Hi,

On a 2007 Grand Prix base, the battery is around 2 years old, and randomly I would see like 12.4 v under load, accessories on, while idling. Other times it would be in the normal range. Sometimes it go up to 13v range. Recently the alternator was outputting low and noisy under load and randomly, so I had it changed. The new alternator the car runs better, and randomly it will drop while under load, and other times it will be normal in the 13v range. I checked the voltage drop its about 0.4v difference.

I did read online and even on this forum, that sometimes the wire harness for the alternator could an issue, I saw that in a recent post. And something about GM's charging system, but I don't know if this applies to the Grand Prix.

I did check some other things, cables are clean, tight, belt is original.

Please let me know.