I have a 2014 Chevy Traverse with 190,000 miles. Three years ago (130,000 miles on vehicle) my wife got stranded with a dead battery. Had to jump start the car to get it home. I charged the battery up with an external battery charger and all was good for a week. Then battery went dead again, same story. I installed a brand new battery at this point. All appeared fine with car running, but noticed on the volt gauge that the alternator would stop charging anywhere after 5-20 minutes of driving. I took it to the dealer and they said it was a bad alternator. I installed a new alternator and discovered that the problem was still there. I eventually noticed that the alternator would work 100% of the time that the headlights were selected on. If the headlights are off, the alternator will stop charging the system after 5-20 minutes and the battery will slowly discharge. Upon turning headlights on, the alternator will start working again and voltage goes back to ~14 volts. We've been driving it like this for 3 years now with no problems as long as we keep the headlights on when the car is running. Any ideas?
Not sure what you mean by " I wrung out the wiring to/from alternator".
But you took it to the dealer and I'm sure that they checked the PCM's PWM voltage control signal to the alternator and determined that it was present so they condemned the alternator.
And they may have been right about that.
Anyway it's interesting that you have ~14 volt charging voltage with the headlights on and GM's "default" charging rate is 13.8 volts.
The BCM monitors battery temp (estimating it from the intake air temperature sensor), as it monitors stuff like the headlights being on or other high amp loads like the heater blower or windshield wiper motor being on, and looks at current going into and out of the battery. It then tells the PCM to either increase the charging rate or decrease the charging rate.
The PCM sends a PWM (Pulse Width Modulated) voltage signal to the L terminal on the alternator as it measures the alternator's output on the F terminal.
If the PCM sees everything looking good here, but the BCM keeps insisting that it isn't seeing that battery SoC (State of Charge) the PCM (on GMs) goes into a default PWM signal of 60%. (13.8 volts charging rate).
I found a graphic from Remy Power Products showing the setup:
I gotta say, 90% of the time the problem is with voltage drop in the cable going from the Alternator to the Battery. Corrosion gets in there. The voltage the alternator is outputting isn't making it to the battery. And this is also the #1 cause of alternator failure so that's why I said the dealer may have been right when they condemned your last alternator (because the alternator gets overworked until it fails).
In your case the 1st thing you want to do is to measure the voltage output at the alternator. Put the system under load by turning on the headlights or the heater blower motor.
Then just grab a $10 multimeter, DC Volts (20 volt range) and touch the multimeter's Red probe to the lug on the alternator that has the battery cable connected to it. Touch the multimeter's Black probe to the alternator's metal case.
Jot down that voltage value and then repeat that same test on the battery Positive and Negative posts.
The voltage shouldn't be more than 0.2 volts different in each test. If it's more than 0.2 volts different, it's going to be a problem in either that battery to alternator cable or the connections to the battery or alternator.
So that's 90% of the time. What about the other 10% of the time?
It's more difficult for a do it yourself guy to troubleshoot because you need a bidirectional scanner to command different PWM duty cycles to the alternator as you watch the voltage signal on a scope at the alternator's L terminal. But you can inspect the condition of that 2 wire wiring connector at the alternator for damage/corrosion because voltage drop here will also mess up the charging rate
Why you have some type of wiring short in the car? Check all the words going to the alternator, especially ground wires on the alternator or the engine. Bad ground wires can do weird crap like that
I initially thought the same thing and wrung out the wiring to/from alternator and checked grounding engine to frame, etc. Everything with wiring was good. I figured there must be a problem with a computer in the system having controlling issues, but I don't know enough about the system to check further. BTW, no computer faults or codes.