One item on the Kenworth dealership service records mentioned an issue with a low voltage light. Their solution was to replace the belt on the alternator and charge the batteries. On the trip home I noticed the battery icon would appear once in a while. It seemed odd since the voltage display was always 13.8-13.9 volts and the alternator appeared to be brand new.
After having looked at the Volvo electrical schematic and the Delco documentation I noticed a couple issues. Someone had connected a wire between the batteries and the 'R' terminal on the alternator. My assumption is someone thought the 'R' was for 'remote sense'. It's not. I removed that wire. Above the output terminal in the alternator pictured is a knockout where an optional 'S' terminal would be. That is for 'sense'.
The electrical schematic showed a wire labeled '14' connected to the 'I' terminal on the alternator. The 'I' terminal is often used for the battery light. This wire was connected to the battery output terminal so I moved it to the 'I' terminal.
I've not had any issues with the low voltage light since making those changes.
Update July 2018 - I've driven the truck some lately and the low voltage light has reared its ugly head. I decided to check the wiring and see if I could fix this issue for good.
When the truck is running there is 14.2V at the 'I' terminal on the alternator. I found where the wire passed through the firewall connector. There was no voltage there. I could track the wire from the alternator to midway down the driver's side of the engine. From there it disappears into a wrapped harness. There was voltage there. Evidently, somewhere between there and the firewall the wire is broken. I ran a new wire to the firewall...which solved the low voltage light problem.