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You have a corroded/loose negative battery wire OR another bad alternator OR something pulling a really heavy load on the alternator (Short to ground). Id check the battery cable first. Both ends of it.
I have the same issue as well. I never noticed this before my hundred thousand miles but now noticed this after my current 115K miles. AutoZone Duralast gold battery and alternator. I heard that it will fluctuate between 11 and 15 as long as it stays between those lines you’re not hurting your vehicle and you should be OK. At times it will tip down to 11 and go back up to 14. I haven’t noticed anything with my engine, shaking vigorously or performance upon acceleration changing. I would think this would only occur when you have a sound system or something else hooked up for an aftermarket. As far as your last sentence goes, I have not had that issue.
Wrong section for your year....but 2005+ rigs have RVC which is computer management of alternator output.
