A/C full of freon and compressor working. What now?

[Shootist]

According to the gauge my compresser is full of freon. The compressor also kicks on. I assume its working. The a/c only blows hot air though. What is the likely culprit?

 

[landtank]

the valve which controls the coolant passing through the heater core. It's located against the fire wall and a few have noticed the cable having fallen off and the valve always in the open possition heating the cold air.