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Interesting. Mind is doing that also, but my alt and batterys are only a year and a half old. The alt is from a shop around here not a parts store one. Of course i did let my truck sit for a while this summer whith out starting it much, maybe that had some thing to do with it.
My voltage is dropping to 11.7-12.3v at idle and as soon as I touch the go pedal it jumps up to 14.5
Is the alternator gone or is it the voltage regulator ?
Hard to say. When mine was doing that a few years ago I pulled the alternator and removed the regulator .. few bolts on the back and you can wiggle it out and the brushes come out with it. If the brushed are gone you can get those separate. In my case the brushes were fine.
Here is what the guts look like inside where the brushes sit and don't be afraid to pull it and look ...
IMO the slots where the brushes sit were worn down too much so I put it all back together, the auto store tested the alternator output, and told me it was good because it was outputting 12.5 volts. I told them they were nuts because our Ford alternators needed an output of 14+ volts.
Long story short, got a new alternator and all is well since.