@db4690 and others. The purpose of this post is to admit that my memory was probably faulty concerning the 24 volt diesel starter. It most surely was a 12 volt. Don’t know where I got it but might have dreamed it for all I know.
I looked through the old service manual and could find no reference at all to a 24 volt starter. The two batteries are wired positive to positive and negative to negative as I remember them. The positive wire for the starter, according to the schematic, is tied in from one of the positive cables, and ground is ground. So there is no way that 24 volts could have been provided to the starter that I can tell. Rockauto too makes no reference to voltage on the starter.
So at any rate if you’re wrong you’re wrong. Going through that manual did bring back some bad feelings though that I thought I was over. I put over 500K on that diesel and think I replaced or took apart just about every item in the manual. I paid $10K for that car and the diesel cost me $800 extra. It did get 24 mpg but I’m sure I spent thousands extra on repairs and engines. I didn’t know when to quit. I still like the diesel smell but just couldn’t bear to buy another. I did have both batteries go at the same time like the OP though and wasn’t the first time.
Autozone shows that starter as 12 volts; not surprising.
A lot of the old military vehicles had 24 volt starters along with 24 volt everything else. Someone in my stash I’ve got some U.S. Army truck and motorcycle blackout lamps.
The truck lamps are made by Guide and are marked 24 volts on the bulbs.
The motorcycle blackout lamps (also Guide) are almost dead similar in appearance but are 6 volts bulbs as is the one Indian lamp I have.
When I was involved in trucking (1955 t0 1995) we went from 1 or 2 six volt batteries running a 6 volt system to 2 or 4 six volt batteries running a 12 volt system to 1 or 2 twelve volt batteries running a 12 volt system.
In the early days we even had some trucks with a single six volt battery with an air starter.
One of the reasons for running six volt batteries on a 12 volt system was that trucking companies had trailers in their fleet that had all 6 volt bulbs so we had a vari-volt switch on the back of the cab that would switch the trailer light plug between 6 and 12 volts.
I even knew one driver that had his string of 6 kerosine lamps that they used before trailers had electric lights. It has been years since I have seen one of the old 28 to 32 foot long flatbed convertible trailers with the curved hooks on the 4 corners and midway down each side.
Every 2 hours the driver had to stop to check the tires and refill the lanterns.
The 2 hour tire check rule didn’t end until after I retired.