Even with my brand new rebuilt alternator, the lights are dim at low revs. Best way to check is with a volt meter across the battery with the car running and at idle... Voltage should be 13-13.5 volts. If it is lower, but not much (say 12-12.5) rev the engine up and see if it comes into normal ranges. If it does come up, I would say you probably dont have a problem. Sure, it could be a pre-cursor to the alternator dying, but that would mean my new alternator is getting ready to kick the bucket (doubt that...) -Nick
