You can ONLY find voltage drop by one or two methods or maybe a combination of methods. First, realize that voltage drop is cause by CURRENT FLOW, and gets worse as current increases. It can be ANYWHERE.......in a connector, where the two halves of the connector "mate," inside a switch CONTACTS, at the switch wire terminals, or even at the junction of a wire -to- terminal point such as the CRIMP on a terminal. And don't be fooled by molded fancy rubber covered ones, they can fail/ corrode internally, as well
1...Shotgun approach. Determine what is the worst symptom, IE the starter, etc, and go over every inch of wire and terminals looking for problems. Either strange appearing insulation as to internal corrosion or signs of previous overheating, "weak" areas which might indicate broken/ corroded wire strands internally, and so on.
2....The methodical approach. Someone mentioned over-charging. MANY times over-charging is cause NOT by bad components such as the VR, but by VOLTAGE DROP. As a long time Mopar guy, this is VERY common on the older "classic" Mopars. Drop in the harness from the battery to the VR, through the ignition switch and various terminals
AND DON'T FORGET THE GROUND side of the circuit
So example, overcharge. Make up a jumper wire, say, no 16 or 18, with alligator clips, long enough to easily reach from the battery to any part of the boat with electrics. If it has to be 30ft, that is fine. Connect the jumper to the battery positive post (you might want to fuse that) and run the wire up to the VR. Connect the jumper wire to one post of your meter, and connect the remaining meter probe to the VR IGN feed terminal, that is the one running to the VR switched power. If you have a separate "sense" terminal, check that too. Turn on the ignition switch to "run" and take a reading
With the meter now connected you are measuring the drop DIRECTLY. For example, if the meter reads .5V that means you have 1/2 volt drop from the battery, through the harness, switch, connections, to the point at the VR that you are measuring, and if this is the SENSE terminal, the VR will ramp the voltage UP 1/2V because of that drop
Example, starter solenoid. Let's say the solenoid won't reliably pull in. You've "thrown money" at it, replaced the solenoid, etc. So the "path" is battery--harness--terminals--etc---key switch starter contacts--neutral safety switch---back through the harness--connectors--etc --to the solenoid and to ground. This of course is just the FUNCTIONAL path, not "electron flow."
So with a helper at the key, or with long enough jumper wires that you can get the meter such that you can read it while operationg the ignition switch, wire to the battery PLUS and to the solenoid S terminal. Crank and take a reading. Whatever you read, again, is the DROP. Now you must "track" along the path that leads from battery to solenoid.
I WOULD START with seemingly obvious culprits------a harness connector or the ignition switch / starter button itself. Measure across the switch terminals, and crank the engine. If you measure .3V across the switch, but ONE VOLT in the previous test, then you have more than one criminal at work.
This stuff is not really as complicated as many seem to think. THE HARD PART is getting documentation and figuring out the wiring.
And I said it before, don't neglect the ground path. Even on vehicles, grounding can "trick" you sometimes
