I had an advance auto parts alternator that lasted about 6 months, and I had to return it for the exact same reason: pulsating lights on idling... Replaced the pulley on the old alt and it's been running for 1.5 years.

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2005 kj CRD, samco, suncoast tc, provent, Kennedy lift pump, GDE ECO full torque, 2nd gen filter head, 245/70/16 a/t tires, mopar light bar, fumoto oil valve, OEM Skid Plates, ARB Front bumper and HD OME, tru cool LPD47391 40k GVW tranny cooler (stock cooler delete), FF Dynamics e-fan and shroud, rocker arms replaced, HDS2 190F thermostat.

Yeah, it sounds like one of the diodes or the field connections might not be so good in that alternator. You may find yourself swapping it out in the sooner-than-later.

Well crap...

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-2006 Sport CRD - White - GDE Eco - Elephant Hose
-2015 Audi SQ5