Wife's 2002 S10 Blazer started blowing warm air. With the switch on max the compressor is cycling on and off every 4 or five seconds. I've got one of those cams, the gauge indicates that the charge is correct. I can feel a slight vacuum on the low pressure port when the compressor kicks in. Thoughts?

 

I let mine run so low on coolant I had to jump start the compressior to get it going again so I could put some in. Don't wait and let it go that low.
I'd recommend a half can or less. Then get your laser temperature gun and check the temp at the vents. Should be in the 40's. . I added a full small can to mine and it is cooling at 45 degrees when it is in the 80s outside.

 

You can seriously damage your AC by just adding some R134a. If you don't have the right tools you should have it checked. Mine stopped working because of a leak. I had the leak fixed and had it refilled by a professional shop. Then it stopped again and it was an $11 pressure switch. If it is full and it is just the pressure switch (they are know to go bad) and you add to much refrigerant you can stress the compressor or blow out pressure lines.

 

Is that gauge only on the low pressure side and is it indicating the correct pressure when the compressor is cycling or not cycling?

If you're only using a low pressure side gauge and the pressure is reading "normal" while the compressor is NOT cycling, then you are low on freon and have a leak somewhere. Also there should not be any physical suction on the low side port even when the compressor is cycling; there should be some sort of positive pressure.

I cannot recall pressure numbers exactly, but when the compressor is cycling (clutch engaged and spinning) is when you should have the "normal" or "full" pressure on the low side. When the compressor cycles off, the low side pressure will shoot up high as the pressure equalizes in the system and that is completely normal.

I am by NO means an AC technician, but a lot of people make out automotive AC st stems like some sort of mystical magic machine that if you touch it in the wrong way it's going to grenade into a thousand pieces and it'll never work again unless you pay someone hundreds of dollars... Truth is, it's a rather system system, that once you understand the basics of, isn't that hard to diagnose and repair yourself. When I bought my 2004 it had a locked up AC compressor because someone didn't know what the hell they were doing and jerry-rigged the compressor to always be on when the blower motor was on, probably because there was a leak somewhere and the pressure got low, the pressure safety switch wouldn't let the compressor kick on to save the system from destruction, and whatever moron did the "modification" bypassed that pressure switch, ran the compressor with no oil (freon carries the oil for the compressor, low/no freon = no oil delivery, which is why the low pressure switch disables the AC when the freon is low), which as you can expect cause the compressor to lock up solid... So I myself, in my drive way, in humid Florida in the middle of the summer, replaced the compressor, ALL of the lines, the condenser, and dryer/accumulator. I used a cheap DIY flush for the evaporator instead of tearing the original out, NEVER vacuumed the system down for leak test, the compressor was pre-oiled, all I did was assembly and recharge with over the parts store counter R134a cans and a cheap low pressure side hose. 3 years later she's still blowing cold as ice, colder than my wife's MUCH new Elantra lol, and hasn't given me a lick of issues. So don't let anyone tell you that you HAVE to take it to a "special" technician to do the work... With a little reading and understanding of the system, basic hand tools, and a little time and patients, it is VERY possible to do maintenance to your own AC system without forking out a small fortune. I grantee you to have a shop replace all the parts I did and fix my AC, it would have cost me well over $1000. I'm in it $300 BUT, if you're not mechanically savvy or simply don't want to invest the time into doing it, then by all means, pay someone else to do it... it's your money...