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?

 

have you checked your belts? what about the bearings in the alternator, they could be going bad. i would change the fuel filter though, if it is clogged it will put more stress on the pump. if your idling too low it will cause your power to drop too. look it over, it's usually something dumb that'll screw ya.

 

Just replaced my belts not a month ago. I have plenty of tention on it. The alternator doesn't make any sounds like bearings going out. I ordered an Accel FP Gauge for the fuel rail. It comes in on Friday, should put some more info down so I can figure this one out.

 

what are u idling at when brakes are depressed and in gear?

 

My car would always do that, in fact it would go as low as 8 volts on the gauge. I would go through a battery and alternator every two years. This was with both the original 85 setup and the 89 serpentine setup. I finally converted to a single wire alternator and that ended my problems...

 

i've only encountered the problem 4 times. 1st, i snapped the upper mounting bolt on my first 3rd gen, 2nd, alt. seized, and third, alternator seized 4th, f*ckin around with the **** boys and somehow my harness pulled loose from the alternator. so if its still running its something simple, could even be a ground.

 

Well, at idle w/ brakes and in gear, I idle around 675-700 or so. Today, out of the blue, the car was charging fine. Then after work on the way home, it went back to being low.

 

are your lights and so on dimming when this happens?
my neighbor's iroc did the same thing and it WAS his alternator taking a hefty dump.
try this...
check the harness/ connections on the alt. try a jumper wire (i.e. about 8 gauge) and hook it up to the alt straight to battery. make sure u disconnect the wire that is hooked up from the factory harness or this test will be pointless.

 

Another problem is the fact that I just bought a serpentine setup for it with all the accessories and it is not ready to be put on the car yet. I really don't want to have to buy an alternator for 2 weeks.