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AC stops blowing cold air

2005 Isuzu Ascender 6cyl–auto-- all power–2 wheel dr-- Air cond blows very little air that is only slightly cooler than the outside temp. This only happens during hot weather( 90 Fahrenheit or higher ) after highway driving for extended periods of time, usually after a few hours. It works as it should during any other conditions. After letting everything cool down, it works properly. The dealer says they cannot get it to duplicate this action and therefor says there is nothing wrong with it. I first noticed this at about 50,000 miles because that was my first trip in it. It now has 70,000 miles.
Any suggestions?
Thank you;

these are usually signs of the vehice engine cooling system reaching an overheating or is in overheating stage - I think those veicles are equipped with some kind of sensor to turn off or disengage the ac clutch when the car senses a raise in water temp in the engine that is too high above normal.I would suggest you check in to make sure the engine water coolant temp is running in top shape before the next step is made which will be to check into the AC system itself.

I’d bring it to a good automotive A/C shop after returning from a long drive and see what they can find.

It sounds as though the evaporator coil is freezing up. Under hot, high humidity conditions, the moisture that condenses on the evaporator turns to ice. This would explain the reduced air flow and the fact that the air doesn’t feel cool. When this happens, you might try turning the air conditioning off for 5 to 10 minutes and then try it again.

I think Triedaq has the solution. When I was stationed in middle Georgia this problem happened to me all the time. When the temperature and humidity are high the problem with freezing evaporator coils is at it’s highest. I used to correct the problem by turning my controls to vent for a few minutes. It always worked for me.