why does my ac fan slow down when i accelerate ?

Hello, You may have a weak charging system or charging system component. When you are hitting the gas hard you are using lots of battery/alternator power to run the various systems of the vehicle, but the alternator is supposed to keep up with this demand. You may have an alternator that is failing, the battery cables or alternator wire harness may have issues delivering power to the battery, bad grounds, a weak battery, or a bad voltage regulator. I recommend checking your grounds and battery cables, including the terminals. If the system is sound, low resistance between alternator and battery, low resistance between battery and chassis (ground side), have the battery charged and tested, and have the alternator tested. You could have this done fairly inexpensively as well.