I have a 2000 A4 1.8T. Usually my volt gauge reads around a 14 but just today when I really step on my gas after a few secs my volt gauge drops down to 12 and the battery light comes on. Is that because of my battery or my alternator? Someone help me.

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2000 A4 B5 1.8T

Sounds like your alternator is going- Any Autozone can testyourbothfor free. I have an A6 and had the same symptoms a few weeks ago and needed to replace my alt. approx. $500 to fix.