I could you some help

[planewhisperer]

With my 72 C10 V8 running at idle, when I turn on the headlights the fuel gauge drops below empty, turn signal indicators become dim when that specific signal is used. My alternator is at 15.6 volts at idle. I have a internal regulator alternator. I was thinking that my system is not getting regulated properly. With the truck running at idle without the lights turned on, when I select any turn signal the gas gauge goes below empty. What do you think...........