After a lot of searching and many helpful responses from this group I have found the definitive information I was looking for.

Anyone searching for information on the correct solenoid part# for your early Johnson it is (as of the writing of this entry) Sierra 18-5807, OMC 378444 or ARCO Marine SW081.

I have also found the answer to one of my big questions. The “I” terminal on the correct solenoid needs a ground signal from the safety switch when the motor is started in order for the start safety (against high speed starting) to be disabled. If the throttle is at the start position or (s)lower then the mercury switch is active and the starter cranks. Any higher than start and the switch is “open” so the solenoid sees no ground and there is no crank.

The question whether or not that terminal sends any voltage at all has now been answered. No voltage sent, just needs to see ground.

Lastly, a question never quite answered but obvious I’m sure to most but me…

The pigtail wire from the mercury switch is to be wired to the long side post on the vacuum cutout switch. The black wire from the motor side wiring harness is also landed on that post. This is the solenoid wire from the boat side that starts out as a white wire on that side. NOTE: If your motor has the vacuum cutout removed, you will want to connect these two wires directly. They are landed on this post because the vacuum cutout is isolated from ground. So a direct connection would be the next best thing. Do not wire the black harness wire directly to ground, doing so will disable the start safety!

And, of course, the magneto wire to the points on cylinder #2 goes on the center terminal of the vacuum cutout switch.

So, I leave this rambling to anyone who is as confused as I was on this topic. Most on this site don’t need this tutorial, I’m sure, but for that one individual who is scratching the hair off their head, I leave you this small bit of knowledge.

Disclaimer:

’57 Johnson 35HP RJE-19M or

’57 Johnson 35HP RDE-19

Only 2 motors I am 100% sure about…

Photo credit to original poster JoeB (Feb 19, 2022)