Hello All,
I have a 2008 3500 2wd, 6.7 cummins with 55K miles. Started getting a throttle control light on a few months ago. Took it to the dealer and they said it was dirty battery cable/connection. About a month later, started getting the light again, intermittently along with the check engine light once. Couple days ago drove it for about an hour and stopped to grab a bite to eat. when i started it up, it barely started like the batteries were dying. on the way home, noticed that it wasn't charging very much. parked it and tried to start it again and it was dead. at that point i figured it was the alternator so the next day i took it out and took it to autozone to have it tested. it tested ok three times. i was very supprised but went ahead and re-installed the alternator. i cleaned the battery posts but didn't charge them. the truck started right up but the engine light stayed on. so i drove it back to autozone where we put it back on the tester while the truck was running. showed under voltage. system was putting out about 12V. the guy said it was supposed to be around 14V. so, i went ahead and ordered an alternator since they didn't have one in stock. Think it's the alternator???

 

Sounds like an alternator until you said you put the old one back in and it fired right up without charging the batteries. I would check all your terminations on your battery posts, engine ground, chassis ground and check the cables themselves. I had one on my 07 on the passenger side battery that had corrosion in the first 6" of the negative cable that had turned the copper nearly to dust. Also wouldn't hurt to load test the batteries and see if one is failing causing the other to run down.

 

Have you figured it out yet buddy? Mines doing the exact same thing battery are good and I'm pulling the alternator off tomorrow to get it tested...

 

Definately LOAD TEST the batteries. That is the only way to tell if the batteries are good. If one is bad, replace both.

 

my alternator tested good and both my batteries tested good. i checked my connections, that i could see and they seemed fine. the only thing i did was clean my battery posts. ive been driving it and so far, so good. pretty funny if that's all it was. hopefully i don't have a gremlin that shows up. guess it could be a cable. doubt it's the starter. i'll keep you posted.

thanks for the comments!