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Turn it off.
The electric fans used to cool the engine are designed to switch on to full speed when the air conditioner compressor is being used. When you turn on the heater/defroster the air conditioning compressor also turns on to help remove moisture from the air. Since both when heating or cooling your car the air conditioner compressor is working the fan will be running at high speed and makes more noise.
A lot of vehicles will activate the A/C compressor in 'Defrost' mode. Either Bi-Level or defroster will turn on the A/C compressor. I know in a lot of GM vehicles, even just the floor selection will turn it on.
The compressor won't turn on because there could be aÊproblem with the air conditioning system. You need to vacuum the system.Ê
Try unplugging your AC compressor and see if it still blows the fuse. When you turn on your defrosters you also turn on your AC.
Do you mean that the AC clutch does not engage the compressor? Or do you mean that the AC compressor won't turn even if the clutch is engaged? If the compressor is siezed and will not turn even if the clutch is engaged, the only choice is to replace the compressor. If you can turn the compressor by hand (understand that the belt pulley will turn freely with the engine if the clutch isn't engaged) but the compressor clutch isn't engaging, it might just be out of refrigerant. There is a sensor, usually somewhere near the receiver/dryer) that detects the "low side" refrigerant pressure. Once the refrigerant pressure at the sensor drops below a specified level, the compressor will not turn on. In other words, if the compressor clutch won't engage it might just be out of refrigerant.
Turn the compressor off.
There are many reasons why an air conditioning compressor might not turn off. There is likely a wire that is not connected properly.
A lot of cars won't let the compressor kick on unless it has enough refridgerant. Try recharging the compressor all the way and if it still don't work, you might need a new compressor.
If it has AC assist and the compressor is shot, cut the belt or disconnect the defrost from the compressor.