Auto Air Conditioning & Heater Repair: A/C Cold when driving, vacuum pump, atmospheric air

Question
1993 Del Sol...  Air conditioner blows cold while driving, but blows hot when car is stopped at red lights, etc.  I was told a sensor was the problem when my son's truck did the same, but he can't remember what sensor.  What do you think is the problem.  We just converted the system, and we are having problems getting the car to take enough freon.  

Answer
When you converted the system over to the new freon did you use a vacuum pump to evacuate ALL of the atmospheric air from the A/C system or did you just put in the new freon ?.

Tell me EXACTLY what steps you performed when you converted it over.


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