Virtually all modern AC systems operate by using a compressor to pressurize and circulate refrigerant and oil through the system to produce cold air. AC systems operate using two different sides, the high side and the low side. The refrigerant starts out as a gas in the low pressure side of the system, and is converted into a liquid in the high side. The constant circulation of refrigerant through the high and low pressure sides of the system is what keeps the vehicle cool.
Because AC systems are pressurized, they must be completely sealed to function properly. Over time, these pressurized systems can eventually develop leaks. Once any sort of leak has begun, they will eventually cause enough refrigerant to leak out to the point where the AC will no longer be able to produce cold air. Once the refrigerant and pressure level of an AC system drop too low, it must be recharged with pressurized refrigerant before it will function properly. Usually an AC system will begin to display a few symptoms when it needs to be recharged.
1. Loss in cooling capability
The most obvious symptom that a vehicle needs to be recharged is that there will be a noticeable loss in the overall cooling capability of the AC system. The AC system operates by circulating pressurized refrigerant, so if the amount drops too low it will eventually begin to affect the operation of the system. You may notice that the air is not blowing as cold as it was before or is not blowing cold air at all.
2. AC clutch fails to engage
With the AC control is set to maximum coldness, you should be able to hear the familiar clicking sound of your AC clutch engaging. The clutch engages with a signal from the AC pressure switch, which reads the pressure level of the system. When the level drops too low, the pressure switch will not activate, and therefore the clutch will not engage. Without the AC clutch engaged, the system will not be able to circulate even the low amount of refrigerant it may have and the system will not work at all.
3. Visible signs of refrigerant leaks
A more serious sign that the car needs an AC recharge is that there will be visible signs of refrigerant leakage. If you spot any signs of a greasy film on any of the AC components or fittings, or any pools of refrigerant underneath the vehicle, then this is a sign that a leak has developed and refrigerant is being lost. The refrigerant will continue to leak until the system no longer functions.
Because the need for a recharge indicates a loss of refrigerant, there is likely a leak somewhere in the system that may need to be addressed before having this service. For this reason, if you suspect your system may need a recharge, first have the AC system inspected to ensure that the AC recharge addresses the problem properly.