Electrical gremlins

[martinphoto]

Hey all, I have a '99 f350 4 door. Problem started a few weeks ago with dead batteries one morning. Jumped her and got it started. Same problem next day. Tested batteries. ok. tested alternator. intermittent. Replaced alternator. After few days same problem. Had to drive from Casper Wy. to Boise Id yesterday so put in 2 new batteries friday night. started fine in morn yesterday. brought the 2 old batteries that were tested fine and fully charged along just in case. About two thirds of the way gauges started going apesh*t and soon after it dies. Replaced one battery closest to alt. started up and ran fine for another 100 or so miles. Same thing. Replaced same location with last good battery and got to within 60 miles of home and died. Called family member to bring me another battery. Swapped again. Got home. Shut it off. Tried to start it and it did start but barely. Today put fresh charged batteries in. Started up and I read 15.4 volts on the batteries so I believe the alternator is working. Any ideas? Cables are all freshly cleaned etc. Gonna check my ground connections to body etc. Anything else. Thanks

[Motomike777]

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.

[Sticker Monster]

just a guess, but I have read of problems with the positive battery cable on the duel battery trucks. you might want to look into that.

SM