Auto A/C Question

I know this isn't exactly the right place to ask, but I'm sure there are a lot of similarities between household HVAC and automotive, so here are my questions:

Is it possible to get an accurate idea of how much refrigerant your system needs using gauges? So in other words if it's supposed to be 35 PSI, and it's 25 PSI, can I use that to top off the system, or is it a lot better of an idea to know that the system takes 2 lbs and just evacuate it, pull a vacuum, and then recharge it with 2lbs? (I do know that if the system is low that it has a leak that needs to be repaired first, but that's not the subject of this topic)

I ask because I've been trying to do it via the first method, and it seems like there is some amount of time you have to wait between adding freon, and getting an accurate reading on the gauges, presumably because you have to wait for the pressures to equalize between the high and low side. If that's accurate, how long would you normally have to wait? It seem to me that it can take 5-10 minutes before I can get a steady reading.

TIA

Yes, it is much like a home system. 1st you have to figure out what refrigerant you have and what the system should be charged at (there may be a spec plate on the compressor). If it is a newer car, it probably uses R134a. You can easily pick up cans of R134a at any autoparts store. The Hi-Lo ports should be either on the compressor or near the compressor. Typically, you just add to the system as needed.

If you have an older car, you can still recharge it, but the refrigerant is more expensive and harder to get. If you are an HVAC guy, it wouldn't be a problem besides the higher price. You could also convert the older system over to R134a. They do sell conversion kits for this. Ideally, you should evacuate the system to remove the old refrigerant and much of the oil. Replace the receiver/dryer. Replace with R134a and add the appropiate oil. The R134a doesn't cool quite as well, but most people I've talked to in hot climates said that it still did the trick.

If you don't already have one, search online for a Factory Service Manual (FSM) for your car. If you do any work on your car yourself, this is about the best tool you can have. This is what the dealerships use when troubleshooting your car and they contain all the specs, wiring diagrams, testing/installation procedures, etc.

I certainly know some of the basics. I know that the system uses R134A, I know how to use and own a set of manifold gauges, I know the ins and outs as to where the high and low side ports are, etc.

What I need to know is more about my specific questions regarding the options for recharging a system, and how long, if any delay there is in getting readings from the gauges. Someone suggested to me that my gauges were faulty because the readings took at least five minutes to level out. I'd like to know if that person is right, or if I am right in assuming it takes at least some time for the refrigerant to go through the system and equalize between the high and low side.

So as for the gauges, let's say my PSI is 20, and I put some refrigerant into the system, the gauges might read 45 PSI, within at least five minutes the readings will have come down and might now read say 30 for example (Obviously depending on how much I put in).

Maybe this will help:

http://www.aa1car.com/library/ac_recharging.htm

You can also find videos and other writeups if you google "r134a recharge".

The system capacity and the target pressures should be either on the compressor or in the FSM. If you think your gauges might be out of whack, most autoparts stores and even places like Walmart will carry a manifold gauge specifically for R134a cars. These gauges are inexpensive and are marked to show the pressure you are shooting for in an R134a system. Might be a good investment. I'm also unsure if your current gauge set has the correct fittings to tap into an R134a system (metric).

If your car A/C just needs a "tune up" , I have successfully used the kits from Kragens. It is a can of R134, with additives, and it comes with a hose and a guage. You just turn on the A/C, connect, and dispense until the guage reads well up in the "normal" area. Pretty basic, but it seems to work. Last summer, both my '94 truck and my '96 Ford Contour seemed to not be putting out really cold air, so I put in a can. Both are still working great!

It's not a cureall, but it's a cheap fix in many cases!