Low Battery Gauge

I have a 1999 Bonneville SLE, when I use the a/c when driving the battery gauge is good around 12-13 volts. When I stop at a sign or a light the gague drops to the red 9-10 volts. I just changed the battery thinking it was that, but it still does it. My next thought was the alternator. Does anyone know if this is just normal for the car or should I be worried? I don't remember having this problem with my other Bonn's. Thanks, Tom

Tom, it doesn't sound normal. Granted, when you're at idle alternator voltage output may decrease a little, but it shouldn't fall on its face. Try turning off some electrical loads ie A/C, radio etc and see how the volt meter responds.

I have a 99 Bonneville doing the same thing only I changed the alternator and have the same problem. I can make mine do it. turn the air conditioner on blower on low, lights on. Shutoff the compressor and I see a 1.0 2.0 volt voltage drop in a about 5 secs it will recover. It was bad enought this morning it caused the low voltage dinger to go off. The battery test good. My question is can the computer decide to turn off the alternator based on some inputs from other sensors on this model. I also notice that it takes close to 10-18 secs before the alternator is putting out 14.00 volts after I start he car. Anyone have any ideas. P.S. when the voltage dops the alternator output at the alternator stud is about 6 amps.