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Can system get any colder?

Scratch on Thu October 07, 2010 12:38 PM User is offline

Year: 1979
Make: Ford
Model: Granada
Engine Size: 250
Refrigerant Type: R12
Ambient Temp: 68
Pressure Low: 50
Pressure High: 100
Country of Origin: United States

I bought this car last month. Before buying, I knew the A/C wasn't blowing cold. I checked the high side valve and saw no freon when I depressed the pin. When I got the car home, I tried removing the orifice tube and low and high side hoses so I could replace the 0 rings. I was able to loosen the high side hose but couldn't loosen the low side hose or orifice tube. I went ahead and pulled a vacuum and found no leaks. I connected the freon tank, started the engine, and revved the engine until the temperature gauge went to 44 degrees. I don't have the equipment to accurately determine how much freon went into my A/C system. When I drove the car, I noticed that the temperature immediately dropped to about 44 degrees but slowly climbed to 60 degrees or higher and remained there. A few days later I added more freon and now the A/C drops to 44 degrees and remains there continuously. Here is my question: Is the car A/C system designed to blow no colder than 44 degrees?

Back in 2000 I decided to replace the compressor in my 1978 Ford Granada because a bearing failed and screeched so loudly that people standing on the sidewalk took notice. I bought a rebuilt compressor, new drier, and new expansion valve. I drained the compressor oil and measured the amount, replaced all used O rings, assembled the new A/C system and pulled a vacuum. After finding no leaks, I charged in the freon and the A/C blew 15 degrees continuously. The air was so cold that I could only leave it on for a few minutes. It could be 95 degrees on a hot summer day but within seconds after turning on the A/C the cold air was 15 degrees. Did I get a miscalibrated expansion valve?

scott johnson

mk378 on Thu October 07, 2010 1:22 PM User is offline

First off, you have a rather major leak. Need to find it. Also a system that has been sitting at zero psi, or is very old like yours needs the drier replaced. When you're running a stock system with the designed refrigerant (R-12), always charge by weight to factory spec.

No system is designed to cool to 15 degrees. If it did that in a humid climate the evaporator fins would ice over, blocking the airflow and stopping cooling. All cars have some mechanism to keep the evaporator above the freezing point to avoid that. Limiting the evaporator to > 32 degrees usually means about 40 degree air at the vents. I think on the TXV Fords it was a thermal switch on the evaporator that cuts the compressor off when it gets to the frost point. If you really have a CCOT, the pressure switch limits evaporator temperature, since the boiling point of refrigerant depends on pressure. TXV's do not regulate the evaporator to a specific temperature. They try to get it as cold as possible, consistent with not letting liquid refrigerant get to the compressor.

Edited: Thu October 07, 2010 at 1:26 PM by mk378

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