Nothing, that is how the defrost works. When you turn on the defrost the A/C turns on automaticaly. If you mean that the air blows out of your "vent" air ducts when you turn on the A/C then you may have something stuck in your air duct plentum or a air duct plentum door is not working. This type of repair involves removing the entire dash board and air ducts to just find the problem.
Sunday, July 6th, 2008 AT 12:27 PM