I beg to differ ....
I noticed a comment to the effect that it should be easy to test for the proper operation of the alternator. Yes, it should be easy, but at least for me, it is not.
I find that there is something different about the design of the regulator located within the alternator and that it can fail in a manner that is not obvious to the normal alternator shop testing procedures.
About the only thought I have is that it has something to do with the interaction of the regulator with the engine computer. In the case of our 3, it was one of the first vehicles to use what Denso or Land Rover calls a Smart Regulator, (and I call dumb).
The regulator gets its instructions from the engine computer rather than the battery. The instructions are transmitted in the form of a digital like Pulse Width Modulated, (PWM), signal rather than just simple battery voltage per regulators of the previous more common design and what we are used to. The reasoning behind the change relates to furthering fuel economy.
Since this is such a "smart regulator" if appears to also have a fail safe mode where it can default to about 13.3 volts, (rather than just quitting), and also allows a certain amount of current to be generated. This amount appears to be enough to run the ignition, radio headlights etc, and provide a bit of a charge to the battery as well. The battery however tends to be last on the list apparently due to unintended software glitches - probably for the same reason the battery never gets fully charged even when all is well. As the battery is last on the list, I think that is why the alternator light on the dash will sometime come on and then go off.
The bottom line to all this magic is that while the alternator can test OK by using the normal alternator shop techniques that have worked since the fifties, those techniques do not necessarily yield correct results on these new PWM regulator alternators.
Ford is now using these same style regulators within the alternators on some of their newer vehicles and recommends a testing procedure that they call PCM/RC where PCM means Powertrain Control Module, similar to ECM per Land Rover.
About the middle of the album per the link below, there are five files related to the alternator and the regulator that may be of interest.
http://bit.ly/1hrIQm5