Alternator Giving Up After a Few Minutes Running

[greywurm]

Long post, need some help... short story is my alternator (regulator actually) fails after engine is running for a few minutes and volts start to slowly drop out. 14.5 volts to 12.5 or less. Replaced 4 alternators and a few batteries, wires look ok, etc. Read on for more info. I am just really stumped on what could be going on and what to do next.




I purchased a 2006 Yukon XL Denali around 87000 miles a couple weeks ago and have been trying to work out a charging system issue.

Initially, it had a bad battery when I bought it, so I picked up an optima red top. On the drive home, the volts dropped to where it was running off the battery. It eventually threw a check engine list, so I stopped and ran the codes from my phone. It showed MAF sensor issue and lean codes, which I understand can happen when system voltage gets a little low. To see if I was stranded or not, I tried starting it and the volts were back to normal, so I continued home.

Over the last week, it's been giving me the same issue, starts up with good voltage, then drops and runs off the battery. This told me the regulator is faulting, so I had the parts store do a system check and everything passed at first, but if we waited a few minutes, the regulator would fail.

So I picked up a new alternator and put it in. Worked fine for part of the next day then started with the same issue. Went and got another new alternator. Same thing. Good for part of a day, then bad. Then tried 2 different new remans with same results.

So I figured the optima gel style battery just wasn't going to work with my system, so I put in an old battery from my old truck. Worked for a while then same deal... Starts up normal voltage then back to running off the battery.

Checking voltage while it is going bad... across the battery posts shows around 12.5 while junction to alternator ground shows a little more, like 12.6 to 12.7. Same with alternator post to alternator body. On first start, the alternator is showing 14.5. This latest alternator I just put in this afternoon, after about 10 minutes driving to the grocery store, it was down to 12.5.

Since I seem to get some time with it working well after the battery is disconnected to swap alternators, I'm thinking that the PCM is discharging and resetting. Then after some time it starts to malfunction.

Am I in the market for a new pcm? I don't know what else to look at. Battery, alternator, and starter all test out Ok (when first started, then alternator fails due to regulator) and the ground wire from the battery looks Ok with no visible corrosion. Same with the primary hot wires from battery to junction and junction to alternator. No visible damage on the pigtail wires to the harness. I am using alternators, both new and reman from O'Reilly because I have had good luck with them in the past, and new from dealer is almost $600 versus the $160 I paid now. The returns test ok on their bench tester at 13.3 volts.

I heard that the gauge can go bad which is why I'm checking voltage with a meter, but can that cause the system to tell the regulator to stop outputting voltage?

[72MARIO]

I have a LS swapped 1968 suburban with a 6.0 in it and it does as you say 14.9 to 12.5-.9 after a few miles. I was concerned so I plugged in the scanner into our 2005 tahoe and it does the same thing... I think I read somewhere its a economy thing.. You might have better luck posting this question in the ls swap area more action over there and lots of knowledgeable guys.

[greywurm]

I just learned about this yesterday after doing a lot more digging. There is a module a few inches from the battery on the negative post that is called a battery generator regulator. It senses battery flow and adjusts the alternator regulator to system demands. It's part of a regulated voltage control system. GM put this on 05 - 10 full size trucks and SUVs (I think they stopped in 2010)

When the engine is in fuel economy mode, and electrical demand isn't too high, this low voltage mode kicks in and drops the alternator output to between 12.5 and 13.1. My readings at the alternator were right in this range.

To confirm, you can turn on tow mode (if equipped) or turn on your headlights and it will trigger the alternator to slowly step up the voltage to the usual 14.x range. Mine did just this, but it is slow.

I didn't connect the dots that by the time I was done with work and got to changing the alternator, it was dark enough for the headlights to come on on their own, so it explains why it was "working fine for the better part of the day" after each change.

Never knew this existed until yesterday. I can't tell if the confusion is worth the fuel economy for day time driving, but I guess it lowers the stress put on the alternator.

Now I feel bad for going through 4 alternators that probably were just fine.

[truckdude239]

I went though 8 in a week on my 02 suburban before i got a good one

[72MARIO]

hmm I don't have battery generator regulator in my 1968 suburban that's ls swapped. I do have it in the 2005 Tahoe. I am curious why and how the suburban voltage is varying.

interesting read on this subject.

http://www.flatrater.com/samples/Battery/rvc.html