Hmmm. Any other symptoms? Where did the alternator come from/what brand? If aftermarket from a parts store, some have been known to be bad right out of the box.


Usual recommended alternator replacement for the 2005-2009 Mustangs is to try and get a 2010-2014 alternator from Ford. Better than the ones Ford originally put in the 2005-2009 cars.

 

I agree, if possible take the car to the parts store, and let them check the alternator. Also, check all your connections to make sure they are secure. If your battery connectors have any corrosion clean them good. Good luck.

 

Make sure it's the Ford "OEM" 2010-14 Denso alternator. Check places like Rock Auto.com for the best prices.