http://www.oxyclean.com/ac-faq.html#q4
Why is my unit icing-up? I have frost on the coil and copper tubing. Why am I getting such little air flow from my registers?
There are several things that can cause frost on your coil and/or reduced air flow. Anything that restricts the airflow through the inside unit will cause frost. As the frost builds up on the coil, the airflow becomes more and more restricted making the condition worse.......
When the frost is also on the outside pipes next to the compressor, you have a situation where damage to the compressor can occur.
1. Extremely dirty air filter restricting the airflow through the inside unit.
2. Extremely dirty (clogged) cooling coil restricting airflow through inside unit.
3. System low on freon, causing coil to freeze up.
4. Check your return grill to make sure that it is not being obstructed.
5. Blower motor overheats and "kicks off" on safety switch.
If you are mechanically inclined, here are a few troubleshooting tips:
The following test is the most important test you can performed on a system that is running but not cooling enough. This test can help determine if your system is low on freon or will uncover several other problems that can cause your system to malfunction. It will tell a lot about what is wrong with the system.
Start by inserting a stem type thermometer in the air supply duct, record the temperature. Now take the temperature of the air entering the return air duct. Subtract the supply temperature from the return temperature. The temperature difference should be 17 to 22 degrees. Make sure there is no frost on the coil before performing this test.
A dirty cooling coil is a common cause of "Not Enough Cooling or "Freezing Up" of the system. You can check to see if the coil is dirty by removing the filter and looking at the underside of the coil with a flashlight. Sometimes a mirror will help you see under the coil.
You can determine if the blower motor is kicking off on overload by setting the thermostat to "fan=ON" and listening to the blower run. If the motor kicks off after a while, then you have a bad motor. It may take a half-hour or more before the motor overheats and knocks on overload........
In addition, you can feel the body of the motor to see if it is extremely hot. Be careful when you touch the motor as they get very hot. Also, turn the power to the unit off before performing this test.
http://www.dslreports.com/forum/r18805132-AC-Freezing-up