379 Charging Issues

Weve got a 2000 379 with a 3406e. I was driving a week or 2 ago at night and all of a sudden the radio shut off. I looked down and the volt meter was resting down at 10volts and the amp meter said it was drawing 50 amps. I also had no headlights at the point from the low voltage. I found a parking lot and stayed there for the night. When the truck was idling the alternator would start charging (the amp meter would read +25ish) and as soon as I started driving the amp meter would go negitive and the bolts would drop again. Went to a dealer the next morning and they tested the alternator and they said it was charging sporadically so they put I new one in. I continued on my trip and after half an hour the amp meter started going negative again and the voltage started dropping. I arrived at my location to load the trailer and let the truck idle for an hour. It charged the batteries and the truck worked perfect for the return 8 hour trip.


Next trip the truck went on after 2 or 3 hours of driving the voltage started dropping again. Got down to about 12 volts then when the truck was idling again it charged the batteries and all was well. I loaded tested the batteries when the truck got back and 3 tested borderline ok/weak and 1 was ok. I’m thinking the weak batteries may be at the center of the issue but why would the truck charge at idle and not going down the highway. It seems that driving at night with the lights on is what triggers the issue. During the day the truck will run fine.

 

When it was at the dealer they supposedly checked the grounds and connections. They said there was one dirty/loose one which they fixed. I checked/cleaned/tightened all of the battery connections

 

Turned out to be a belt issue. A few days after I posted this thread the belts broke. Drove to a shop and they put in 2 brand new belts and it fixed the problem. I guess the originals were too loose so not spinning the alternator when the rpms are up.

It’s weird the problem persisted after the new alternator was put in because you would think the shop would have tightened the belts correctly but I guess not. All together the truck got a new alternator, new batteries, and new belts and now has no more issues