Hey guys, sure could use some help. 2013 silverado 2500hd, 6.0. Noticed the volt gauge indicated overcharging....literally pegging out the gauge, and causing the headlights to flicker. And I checked with a volt meter, same. I am getting the "battery charging system" message. The alternator is new, AND I had it tested at 2 different places. Charged the batteries and both load tested good. Cleaned both ends of all battery cable connections, and replaced starter. It is still over charging and the headlights flicker at high Idle. Any help would be awesome and appreciated.

 

Why do you have 2 batteries? A 6.0 wouldn’t have had 2 batteries from the factory. If it’s been added for accessory reasons, was the alternator upgraded to accommodate?

Start by checking for codes - they can be stored even if the CEL isn’t lit.

I’ll review the wiring diagram later today and advise you on the next steps.

 

This may sound crazy , but if one of the batteries is defective it can affect the entire system and charging function.

Suggest you remove one battery from the system at a time and see if you get different charging results.
If one battery for whatever reason is calling for charging, the battery that does';t need it will get overcharged.

A poor connection where the alternator gets wrong info can result in an overcharging condition.
Might be a good idea to check all the connections that supply info to the alt.

 

To amend one of my earlier statements - if your truck has RPO code TP2 (check label in glove box), it would’ve been equipped with dual batteries from the factory. This is most likely the case if it’s a cab/chassis, work truck or other utility type vehicle that would’ve been expected to have additional accessories or equipment installed on it. Decommissioned police and/or emergency vehicles would be included for this RPO.

Start by unplugging the 2 pin connector at the alternator and checking voltage on the brown wire. This is the control wire from the ECM.

Key on, engine off, there should be 3.0-5.5 volts.

Diagnosis can go several different directions depending on the results of this test. Since the ECM should be controlling alternator output, verifying circuit integrity is the first step.

To build on/add to what hanky said -

One bad battery in a dual battery system can wreak havoc with the electrical system. Disconnecting/isolating one battery at a time to observe the alternator response is a good approach and can be achieved by simply removing the ground cable from the battery to prevent accidental short to ground from the positive cable.

Bad batteries can test good, regardless of the test method!

So, here’s some other things to look for:

1) Acidic discharge around the caps/vents, regardless of quantity. May or may not be accompanied by a rotten egg smell, which is hydrogen gas and extremely flammable. Use caution when disconnecting terminals as arcing could start a fire. PPE highly recommended.

2) Bulging of the battery case, indicative of a burst cell. Not always visibly obvious, so it helps to run your hand/fingers along the case and feel for a bulge/convex shape. It can be flat on one side and bulge on another.

3) Disconnecting the battery overnight and load testing or checking the voltage in the morning. A bad battery can discharge overnight when it’s not connected to anything.

 

So I changed out both batteries, and it fixed the problem.....for a short time. It is still overcharging and the lights flicker. There are 2 batteries because we have added a plow to the truck. I have disconnected all aftermarket wiring, and cleaned evey connection. Still not working correctly

 

Did you check the voltage on the control wire, as I suggested in my last post?

 

Yes I did. 5.0 volts. Took the alternator to a 3rd tester. He determined it wasfaulty