This could happen to you;
During my last trip the voltmeter did some strange things. Volts fluctuated from 12 - 15v then settled down. This trip it was steady at 13.5v for the first half of the 1300mi trip.
On the first 200mi leg of the return it started out at 12v and ended at 11.4v three plus hours later. In the past it has not unusual for it to run at 12.0 - 12.5 for about 30-40mi then kick in at 13.5v. However, this time it never did and kept decreasing. To make matters worse the weather was cold so I had to use the original monster heater fan motor plus running lights for an hour and then all lights for another hour.
To make a long story short I found that my year old alt belts were loose. I could turn the alt pulley by hand. This made no sense to me because the weather is much colder here than usual so I expected the belts to shrink and get tighter. Maybe the whole engine and mounts shrunk instead - who knows?
I've tightened the belts again and assume it will work fine tomorrow for the remaining 450mi.
This does explain the Volts fluctuated from 12v - 15v on the last trip.
This is a 4104 with 671 engine and old gen converted to a 160a truck Leech-Neville alternator.
Thought this experience might help solve someone else's mystery alt problem sometime down the road.
Cold weather only applies if the pulleys are cold...
The friction of the belts causes the pulleys to heat up and belts to
stretch under heavy loads.
Technically you would tension the belts when warm, not cold.
Just a technical glitch-splantion...
Oh. No a NEW word.....
Dave....
This is a 160a truck alternator in a minimum electrics 4104. I doubt it has ever been under more than a 30a load since I've owned it.
I couldn't figure out the fluctuated from 12v - 15v until after I found the loose belts, had never had that happen before?
I forgot to mention that the bus batteries are two Group 31s from Sam's Club, don't remember the brand. I installed these batteries to replace the monster 8Ds that came in the bus to make room for two Group 31 house batteries.
These four batteries do all I ever need and more.