This happened to me not too long ago, on mine, the alternator was just old.
The book calls for 5mm on the brushes attached to the voltage regulator. Mine were bad, and a new one costs about $90, special order. A rebuilt alternator costs $110 with a core charge, so I just got that since the alt had other worn parts.
VW TDI forum, Audi, Porsche, and Chevy Cruze Diesel forum
A forum community dedicated to Audi, Volkswagen, Porsche, and all turbo diesel owners and enthusiasts. Come join the discussion about performance, modifications, troubleshooting, TDI, maintenance, and more!