I need help finding why my car's air conditioning (2011 Ford Mustang v6) is only blowing cold air at idling and not while driving on the highway when it's 100°F here in Nashville. In the mornings while driving to work it blows cold air since it's maybe 78° outside but once I get off work it's 100°F and it won't blow cold air. Why?
Bryan, I must admit I'm very confused by the way you are asking your question.
(1) I think you are saying that your car's air conditioning works well when the outside air temperature is at 78°F, but it doesn't work when the outside air temperature is 100°F.
But its also possible that you are saying...
(2) My car's air conditioning works well when I first start the car, but after the engine warms up, it doesn't cool off the interior of the car anymore. In fact the vents discharge warm air!
(Sorry, I got lost in your description of which 'it' is which!)
So there are two things possible here.
If we are talking the first case, that could be just about anything related to general A/C performance. Appropriate pressures, no refrigerant leaks, correct A/C clutch air gap, correct cycling times. Here I would strongly recommend you pick up a Service manual (Haynes or Chiltons) and review the A/C troubleshooting guides. Best is to use a gauge set and read the high / low pressures on the system while it is running. That way you'll know if there is too much refrigerant, too little refrigerant, or perhaps other troubles. And yes, Condenser fan is a possible. I will say for the root cause of condensor / engine cooling fan inoperative what you will see is terrible A/C performance at idle and good performance at 60 MPH. (hint: from what you've said, I doubt its the cooling fan...Also, if the cooling fan was inoperative, the engine temperature gauge would be quite high.)
Now if your issue is the second case (good A/C performance when engine is cold, bad performance when engine is warmed up) that would indicate that the temperature blend door is stuck partially open. In your car, engine coolant flow into the heater core all the time. They use a small plastic door to control the air flows into the core. That door is driven by a small motor and plastic gear set, controlled by the Red/Blue (Hot/Cold) knob on your dashboard... The door is fully closed at the blue end of the dial, and fully opened at the red end of the dial. If that isn't set to full cold (or is not getting there for some other reason) that would definitely cause bad performance when the engine warms up. Air is leaking thru the hot heater core, and that will definitely overwhelm A/C cooling.
Does that make sense?