'98 Lumina A/C performance...

My wife recently got a terrific deal on a mint '98 Lumina, which she purchased at an auction for her company's older cars ($625!!). The car has high mileage (138k), but it looks and runs GREAT.
Last week she complained that her a/c had started to blow warm air. Upon checking it out, I noticed that the compressor clutch was not engaging. I thought it might be due to low coolant level, so I checked it at the low-side port with an a/c pressure gauge: it read 44psi, which I believe is just about what it's supposed to be @ full charge. So I assumed an actual low coolant level wasn't the prob.
Next, thinking it might be a defective pressure switch which was incorrectly "telling" the compressor clutch that the coolant pressure was too low, I attempted to "jump" any 2 of the 3 wires together to see if I could effectively bypass the switch and "fool" the compressor into engaging. No go. Of course, I didn't have a wiring diagram, so I was really just shooting in the dark here.
After checking all the involved electrical connections for continuity, checking the compressor fuse, swapping the compressor relay for the identical fuel pump relay, and a bunch of wire-jiggling and other general mucking about, I finally gave up on getting the compressor to engage. I put everything back together, and then went inside for a well-deserved Corona w/lime.
OF course, later when I started the car to move it outta my work area, the compressor started to engage and dis-engage correctly as I worked the interior AC controls - although it was still only blowing hot air. Now with the compressor running, I checked the low-side pressure again, and this time it read only around 24psi.
At that point, I decided to take a $6 chance & try adding a can of R-134a. The system took the whole can, and the pressure now read 44psi with the compressor running. Gradually, the system started blowing (somewhat) cold air.
I thought I was outta the woods, but after using the car for a few days in 95 degree/105% humidity weather, my wife tells me that the a/c isn't NEARLY as cold as it used to be - she says now needs to have it on MaxA/C, fan=4 to cool things off. On checking it out, I had to agree with her: it's pretty lame.
Any suggestions? Should I bleed and/or purge the system entirely and then add 3 new cans of R134a? Would any of those additives that I've seen (ArcticFreeze, MaxiFreeze, link below)work to lower the cooling point, given that it's a hi-mileage car with a hi-mileage compressor and evaporator?
http://www.id-usa.com/catalog.asp?CID 
Any help will be MUCH appreciated! As you can probably tell, I'm not a very accomplished weekend mechanic...
JohnB
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Your system needs to be properly evac'd, vacuum'd, cleaned, and refilled, then leak checked, before you can make any assumptions.

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My opinion for every A/C related problem is to take it to an A/C shop and have them look at it. But that's me, it's just one of those things. I take it to the A/C shop and get them to tell me what the problem is, then I fix it. Maybe it's because I'm in Canada and any A/C work has to be done by the pros, A/C equipment is just not available to the DIY'er.
Steve

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You sound pretty smart, don't add any more refridg, have the system evacuated, change orifice tube, have it recharged. Use a good thermometer, write down air temperature, check pressure engine off, and again with compressor running [ low side ]
Now you have a baseline for recharging, take temp / pressure readings as weather changes. Now you know, if it's X out, my guage should read Y pounds .
I urge people not to slam more refrigerant in, I run my old systems a bit low, so they don't blow up.
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You guys were exactly right: I took it to my local mechanic (who does a lot of A/C servicing) and he determined that we had a clogged orifice tube. He then did an evac/vav/fill/leak check and it works fine now. This would explain why the pressure was all over the place, and why the compressor would only kick on intermittently. Just slamming more R134a in there would've done no good.
I guess with certain things, like A/C, it's sometimes just easier to bite the bullet & go to the experts.
Thanks again, JohnB
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I had some morons (A/C technicians) replace my compressor and refill the stystem. It had the same problems you described. He checked the presure on high side and it was fine, low side screwed up.
To me, standing there, it was obvious, something was blocking it. I asked them if they had replaced the $1.85 orifice tube like I had asked them do and they assured me they would, and they said yes. I went for a walk, and came back right away, and caught them replacing the old crummy orifice tube! It worked fine. But they were sullen and didn't pump it down long enough and thought they'd wasted too much time already and were pissed they got caught in a lie and a cheat and incompetant and so it runs fine now, but not as cold as it should. Ass wipes. With a few tools and a tiny bit more knowledge I could have done it all myself and done a good job instead of the shitty 'professional' job they did.
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