Cadillac Repair: alternator possibly going bad...??, northstar engine, hello bill

Question
Hello Bill,
I own a 1993 Eldorado with the Northstar engine, and recently a friend told me he had to have his alternator replaced, so I asked him if where there any signs that the alternator was about to go, he then said that the only thing he notice was that his lights would go dim whenever he steped on the brakes. So I decided to try it on my car, and yes, the headlights would dim a little ,for a split second, when I stepped on the brakes.
My question is,  is the dimming of the headlights ,when you step on the brakes, a sign that the alternator is about to go bad? if not, are there any signs?

any and all help is greatly appreciated.

thanks
Manny

Answer
Hi Manny, If you are at idle and turn on something that requires 5 or more amps the headlamps are going to dim briefly because the alternator needs to be turning a minimum speed to produce its rated output. So at idle it might be able to make 40 amps with the rest being provided by the battery but at 1500-2000rpm or at 30mph the alternator can produce 140 amps. So just because the lights dim when putting on the brakes at idle doesn't mean the alternator is bad. There was a buletin published to explain this back in '99 or '00. Signs that the alt. is going bad is dead battery in the morning or slow cranking speeds due to a batty drain through the alternator, it will make a grind/growl noise with the engine running, the alt or charge light will glow or turn on bright, or your car could just quit while driving and if you had the headlamps on they would start to get dimmer the longer you drove. Hope that explains it. Bill