[V8] Charging question

[Ingo Rautenberg]

Lately I've noticed that since I got rid of all my vacuum leaks and (more importantly) replaced the FPR, my idle hovers ~650 rpm ('91 V8 5 speed).  When the headlights and fan are on the headlights will dim somewhat, the dash lights will also dim, and the fan motor will slow down, with the battery warning light displayed until I resume an above idle speed, when all is back to normal and the dash warning light goes off.  So does that indicate the regulator/brush pak?  Normally I would think that the alternator pulley was the wrong size -- but I couldn't have the wrong size alt pulley, could I?

Ingo