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I just replaced my FICM, batteries, and alternator about a month ago and this morning the voltage reading on my scan gauge dropped from 13+ to 11.9-12. Then it stayed at 11.9 and the battery light came on. Am I seriously going to have to replace the alternator again? What would cause this to happen?
 

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That's not likely the alternator causing that. You're seeing battery drawdown from the GPs. I'd put my money on one or more batteries being weak. If you're putting out 13.5-14V going down the road once its warmed up the alternator is doing its job.
 

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Thank you for the reply, but did you read the post? "The voltage reading on my scan gauge dropped from 13+ to 11.9-12." Indicating that the alternator was out. I had the alternator bench tested and it was indeed bad. My question is what would cause it to go bad in less than a month? By the way how much money were you putting down? haha
 

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Yes, I read it, but you are misunderstanding what you are seeing. The amp load when starting a 6.0 and while the GPs are lit, which is for several minutes after starting, is extremely high. It is too high for a stock output alternator to fill the void created by the drawdown of the batteries. So what you are seeing is the load draw the batteries down to a degree for which the alternator cannot compensate. Once the GPs go out the alternator can fully recharge the batteries, which is why you see the reading go back up. A weak battery exacerbates this effect.

This is why 6.0s seem to go through batteries much faster than other vehicles. My stock batteries lasted all of two years. I then switched to Interstate Megatron II batteries from which I typically get 5 years. My stock alternator has 194k on it and still does the job.

An option is to swap in a higher output alternator like one from DC Power, but they are spendy ($300+).

Still another option is to upgrade the charging cabling from that alternator by adding a 1 gauge wire from the charging post to the + terminal on the RH battery, and add a 1 gauge ground from the alternator case to the chassis ground lug on the crossmember. I did that upgrade about 74k ago.
 
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