Not only is overcharging your system potentially dangerous, an overcharged system will not function properly and can seriously damage your A/C compressor or other component parts. The symptom of an overcharged A/C system is exactly the same as a system that is undercharged: warm air coming out of the vents.
In order to prevent overcharging your A/C system, it is imperative that you measure the pressure before beginning to charge it. To measure the pressure in the A/C system, attach a pressure gauge to the low-side port. Your car must be running with the A/C system set to maximum cool and the fan at its highest setting. It is important to note that the proper pressure in your A/C system is dependent on the ambient (outside) temperature. To determine the proper pressure for your system, consult a pressure chart, or set the bezel arrow on your pressure gauge to the corresponding ambient temperature and charge to the pressure corresponding to that temperature setting. If the outside temperature is at or below 55°F, do not charge your car. Also, never charge your vehicle above 55 psi.
There are a number of possibilities. Your compressor may be damaged and not working properly. You can check to see if it is cycling on and off properly by looking at the clutch faceplate in the middle of the compressor pulley. Watch this video of a compressor that is cycling on and off properly. If your compressor is not cycling on, then you should seek the help of an A/C professional immediately. Ignoring the problem will not make it go away and, in fact, it could make matters worse. Other possible causes are blown relays and fuses, defective sensors or a stuck blend door. Seek the help of an A/C professional to assist you in determining the cause in your particular case.
The most common reasons for the pressure gauge to indicate a red/high-pressure reading are the car is not running, or the A/C is not on with the fan switch set to high and the temperature at its coldest setting. In order to obtain an accurate pressure reading, the car must be running, the A/C set to the coldest position and the fan set to high. Also, as the car’s compressor cycles on and off, the needle on the gauge will move in and out of the red/high-pressure area of the gauge. This is normal. Watch this video to see an example of a car compressor cycling on and off.
When checking low-side pressure, you will get two different pressure readings, one when the compressor cycles on and one when the compressor cycles off. Use the pressure reading when the compressor is cycled on and the system is operating at its highest settings (temp to MAX cold and fan on high). When the compressor cycles on, you should see the needle on your pressure gauge begin to drop. When the compressor turns off, you will see the needle move back to a higher pressure. Take the reading that occurs when the pressure is at its lowest point, when the compressor is cycled on. To tell if your compressor is cycling properly, take a look at this following video.
Ambient temperature is simply the temperature of the air surrounding the car. For instance, if the ambient temperature (outdoor temperature) is 75°F, the corresponding low-side pressure should be between 35–45 psi and if needed, the corresponding high-side pressure would be between 150–170 psi. Our automotive A/C chart outlines the proper pressure readings for a particular temperature range.