Once you've finished your repairs, you'll need to evacuate the system and recharge it. To do this, you'll need a vacuum pump and a set of AC gauges. You'll also need the correct amount of refrigerant for your application. Many over-the-counter AC recharge kits come with a gauge on the can that can be used to monitor the low-side AC pressure when charging the AC system. This can work, but it's not the ideal way to do a recharge, as the high-side pressure is equally important for proper operation. For that reason, I recommend a full gauge set when recharging an AC system. Besides, having that tool around will help you with other AC problems that might pop up in the future.
The first step is to evacuate the system. You do this by hooking a vacuum pump up to the system and letting it draw the air out of the system for about 30 minutes. I then recommend you let the system sit for another 30 minutes while you watch the gauges. If the system still has a leak, it will begin to loose vacuum. If the system can't hold a vacuum, it certainly can't hold pressure.
This is a great test to perform before you commit to adding refrigerant. This way, you avoid wasting refrigerant on a leaking system. The system should be able to draw about 29inhg when under vacuum. If it can't reach that level, you likely have a leak somewhere. Find and repair the leak, then pull a vacuum on the system to see if it's been repaired. Be sure to check the service fittings, as this is a common leak point.
In addition to testing the system, putting a vacuum on it also helps remove any moisture that might be left over from when the system was open to the atmosphere. Moisture is always present in the air, especially humid air. The AC system must be free of moisture in order to operate efficiently. Vacuuming out the air in the system is the best way to remove this moisture and ensure the system is intact and ready to accept a refrigerate charge.
If the system is able to hold vacuum for at least 30 minutes, you're ready to start adding refrigerant. You only add refrigerant to the low side of the AC system. This helps draw in the refrigerant and prevents damage to the compressor.
- Start the engine and turn on the AC.
- Open the low-side pressure port on your AC gauges. The center hose should be hooked up to your refrigerant supply.
- Once the low-side port is open, the refrigerant will be drawn into the system. At some point, the AC compressor clutch should kick on and the system will begin to run.
- Keep putting refrigerant in until you've installed the correct amount.
A refrigerant charge is measured by weight. You can look at the outside of your refrigerant supply and see how much is in the can. As for how much goes in the vehicle, that depends. Look for a sticker under the hood that tells you how much refrigerant to put in. You can also use the pressure gauges to see when you've installed enough refrigerant. This takes some experience, however, because the pressures will vary depending on ambient temperature and humidity. Generally on a 134a system you want to see low pressure readings near 20 – 30 psi and high pressure readings between 200 – 300 psi. Your readings will vary a great deal depending on the heat load to the AC system. Most manufacturers don’t give exact readings but rather a chart that shows approximate readings under different temperature and humidity conditions.
One last test is to check the vent temperatures when filling the system. If I'm getting a 30º to 40º F drop, I consider that good. Close your service ports, remove your gauges, and go for a nice cool drive.
As I stated earlier, the most common place for the AC system to leak is at the service fittings. This is especially true after servicing the system. Think about it; these valves haven't been touched in years sometimes. Disturb them during the course of service, and they start to leak. You will normally see a few bubbles when you first remove your gauge set, but this should only happen for a short time and then stop. This is the boiling off of excess refrigerant that was trapped between the service fitting and your gauge hook-up.
If you have a valve that's leaking a little, try just blipping the valve with a screwdriver for a split second. This might clear any debris and allow the valve to seal again. It works, sometimes. As a general practice I often install dye when recharging a system. This way if there are any leaks later, I won't have to go through the trouble of installing dye to check for where the leaks are. In fact, some manufacturers install dye in the AC system at the factory for this very reason. Here's a video covering how to charge an AC system. It also covers adding dye and oil to the system.