I've done some searching for my problem already here in Z28, however haven't come across anything that seems similar. Hopefully you gurus out there can give me a hand.
I have a 93 z28 m6. When I first start the car, the voltage reads right in the middle of the gauge and seems to be ok. As my engine comes up to operating temp. (160) there is some strange voltage happenings.
When at idle my needle moves toward the center hash mark. When I'm in first and heavy on the gas the voltage drops significantly to where the "check gauges" light comes on. In between first and second shift it moves just a touch toward the middle and when I engauge 2nd it just goes back down. It's not until 5th or 6th gear that it stabilizes and moves toward the middle, however never reaches it.
This is constant while driving. I do a fair amount of city driving and sometimes the voltage doesnt recover as quickly and will stay in the shaded area for upwards of 10 seconds at idle when I stop.
Not sure what could cause this malfunction. I would say alternator or the belt slipping, howeve it doesnt make sense if I am at idle and the needle is moving towards neutral and loosing voltage under load, unless the alternator is just not putting out the desired voltage. Guess this weekend I'll check.
If anyone has any bright ideas, please share. Thanks.
Steve

 

Get the alternator checked out. Most places can do a load test to see how many volts and amps its putting out. Autozone does it for free but make sure you get somebody who knows how to use the equipment like an older guy.

 

Could be belt slippage. The tensioner may be wearing out a little. How old is the alternator?