Well, for some time now, I have had something weird going on with my charging system. When I start up the car, the battery gauge reads full. After a bit of driving, the battery gauge starts reading lower. When the fans kick on, or I have any accessories running, the gauge goes down to like 10 or so. But it only does this at idle. I can hold down on the brake andgive it some gas and it goes right back up to 13. Just replaced the battery (optima Red Top) and the alternator checks out fine at 3 different auto places. Also, when the gauge goes down, the fuel pump makes a humming noise. I don't know if that is related, but it might be. Could it by chance be a clogged fuel filter or the pump taking extra voltage or just a bad alternator?

 

I fixed a somewhat similar problem by cleaning the the engine harness ground connections. On my car they are under the upper Alt bracket and at the back of the Pass. side head. Just had to remove the mounting bolts, scrub the surfaces clean, and bolt together.