Join Date: Feb 2013
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Did you test the voltage on the batteries separately? if you don't isolate to test you will get a false positive till they are REALLY dead. That's an aside regardless, since you've replaced the batteries you obv have other issues.
If they are dying while driving, it really sounds like even if the alternator is working, they are not getting charged by it consistently. I would test the batteries when the truck dies. I'm new to diesels, but I wouldn't think getting 15.4 on two fresh batteries after just putting them in was even if the alternator was dead. When it dies is when you know if they've been charging properly and consistently. I'd dig deeper on the electrical and check continuity on wires and clean up connections.
2003' F350 7.3L Lariat Crew-Crab 4x4 6spd Manual Short Bed. 3.73 ratio and that's about all i know... so far.