The voltage gauge in my 85 Camaro works fine when the car is cold. Once the car/engine/interior warms up the gauge will sometimes read at 13 volts or drop down to near the bottom and stay there.

I have replaced the gauge and the exact same thing happens. The cluster has been out a few times to replace bulbs and it seems to be securely fastened.

Where does the voltage gauge get its signal from?

I should note that a voltage tester on the battery terminals show a constant voltage when the gauge is going up and down.

 

It comes from the 10A Gages fuse. It's a Pnk/Blk wire at the cluster. It powers your coolant temp, oil press. and Tach gauges. Are any others bouncing around or reading inaccurately?

 

All the other gauges work fine. Oil pressure rises and falls with engine speed but that is a different story.

The voltage gauge has been going up and down even with 2 different alternators.

 

I'd pull the cluster and check voltage at the gauge itself, when it happens substitute a known good B+ source and a known good ground source to see if you've got a bad B+ or bad ground to the gauge or just a gauge problem.