i have the same problem at the moment as well. If I start the car, it idles at 14V. I can leave the car idling for 15 20 minutes and the voltage wont change. However if or when i start driving the voltage will fall to 12.6V as well and i could drive over a bump or something and the voltage will move around.
Possibility:
Im investigating the thought that it could be a slipping Serpentine belt due to a loose tensioner. il come back to you on this.
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