I have a 2004 mercury marquere. When I drive about 45 to 50 miles per hour it puts out cold air. when i get on the interstate and go about 60 or more miles per hour it puts out hot air. as soon as i get off the interstate and slow down to about 50 miles per hour or less it starts putting out cold air. i have had it the shop 2 times and they cannot find anything wrong with the air conditioner.
It is likely a vacuum problem. On many cars the blend doors that control the temperature of the air are controlled by vacuum. The default position is heat so if the vacuum goes too low, it causes the system to go into heat mode. It is normal for a car to have less vacuum at high power. You should get the same hot air if you are accelerating fast or going up a steep hill.
There may be leak somewhere in the system causing the vacuum to go too low to keep the A/C on at the high speed. It may be a vacuum hose that is cracked, defective, loose or even pulled off.
This is a very odd problem. It is not a vacuum and blend door because they are electrically-controlled in these cars. Vacuum affecting some other aspect? I can’t think of any. The doors that control the air flow, dash panels vs. floor vs. defrost, as well as fresh air vs. recirculate, are vacuum controlled, but even at 60 there should be plenty of vacuum.
You don’t have your foot to the floor to maintain 60 mph, do you? That could account for it, but that would indicate a deeper problem with the engine. I suppose that a bad throttle position sensor might do this as well since the AC is shut off at full throttle. I don’t know what else would happen if the throttle position sensor is bad in this way, indicating full throttle at starting at significantly less than that opening.
If you get no more response here try posting at www.crownvic.net. There might be someone there that has some ideas. Be sure to post whether you have manual controls (knobs) or EATC (electronic controls with digital readouts and buttons).
It is not a vacuum and blend door because they are electrically-controlled in these cars.
That blows my theory.