If you've ever pointed a compressed air hose at your face, or held a compressed air tool after long use, you've already got some idea of how compressed gases rapidly cool down while expanding out to the atmosphere. Your AC system works the same way, using the cooling effects of expanding gas to chill the air in your cabin. But that gas needs some room to expand, and low pressure to expand into; overcharge the AC system with coolant, and you wind up with a system that acts more like a big oil pump than a nozzle blowing cool, decompressing air into your face.
The most obvious sign of an overcharged AC system is poor cooling. Without room to depressurize the coolant, the AC system simply stops functioning. You might still get a cool breeze coming from the vents, but just as often you could get room-temperature or even hot air. Some systems will shut themselves off if they detect improper pressure release in the system. Be glad if yours does; it may be saving you from a hefty repair bill.
The compressor might make excessive noise, the sound of it struggling to shove liquid coolant through a tiny nozzle designed for gas molecules. This doesn't happen in every case, since some AC compressors are designed with a bypass to prevent excess internal pressure from straining the pump. If you've ever heard a power steering pump going bad, then you'll have some idea of what a struggling AC compressor sounds like.
High Pressure Readings
An overcharged AC will show higher than normal pressure and temperature readings. The compressor works harder to try balancing out the system, which results in more pressure. The higher the pressure, the higher the temperature rise as a result. This causes even more pressure to build, resulting in a cascade failure of cooling capacity.
An AC compressor can draw a significant amount of power from the engine, never more so than when it's trying to push a gallon of coolant through a one-ounce nozzle. This excessive power draw can cause the compressor's drive belt to slip and screech, which generally precedes a loud snap and belt failure. Or, the belt may hang on, and sap power from the engine. In this case, you may end up with a low or fluctuating idle, sluggish acceleration and excessive loss of fuel economy when the AC is running.
This is often the inevitable end result of a chronically overcharged system. The compressor is your AC system's heart, and eventually it's going to respond the same way your heart would if you had chronic high blood pressure. Internal seals will fail, valves will cease to function, the pump will fail and the system will ultimately flatline.
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