This is exactly backward from what I've been reading. Just to double-check, your voltmeter's negative probe should be on ground or the battery's negative post, not on the positive post.
After conferring with a former student, there will not be a steady DC voltage on the brown / gray wire feeding the field winding in the alternator. That will be a square wave and its "duty cycle" will be varied to adjust output current. That will confuse a digital voltmeter so it won't read correctly. The red / gray wire carries the signal back to the computer that tells it the alternator is working. Either the 12 volts you found there is saying the alternator is working or you have the voltmeter grounded to the wrong place.
What is the voltage right on the large output stud on the back of the alternator when the engine is running?
Wednesday, November 25th, 2015 AT 5:28 PM