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wbeaton.
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November 29, 2015 at 1:58 pm #3079
I’m converting a 1958 Johnson RD to electric start. The motor only has a single mercury switch like the previous RD’s. After looking at the wiring diagram I don’t understand why there is a second mercury switch for the electric start motors. I understand the purpose of each switch. What I don’t get is why they don’t share a mercury switch considering that they are stacked and appear to do the same thing at the same time. I doubt OMC would waste money on a redundant switch so what am I missing here?
Wayne
Upper Canada Chapteruccaomci.com
November 29, 2015 at 3:32 pm #27802in the "BOOK" they refer to one mercury switch as a "starting range" switch which prevents the engine from remote starting if the grip handle is set to high
on one diagram one switch is the cut-out for the grounded side of the selenoid and the second mercury switch as Something to do with the magneto circuitry see diagram page 531
http://boatinfo.no/lib/johnson/manuals/ … on.html#/0
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November 29, 2015 at 3:51 pm #27803Thanks, Crosbyman. My question is why two separate switches if both switches appear to do the same function, which is completing the circuit(s) to ground when the throttle is advanced? Why couldn’t both circuits be connected to the same switch?
Now that I’m thinking about it, it probably has to do with the key switch. I added the double mercury switch from my donor motor so the motor is now factory correct for a 1958 electric start.
Wayne
Upper Canada Chapteruccaomci.com
November 29, 2015 at 5:10 pm #27810dont know but I can only assume it was to prevent some form of electrical relation between both circuitry
the other possibility could be for double safety reasons in case one mercury dropplet get stuck for some reason… better safe than sorry !
worse thing s to worry about in life 🙂
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November 29, 2015 at 5:25 pm #27812Crosbyman…..
The second switch completes the ground from the starter solenoid pull-in coil. It’s purpose is to prevent starting the motor at fast throttle settings as we know. Straight forward on that one.
The first switch completes the ground for the vacuum cut-out switch… which prevents motor run-away under no-load conditions like when you shear a drive pin. It only does that function at low throttle positions, however. That is what the mercury switch does, only allows that to happen when the throttle is at low. Otherwise, the vacuum switch would cause mis-firing of the motor at cruise to high throttle settings.
So perhaps that is why Wayne? Would they interfere? I’m not smart enough to know. Especially on a Sunday morning.
November 29, 2015 at 8:39 pm #27824Actually, the 1957 did the same job (s) with a single mercury switch. The dual setup on 1958 models allows a slightly different make/break time for the different jobs. Is there a good reason not to use a single switch on a ’58? Not that I know of, if you adjust it properly. Just use the ’57 wiring diagram if you elect to do it that way.
BTW, later on they went back to a single switch to do both jobs, but used a micro switch instead of a mercury switch. Micro switch was a big improvement.
November 29, 2015 at 9:43 pm #27826Thanks, everyone. I did notice that one switch is offset to open before the other. I guess we’ll just chalk this up to one of those engineering changes that was better on paper. I used the double switch because it was quicker and easier than splicing an extention to the solenoid lead. The owner has a ’58 external harness that he will be using with it.
Wayne
Upper Canada Chapteruccaomci.com
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