The AC starts out nice and cold then after about 20 min it's just cool and humid and finally turn hot. If I turn it off for a while or turn off the engine, it's cold again when I start it up.
I added one can of freon (bringing the low pressure side to about 35 psi not correcting for ambient temperature) and it's better but not constantly cold.
I can hear the compressor come on when it's parked.
It's an '01 with a little over 200K miles, no A/C work ever performed on it.
Any ideas?

 

Does your blend door work?

 

Meaning I can adjust the thermostat? I can try it tomorrow. I usually keep it set a max cold because the heater core is leaking.

 

Eh, something like that.
The blend door changes the amount of heat and cold you're getting through the vents.

 

Does it work off vacuum? If so, that would fit what I'm seeing. If it does work off vacuum, is the hose easy to get to? I have a vacuum gauge.

 

It's an electric motor.

You might have wild vents, do a search here on DF for more information.

 

Try measuring and adjusting the air gap on the compressor. SHould be less than .55 if I remember right.

 

^^^that. Also make sure it's draining properly and not backing up. Does it happen when you go down the road at a constant speed for a while without stopping? Make sure it's not freezing up also.

 

Fixed the same issue on a friends truck, he installed 2 new check valves from the dealer.

 

I did a little more troubleshooting...
*The air blows out the right vents when I change the selector, even after it stops cooling.
*It stayed cool almost all the way home today (25 min). The last 5 minutes it stopped cooling. I left the engine and the A/C on when I parked in the driveway. The compressor was NOT engaged.
*I turned off the A/C and the engine. About 30 minutes later I started the engine and turned on the A/C and the compressor engaged right away.
*My commute is mainly highway/constant speed. Once it stops cooling, it doesn't start cooling again until I turn off the A/C or the engine for a while.
*I don't believe it's clogged or freezing up.
*Can you elaborate on the air gap? Is it the space between the pulley and the compressor housing?
*I'm thinking it's a bad compressor switch or a short in the wiring to the switch.
What do you think?
Thanks