Jeep Repair: 1997 Jeep A/C Problems, refrigerant leaks, jeep sport

Question
Hi Russell.  I have a 1997 jeep sport 4x4 auto trans and 4.0 6cyl. I am having problems with the A/c working intermittently.  It will work fine when I start the jeep up after it has set and cooled for  a long period of time or overnight but when I drive it and it sets for a while like at the grocery store or to shop somewhere, the a/c will not work and blows warm air out.  I have replaced the relay for the a/c and also have replace the high pressure switch on the a/c line nearest the a/c dryer an it helped in that the a/c runs for a longer period of time when it runs but like I said, after it sits for a while and when the engine has warmed up, it won't work at all.  When I raise the hood the compressor clutch is also not engaged.  I have had it checked for refrigerant leaks 2 times by different mechanics and they can't find any leak by using dye.  I have also watched the coil under the dash to see if it freezes up and it doesn't.  The mechanic has also purged my a/c system and refilled it and it doesn't change any thing. Is it that I just have an old a/c system and it needs a total replacement?  The vehicle is a 97' and not worth much.  Just trying to get one more summer out of her before she is sold or traded.

I appreciated any advice you could offer.

Thanks for your time.
Scott in TN.

Answer
There are two likely causes for your A/C system to stop working as you've described. The first would be a failing Low Pressure Switch, which is located on top of the accumulator/dryer assembly. The second would be a failing Oriface Tube which is in the A/C line which runs from the evaporator to the condenser. To replace the tube you will need to replace that line as it is not replacable seperatly. I'd like to know what the pressure readings are just before the system quits before recommend which component needs replacement.