92 Bonneville SE. When I have my lights on and hit the breaks I notice that my lights dim (just for a second). I have replace the alternator about 2 months ago. What this would seem like is a bad voltage regulator, but I'm not aware of this vehicle having one. Anyone have any ideas? I notice my guages are slightly off sometimes. The voltage one day will show 14 then the next day be just a little under, same with fuel and oil.

Where did you get the new alternator?

I had the same symptom on my 97 Bonneville SE, and my wife'* 88 Grand Prix SE. On both cars it turned out to be corroded battery cables. They looked fine externally, but inside the rubber insulation they had corroded. I also had a cracked and leaking battery in the Bonneville, which I noticed because my car wouldn't start. After replacing the battery I noticed the dim lights when I hit the brakes. A local shop replaced my alternator. Still had the same problem. After it wouldn't start again, they got it right and replaced the battery cables.

On my wife'* Grand Prix I first replaced the alternator on my own. Still dim lights when I hit the brakes. The shop replaced the battery. Still dim lights, later it wouldn't start. Finally, they replaced the cables. A few weeks later the starter died.

I mean, there'* only a handful of parts it can be: Battery, Alternator, Cables, & Starter.
Incidentally, the voltage regulator is a diode-pack built into the alternator.

Remember that a bad alt can kill a battery in short order. You may have over-stressed the battery, and now it may try to kill the alt. Without a full electrical diagnosis, I'd be tempted to replace the battery with a new one, and the cables, too. They're cheap enough. Otherwise, you may be killing the new alternator soon. The Voltage regulator is built in to the Alt, btw.

The other thing to consider is how many amps your new alt is. I had a similar prob with a new 105amp, and just switched to a 160 amp and new battery.

I had bought the Alternator through Johns Auto (it is a rebuild with 2 year warranty). I do have the 105 amp. The battery and cables appear to be fine, but I have not actually done a thorough look, which I will do. I will do an electrical test and see what comes up.

Um, it'* "brakes," not "breaks"...

It'* built into your alternator, but that'* not the issue here anyway.

You've got one or more bad grounds. Current flow is "backing up" somewhat at the ground connections, and seeking a ground path through other circuits connected to the same ground, which serves to lower the overall voltage a bit, though maybe for just a second or two.

Start with the battery cables, follow each wherever it goes, and remove and clean the connections. I know the lower ends are a pain to get at, but they may well be the problem areas anyway.

I fixed this exact problem just a month or two ago on a BMW: you step on the brakes at night and the dash lights would blink, as would the Brake Lamp Out warning. I needed to do nothing other than to clean some really cruddy ground connections under the hood where about 10 circuits all converged at one big bolt on the fender, and everything was fine after that.