How To Make Sense of AC Pressure Readings

Auto AC systems are extremely difficult to understand, the system not be blowing cold air but most mechanics have no idea why.  Most mechanics will take a few minutes with the vehicle and say the old “the AC compressor is shot” buuuuut honestly they are just guessing. Usually changing the compressor it will fix the problem…but not for the reason you think. Not to mention it will cost a lot more than it needs too.  The reason that the AC works is, the replacement of the compressor also means that the mechanic changed all the other AC parts in the system. Also they refilled with the correct amount of Freon and oil which may have been wrong. With the whole system changed you are back enjoying your ac system, little do you know you just spent way too much money.

Air conditioning systems are very easy to diagnose if you know what to look for. The easiest way to diagnose an AC system is to get the pressure reading from the system with a pressure gauge. Issues like air in the system or too much refrigerant all have specific symptoms and can be diagnosed simply by using a pressure gauge.  Remember all the readings are set for an average day at around 75 degrees so reading may fluctuate a few psi depending on temperature.

Issue = readings and symptoms

Air Entered the AC system = both the high and low side pressure readings are high (low sire 35 – 43 psi, the high side is 284 – 355 psi) also the low pressure side will not be cool to the touch.

Too Much Refrigerant = same symptoms as air in the system but bubbles cannot be seen in the sight glass.

Expansion Valve Issues or Improper Heat Sensitizing tube installation = same symptoms as above but frost or dew on the low pressure piping.

Faulty Compressor = low side too high and high side to low.

Low Refrigerant = both high and low side readings are low.

Moisture Entered the System = low side pressure will be a vacuum.

Refrigerant Fails to Circulate = vacuum on the low side and low pressure on the high side.

If you follow these readings, the issues with your system can be worked out before the AC is opened up. In other words the mechanic can’t take you for a ride, you know more than he does. Being prepared is the best way to avoid spending more money than you have to. In an economy like the one we have today a little knowledge can be a valuable thing.