You cannot seat of the pants troubleshoot these systems. If the light is on GET THE CODE. And read up on how this system works--it's VERY complicated--like nothing you've ever seen before.
Once you get the code, follow the troublshooting in the shop manual.
And read this article on how the darn thing works
The fact that both alternators are overcharging make me suspect the gauge. A gauge that is not properly calibrated would be the simplest explanation. I would put a multimeter on the battery terminals while the truck is running to confirm if I have a problem. It should be running about 14 volts with the car running. The battery should be at about 12 volts when the truck is stopped. You could stop the truck, measure the battery voltage with the multimeter and look at the gauge with the key turned on to see if the gauge agrees with the multimeter. You can buy a cheap multimeter for about $12.
The alternator should have an internal voltage regulator which determines the voltage output. You could take off the alternator and take it to an auto parts place to put it on a bench. Then check the voltage on the bench to confirm the alternator is overcharging. Normally high voltage is related to the voltage regulator. You could take it to an auto electric shop, they may be able to adjust the voltage regulator.
Don't replace things without first doing actual tests.
You either have an ammeter or voltmeter, and neither actually tells that much alone.
The gauge could simply be getting a weak spring and reading higher than it did.
Harbor Freight sells a voltmeter for $3.
The battery should read 12.5 volts, and when you rev slightly, you should get around 14.5 volts.
You do not have an alternator problem unless you get over 15 or under 13 volts.
If your dash gauge is an ammeter instead of voltmeter, then higher reading could indicate your battery is not holding a charge very well. There is no way to test a battery without causing it some damage, so do not have it done. Every time you start the car, you are testing the battery. Just keep it in mind.
And while Greg in general is correct, you have not been able to adjust voltage regulators in decades. It is all solid state these days, and so small that you can't likely even find the regulator.
2011 silverado engine light came on in dash, I notice the alternator hand was really over charging, had it tested and installed a new alternator, no help, it is still over charging, the battery checked out good. what is the most likely cause for it to be over charging.?
