This may seem strange but here goes:

I have a 2000 Bonneville SLE (80000 mi) and something odd appears to be happening with the ammeter (alternator voltage) guage. I sometimes get a "drop" in voltage and dimming lights at a certain engine RPM. It is not a fixed RPM value (although it seems to be from about 1200 to 2000) and it is not constant. When the engine gets warmed up, it seems to disappear, but there has been a few "hot engine" incidents. I thought it was the alternator, but if it was going south, I figure it would be constant and not RPM dependent. I replaced the plug wires (in case it was an intermittent short or cross over caused by resonant vibrations at a given RPM). Still seem to be getting the problem. Could it be a voltage regulator? Alternator diode gone? Kind of baffled. Any help appreciated. Thanks.

It could possibly be the voltage regulator, but you may want to look into whether or not your belt may be starting to slip, or be stretched. At 80K miles, you are due for a new one.

The reason I think it might not be the voltage regulator is that I have had them go out on me in the past and always had the opposite happen...i.e. the volts increased with engine RPM.

Jay

If you are able to check the state of charge on the battery after driving car for a while, and it'* indicating full, or near full charge state, then you may rule out the battery, alternator,and charging system as being faulted. The specific gravity of the electrolyte tells the whole story. Specific gravity in a lead acid battery (6 cells) should be 1.280 at 80 degrees faranheit. If the ambient temp is above or below the 80 degrees, there is a correction formula to solve for that to give you a corrected specific gravity. The problem with todays auto batteries we all know is they're sealed. Maintenance free, yeah right.
Some auto service places have "battery checkers" that actually check the battery under load so that they can determine "drop & recovery" so to speak.
It sounds like you had some intermittent cycling load on the electrical system for a short duration, then goes away. That is what you need to isolate. The alternator must put back into the battery all ampere/hours that were taken out by the cars electrical system (ie. starting engine, radio, A/C, lighting, other acessories) then float the battery. In other words when the battery recovers form the initial starting of the engine (alternator charging) they're running in parallel, if you notice lights dimming momentarily, it means that something is heavily loading the system. Hope this helps a little, didn't mean to get carried away, just an old submarine electrician here.
markwb

Los Angeles, Trident, Seawolf, Virginia Classes. I only work on them, I do not serve in them.
markwb