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Author Topic:   70-HP Johnson Alternator
westcoastwhaler posted 11-13-2007 02:58 PM ET (US)   Profile for westcoastwhaler   Send Email to westcoastwhaler  
1988 70HP Johnson VRO

I was out last week and the battery switch must have vibrated to the off position while underway. I have since removed the switch as I don't want this to happen again.

Now the voltage meter shows about 12 volts underway (less than the app. 14 volts it used to show) so I am thinking that the Alternator is shot. Need advice on where to start on fixing the charging problem.

Also, is there any problem running the engine with a blown alternator?

minitauk85 posted 11-13-2007 04:25 PM ET (US)     Profile for minitauk85  Send Email to minitauk85     
westcoastwhaler- Not sure if this will help- but it might:
Good Luck!-k
westcoastwhaler posted 11-13-2007 05:26 PM ET (US)     Profile for westcoastwhaler  Send Email to westcoastwhaler     
Thanks for the read. With your trial and error, I am thinking rectifier or stator.
jimh posted 11-13-2007 09:43 PM ET (US)     Profile for jimh  Send Email to jimh     
The charging circuit on your two-stroke outboard probably consists of an under-flywheel coil and a separate rectifier assembly. Some of these smaller motors did not employ a regulator. The charging current generated was delivered to the battery, and the battery itself acted as a crude regulator of the output.

When your primary battery switch moved from the ON position to the OFF position while the motor was running, the load on the charging circuit was removed. This may have created a voltage transient which caused the rectifier diodes to fail.

Investigate and check the AC voltage generated by the under-flywheel coil. Check the rectifier. If the engine has a voltage regulator on the charging current output, check this, too.

A simple check on the charging system can be made by observing the battery terminal voltage when the engine is not running and comparing that to the battery terminal voltage when the engine is running. The voltage should rise when the engine is running. Typically an engine will produce about 14-volts at the battery terminal if it is supplying much charging current.

Chuck Tribolet posted 11-14-2007 12:00 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Most likely rectifier, but possibly stator or regulator.


westcoastwhaler posted 11-14-2007 01:07 AM ET (US)     Profile for westcoastwhaler  Send Email to westcoastwhaler     
Jim - I will check the engine out with a voltmeter in both states to make sure.

Chuck - Does the flow of electricity go as follows: Stator, rectifier, regulator? Is the voltage as follows: Stator (AC), Rectifier (DC), regulator (14 volts)? Any idea on the AC voltage?

Just to clarify: It was an on/off switch that can be rotated 360 degrees (with on and off every 90 degrees). Since it is mounted on the battery box which is under the rear deck (15 SSL) away from everything. I am guessing that I did not have it totally in the on position and it vibrated to the off position.

jimh posted 11-14-2007 09:21 AM ET (US)     Profile for jimh  Send Email to jimh     
To learn more about how electrical power is generated in outboard motors, you may find this article of interest:

Boat Electrical Power Generation
by James W. Hebert

Since you most likely have a permanent magnet alternator, you may also find this article will explain its operation in even more detail:

Permanent Magnet Alternators
by James W. Hebert

westcoastwhaler posted 11-14-2007 02:08 PM ET (US)     Profile for westcoastwhaler  Send Email to westcoastwhaler     
Jim - Good stuff. My outboard charging knowledge has been increased and it confirmed my current flow suspicions. Time to get the voltmeter out and start testing. I will post progress as I resolve my issue.


where2 posted 11-14-2007 10:55 PM ET (US)     Profile for where2  Send Email to where2     
My money is on the rectifier being shot. I bet your tachometer is non-functional as well. (for the same reason, dead rectifier)
westcoastwhaler posted 11-29-2007 08:17 PM ET (US)     Profile for westcoastwhaler  Send Email to westcoastwhaler     
It was the rectifier.

Thanks for all of the guidance. I am now well versed in the charging system and also an owner of the service manual. I can't imagine ever owning another outboard without one.

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