the same thing.
If I shut off for a few hours it stars the process all over.
Also I notice that when i turn control to defrost the ac works , even if then button is off

 

First off, the Defrost activating the A/C is normal for every A/C system.

Second, assuming the pressures are good... Have you checked with the mechanic who repaired the A/C in the first place?

It could be an accumulation of junk between the A/C Condensor and the Radiator.

 

All cars with AC turn on when the defrost is activated, the vehicle is using the AC compressor to create dry air to blow on your windshield, yes it's cold... but it's because it's dry that it does it.

Is the condenser free of debris? If the system overwork itself from not being able to dissipate the heat you'll get the symptoms you're describing

 

You type faster than I do

 

if the air discharge out of the vents is 30 degrees that is too cold. Off the top of my head, there is a possibility that the evaporator has frozen over and you have to wait until it thaws. When it stops working check the evap drain to see if it is pluged or is still draining.

 

i would double check if your system is filled correctly. too high or too low gives some erratic HVAC issues.

 

Definitely take it back to the mechanic and tell him: "you know that thing you said would fix my problem? It didn't fix my problem and I don't think I should pay for that."

 

Did he replace the Dryer as well? - Sometimes, you will never remove all the moisture that accumulated in the system when there is a leak (though it sounds like yours didnt totally evacuate) and some gets stored in the dryer. Purge and fill doesn't always result in factory like results.

 

Sounds like the evaporator coil is freezing up. Have you checked your condensate drain?

 

The thermistor on the face of the evap has failed allowing the evap to ice over as other suggest.

The evap should operate about 34-36* then the thermistor shuts off the compressor till the temp rises to something around 40*, then allows the compressor to engage again.

The ice that forms stops air flow. You can make it last a 'little' longer by selecting outside air instead of recirc until you get it fixed.

 

Thanks for all the input . I will look into these suggestions and get back to you. Good news it does not sound like it will be too expensive.

 

the average temp is 40 so I always run outside till it warms up.
I will have to look up where the evaporator thermistor are located