what do you mean by max cold setting? As in on the selector bar? And by just vent I take that to mean that you do not have the A/C on. If this is the case, the vent blows in air that is the same temp as the outside. If it is cold out it will be cold, if it is hot out it will be hot - even if the selector is put on cold. The selector just controls if air is blown through your heater core - if you put it on warm or hot it will blow air through your heater core which is like a mini-radiator - using your engine's heat to warm the air.

Now, if you have the A/C on then you probably need to have the system looked at. If it is really hot out try using the recirculate button to only cool the air already in the car.

Hope that helps.