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Battery light coming on after starting
HELP!! I've been fighting this for almost 2 years. My '04 6.0 was built in July of '03. It has the single alternator is very stock. It has only 52,300 miles and I have a scan gauge on it. 3 years ago the light kept coming on and I was able to determine that the belt was old and slipping. I changed it and the light stopped coming on. Almost 2 years ago, it started again. Here is what it is doing:
Start the truck in the morning. Voltage on the scan gauge reads about 11.7 for about a minute. Bumps up over the next 2 minutes to 12.8 / 13.5. Settles in at 13.8 to 14.2. I only drive 2.9 miles (about 10 minutes) to work. I know this is not good on the system. The light will go off and on a couple of times. I leave 3 to 4 hours later and the same thing. When I drive a greater distance (more than 10 miles) it will stay off. I drove home last night (104 miles) and 12 hours later, the light came on this morning. I will be checking the idler pulley this week. It may be that it's dragging until it gets hot.
Here is what I have done:
1. Had the alternator tested. Good
2. Swapped the alternator with a rebuilt. No change, put the original back on
3. Pulled the batteries (2 years old) and had them load tested. Good
4. Charged the batteries all night. No change
Does anyone have any ideas what else to look for???
The only other thing I have done is have the ficm rebuilt with an output of 54.5 volts.