CORRECTED DRAWING ATTACHED
This time in parallel without the dead short to ground.
The only thing this will do is measure the current flow between the batteries (likely nothing unless one is bad).
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CORRECTED DRAWING ATTACHED
This time in parallel without the dead short to ground.
Also glad to help, I am actually in the process of figuring out where to wire an ammeter on my snowplow truck so this thread is helping me think deeper about it also. Except I have a few more conditions to think about (two alternators, two batteries, in-cab battery selector, already pre-hacked wiring by a PO, and a snow plow pump that draws the same current as a starter, lol)
The bottom drawing below 'option 1' is essentially what you are looking for, the tough part is finding just 1 wire that supplies all of the vehicles power then branches off after that. Unless you make a junction box of some sort that will take all of the current positives (small wires on the starter solenoid, alternator output, all the relays on the firewall, etc). Then take just one wire from the batteries and run it through the ammeter then to that junction box.
Well, like I said earlier, Generally I like dealing with negatives in these kinds of circumstances however in this case trying to isolate just the starter from the rest of the devices is virtually impossible to do through grounds... if you think about how many short ground wires there are going to the frame or body and how many things ground themselves through their mounting surface without any wires it just isn't possible to do. With the positives though, you should be able to find everything pretty quickly coming out of the battery cable and starter solenoid.
And I never thought about my plow truck like a yacht before, lol, but I guess it is somewhat nautical. It does move water like a boat, just frozen water instead.