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I have been a master auto tech for over 35 years and have seen this happen many times before.
The problem lies with a diode inside the alternator that is failing. There are 3 diodes on this particular alternator and when one fails the light will do on and off just like you are seeing. It will still put out enough charging voltage but it puts a load on the remaining diodes and as soon as another one fails, it will not be able to charge the battery anymore. Bot***** *****ne is the alternator is on its way out and needs replaced.
The failing diode can be noticed only on an oscilloscope so it wont show up on regular volt/amp meters.
Replace it and it will take care of the light coming on and prevent problems down the road.
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