Question : AC needs help

[Jesster]

I have a 98 Lumina with a 3100. I replaced the LIM gasket late last summer and, at that point, I didn't need to use the AC. We are starting to get the hot weather and I found my AC is not blowing cold. I figure top it off as your supposed to run the system 5 minutes a month to keep the seals in good shape, I didn't run it this winter. Prior to charging, I decided to check the low side pressure to see approximately how much I need to add. I have checked with two different gauges to be sure, but the pressure is 50-52psi (depending on gauge) at an ambient temperature of 80 degrees f. My reading of the gauge 1 information says I should be at 25-45psi @ 75-85degF for proper fill; the gauge 2 information says that I should have 40-50 psi @ 80degF for proper fill.

The high side pressure is 52psi, but should be 175-250psi @ 80degF. On the gauge's troubleshooting information, it says if the low side is running high and the high side is running low along with a wildly vibrating gauge needle would be the reed valves in the compressor and the compressor needs to be replaced. I didn't notice the gauge needle vibrating at all and really was consistent.

Could this be a bad compressor (it doesn't make any rattling or whining noise, it sounds normal) or is it something else-->dryer, evaporator, clogged orifice tube? I have not opened the system at this point. Any suggestions would be appreciated.

[95SleeperAcheiva]

Like you know, low side pressure can be effected by ambient temps. Also, keep in mind, low Freon can also show a higher pressure in the low side and adding Freon can drop your pressures to where they belong. But, lets back up a bit.

An A/C system works by changing Freon from liquid to gas and back to liquid, hence the names of evaporator and condenser.

This change in state and temperature produces moisture/condensation. A low system has a leak somewhere; where Freon can come out, atmosphere can go in, atmosphere contains moisture. The temps that Freon operates inside, freezes said moisture and can literally clog the system. "How do you get it out?" rather simple.

So few things. If you're trying to get off cheap and I think you can considering the system is holding some pressure, we can get away with a quick recharge and worry about whats leaking later. Otherwise, if you have to replace anything, hose, condenser, etc, replace the dryer/accumulator. They are usually cheap, about $20, unless this car is asian, then it's usually one piece with the condenser from age and expect it to run you in the $300 range. (bet you wish you bought american now, for those foreign car owners reading this)

Harbor freight is your friend! For $15 you can purchase an air compressor operated vacuum pump, works great! This is a MUST!
Hook up your gauges to high and low, open both of them at the quick disconnects, put the pump on the yellow hose, open low side valve at gauge, plug in air line to pump. Let that suck it down to 29-30 Hgi (vacuum).
I've used a shitty craftsmen 20 gallon compressor on this for years, sometimes I had to close the low side and shut off the pump to let the air compressor catch up and do it again.
Once the pressure is down to 29-30 Hgi, close the low side at gauge, disconnect the pump and wait min of 20 mins, optimal 2 hours.
Here's whats going on inside the system. Any moisture in the system will boil off, just like water in space/vacuum, no pressure and its boiling point drops. The pump then removes this, so it maybe necessary to run the vac pump again in about 15 mins. And/or you may have a leak and are pulling air into the system. So check you gauge in 15 mins and fire up the pump again if it dropped vacuum. Check your gauge again in another 15-20 mins and see if you lost vacuum again. If you did, you 2 choices, charge the system anyway and hope it lasts a few months or live without a/c. If it holds vacuum, let it sit for 2 hours optimal, 45 mins is about what we do in the shop.
The next step (if it has a leak), doing it right with expensive tools, would be to charge the system with nitrogen or inert gas and go around with an electronic sniffer looking for the leak. Staying cheap, lets do some math. On your cowl somewhere will be a sticker that tells you how much R-134a your system holds. Figure it out to ounces, 16 ounces to a pound, and buy the correct amount of 12 oz short and squat cans of Freon w/ dye. DO NOT ADD ANY FREON WITH LEAK SEALANT! It clogs evaps and condensers. Next start the car, open all windows, turn on a/c, full blowers, recirculate on, and put on vents. If you have a meat thermometer style gauge, drop it in the center vent. Put a can of Freon on the yellow hose, fill the gauges, and open the low side on the gauges, in a few seconds the compressor should kick on and start to flow the Freon. Most cars take 2-4 cans of 12 oz Freon. Over filling the system will NOT make it blow colder and can actually make it blow warmer.
During your 3rd can or the can that will bring you up to you capacity, Try to hit your proper low pressure gauge ranges for your ambient temps, that you have info on already. I open the low side for 1-2 second counts and close, wait to stabilize and see what I got till I hit my pressure mark. Let the system run for a good 20-30 mins. Certain things should be happening at this point. The compressor should be on full time, no cycling. Both of your radiator fans should be running full tilt. And high pressures should also be in range. IE low at 25-35 high should be 175-225 (assuming ambient temps are 75-80 deg F).
On your meat thermometer you should be 30+ degrees below ambient temps if the system is working 100%. So ambient temp is 80 your air should be blowing 50 degrees or colder. If not, check things like externally clogged evap, dirty cabin air filter, if you have one. If she is blowing hot and your pressures are off, then we have another problem somewhere else and the list is long. Detail that and we can go from there.
Good luck.