What's wrong with my Air Conditioning? - Here are solutions to common problems, written by the staff of http://www.techchoiceparts.com

Leaks / Low Refrigerant
No A/C system is completely, 100% sealed. Over the years, small refrigerant particles will leak out, up to 1/2 ounce per year. On smaller-capacity systems, this will become noticeable a lot quicker than a larger capacity system. In this case, the most practical thing to do is simply add a bit more refrigerant. You will want to hook up a set of manifold gauges to make sure that the system has at least some pressure in it before you add refrigerant, because if the system is completely empty, you might have a most severe leak. At best, you will still have AIR in the system, and you want NO air in there.
Symptoms of low refrigerant include rapid cycling of the compressor clutch, or clutch will not engage, little or no cooling, very oily spots on A/C components.
A severe leak will require repair or component replacement. To find a leak, there must be some refrigerant in the system. An electronic 'sniffer' or dye are the most effective ways of finding a leak. Most A/C shops can perform this service for a minimal charge. Some retail auto parts stores also loan or rent tools for this.
Once the leak is identified, the component should be repaired or replaced, and the system serviced.
*Tip* - Evaporator leaks can be difficult to find, since the evaporator is usually not easily accessible, located in the dash of the vehicle. With dye in the system, place a white, preferably unused baby diaper under the evaporator drain tube. Moisture will drip on to the diaper. If there is dye in there, you'll know it!

Poor Airflow
One of the most-missed problems is poor airflow. Your A/C comes on, but it's not very cold. If you have a gauge, the high side pressure seems awfully high.
There are a couple of quick, easy things to check. First, make sure the condenser is debris-free, and that the fins are not all bent over. The condenser is located right in front of the radiator. Bugs, rocks, plastics bags, etc. covering the front of it prevent air from passing through, cooling not only your radiator, but also the refrigerant in the condenser.
All engines have either a fan clutch or radiator / condenser cooling fans. Fan clutch go out all the time. The fan clutch is located between the radiator and the engine, and has a big fan blade attached to it. With the engine on, that fan should be turning very fast, and moving a lot of air, TOWARDS the engine. Running correctly, you will not want to get your fingers near it. I recently repaired a 1996 Chevy truck that had blown the compressor shaft seal. While diagnosing the system, I noticed that the fan was barely turning. I could actually stop it with my hand. (not recommended for the non-professional) Because there was poor airflow, the pressure (pressure is temperature) built up from the condenser to the compressor, and was more than the compressor shaft seal could handle.
If the vehicle has an electric fan, make sure it (or they) comes on, and that the air is fast, and moving TOWARDS the engine. If the fan(s) does not come on, check the fuse. If that's not it, you may need to replace the fan.
*Tip* - You can confirm proper airflow by TEMPERATURE TESTING. A good thermometer that can be touched to the inlet and outlet of the condenser will confirm proper airflow. You should see a 20 to 40 degree drop from the inlet to the outlet of the 2 condenser pipes. For example, if you measure the temperature at the inlet (where it comes from the compressor) and have a temperature of 160 degrees, the outlet should be 100-140 degrees. Less than 20 degrees difference usually indicates poor airflow. MORE than 40 degrees usually indicates a restriction in the condenser.

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