'87 Johnson Sailmaster charging circuit - SailNet Community

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Old 07-14-2013
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'87 Johnson Sailmaster charging circuit

Me again... '72 Pearson 26 from hell... ..

Actually, things are going very well. The new compression posts, mast support crossbeam, and bulkheads are installed, the mast is rewired, and the mast step is sealed and in place. The outboard mentioned in the subject has had its bottom end rebuilt down to the last bushing, bearing and seal, and, after running poorly, just been the recipient of a carburetor rebuild. I haven't learned this much in so short a time since college. I won't tell you when that was.

Anyway.. does anyone have any knowledge about the charging circuit of the aforementioned outboard? I have the shop manual, but it barely mentions the alternator circuit.

Here's the situation. This is a manual start engine. There's two wires coming off the charging circuit of the motor that are connected directly to the battery via the battery master on/off switch and a 1 amp fuse. At idle, the wires output about 4 volts, and the voltage rises to about 16v at full throttle. When connected to the battery that shows a voltage of 12.58 at rest (a new basic deep-cycle battery), the voltage read across the battery terminals increases as the engine is revved but not dangerously so... maybe to 13v... but reads lower than the battery at rest when the engine is idling.

There are no regulators anywhere in the line, other than whatever is under the hood of the outboard.

Is all well? Is something wrong? I'm concerned about the huge swing in voltage output between idle and full throttle... 4v - 16v or more. The 1 amp fuse seems a little low to me, too. The original fuse was blown when I took possession of the boat; I temporarily replaced it w/ a 5 amp just to run a quick test or two. 2 amps, maybe?

Thanks in advance for any insight. This place has helped me more than words can ever say.

Best to all.


Thanks in advance for any input.
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Old 07-15-2013
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Re: '87 Johnson Sailmaster charging circuit

Don't worry about idle voltage. The alternator is too small to overheat the battery. If the charging voltage is somewhere between 12.8 V and 15.5 V you are gaining some but not too much. There is no voltage regulator. I would use a fuse somewhat larger than the rated output of the alternator which is probably about 6 amps.
Riding lawnmowers use a similar system.
Seafarer 29 KCB
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bblument (07-15-2013)
Old 07-15-2013
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Re: '87 Johnson Sailmaster charging circuit

I had about the same output on my Evinrude 35 HP 2 stroke. The charging capacity was pretty wimpy. Agree on the fuse.

Paul T
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