1986 Kenworth 3406b starter wiring

Verify that all batteries are getting the same voltage. A bad connection at the batteries can show symptoms as high voltage.

If all batteries are within 1 volt of the alternator, I would agree the alternator has failed, one battery at 16 or 17 Volts and the others at 12, you've got a connection or cable issue

 

Easiest way is All the gauges if still original in the dash would be marked on the backs of them “+ GND”

Positive ground was common in same makes of trucks up thru the 80’s. Most were 24v start and 12v run with series/parallel switch. But not all were. There were some up into the 80’s that were just 12v. They can be converted to negative ground and most were. The 12v Delco starters were the same and didn’t care about polarity but the alternators had to be different for the polarity. So if someone swapped over to neg ground just by simply changing the way the switches,gauges, starter, and battery cables are posted but did not change to the correct alternator the system will have gremlins.
I’ve rewired and changed over two trucks from scratch recently. A White and a Diamond T that were pos ground. The Diamond T had a generator and external voltage regulator. I had a rebuild shop go thru it and change polarity for neg ground and they tuned and set the regulator to match it. The White I just installed a standard Delco 34Si on it.

 

A bad cable or connection on the alternator can cause high voltage. Same as before with a meter. Get truck running, turn on all lights and accessories. Check voltage drop between alternator stud and the appropriate battery post. Then alternator frame to the ground post on the battery. Voltage drop should be nearly zero. If there is a volt or a couple of volts then the alternator needs to produce higher voltage in order to push past the resistance.