This is a question that often gets asked, and it is akin to asking “how long should I cut a piece of string?” Here’s why: every car A/C is different. Every situation is different. If you can tell me precisely how much refrigerant leaked from your system, then I can tell you the amount of refrigerant that should go back into your system. Here’s the problem: you can’t tell me precisely how much refrigerant has leaked out. I can tell you the amount that your system holds if it has been completely evacuated, but I can’t tell you how much should go in if I don’t know the amount that is already in it (i.e. the portion that is left that has not leaked out).
So how do we know how much to put in? Great question. The only way to really know how much refrigerant is in your system (and therefore how much we should put in) is by checking the system’s pressure with a pressure gauge. To check your system’s pressure you will need to know location of your low side port on your A/C. If you know the location of your low side port keep reading. If you don’t, go here for help locating it. Once you have found it come on back and read the rest of this entry.
Typical A/C Pressure Gauges—Here is a typical pressure gauge that can be used to check the low side pressure. Chances are if this is the first time that you have ever checked the low side pressure in your vehicle’s A/C system a gauge like this will be the likely gauge type that you will use.
Take a look at the face of the gauge. Here you will see pressure numbers that are represented as pound per square inch or “psi”. Directly underneath those numbers you will see numbers that correlate to the outside temperature in Fahrenheit. On the face of the gauge you will see an arrow, and two lines that form the shape of a “V”. Simply rotate the gauge so the arrow points to the outside temperature. The area between the lines in the “V” represents the properly filled range for your system. If the needle is anywhere in that range you are ok…even if it is on the low side of the range. If the needle on the gauge is below the range your system needs refrigerant. If it is above the range then your system is overcharged or possibly has some other serious problem. If that is the case you will need the help of a qualified technician.
Checking the Pressure— Taking a pressure reading is a lot like checking the pressure on your car tire. Find the low side port, unscrew the cover off of the port, and attach the pressure gauge. However, when we check the pressure on an A/C system we need to know the outside temperature to determine the proper fill amount. Once we have determined the system pressure, we can determine the amount of refrigerant needed to replace the lost refrigerant. There is one thing that you should always keep in mind when charging your system: “More refrigerant is not always better.” Overcharging your system can be harmful to the components of your system. When the pressure gauge indicates a pressure anywhere within the “V” bezel marked on the gauge then should have enough refrigerant in the system.
Here is a video that will show you the proper way to read the pressure in your car’s A/C system… Enjoy.