Hi bestaimee. Welcome to the forum. There's two possibilities. Since this seems to occur with regularity, suspect the system is over-cooling. It is only supposed to lower air temperature about 20 degrees. The real comfort comes from lowering the humidity. The moisture in the air condenses on the evaporator in the dash and is supposed to drip into a drain pan and onto the ground. The flow of refrigerant is regulated so the evaporator never gets colder than around 40 degrees. If it gets too cold, the water condensation will freeze into a block of ice that blocks air flow. The ice will melt when the system is turned off for a while. If this is what's happening, the problem may not show up if you set the temperature higher. You can also stick a thermometer in one of the ducts. If the air temperature gets down to 40 - 45 degrees, it's too cold. The cause is likely to be a mispositioned temperature sensor on the evaporator that isn't making good contact so it is allowing too much refrigerant to enter. When it starts blowing warm air, look under the hood at the hoses. If one of them has ice on it, too much cooling is taking place. The same ice will be found inside the car.
Another possibility is the valve that controls how much refrigerant enters the evaporator is sticking. They can stick open and allow too much refrigerant to enter which results in over-cooling, or they can close in response to normal operation, then stick and fail to open when it's time to circulate more refrigerant. This is less likely to be your problem because while it is an intermittent problem, it will not occur on such a regular basis. The valve could work properly for days or weeks before it acts up.
The system could be low on charge too. As the low side pressure drops, a low pressure cutout switch turns the compressor off to avoid potentially drawing air into the system through a leak. The moisture from the humidity that could enter the system would mix with the remaining refrigerant to cause corrosion to metal parts. When the system is low on refrigerant, the cooling usually doesn't stop completely. The air temperature typically rises slowly and you can feel the increase in humidity.
Friday, June 11th, 2010 AT 10:23 AM