379 sporadically won’t charge

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 testedborderline 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.

 

They didn’t put new belts on but I did check the tension after they installed the alternator they seemed good. The fan clutch needs a rebuild and squeals but I can’t imagine that would effect the alternator. I didn’t think about the slipping belt theory though that’s an interesting idea I’ll re check the belts

I agree and new batteries will be put in after I get back to work after thanksgiving. But why would it start charging at idle. Even with the lights on it will charge a little at idle. Obviously it charges much faster with the lights off