No alternator output?

[sloguy]

I recently received a 2004 25 HP Johnson outboard with about 50 hours total time. After initially checking the battery (12.58 volts) and running the engine for about 10 minutes twice, I found the battery lost voltage down to 12.52 volts. Next, I checked the voltage at the battery while the engine was running about 1/3 throttle and found the voltage reading was 12.50 volts. It appears that the alternator is not having any output to keep the battery charged. I need a couple of things - any suggestions as to the source of the problem? - where do I find an online schematic showing wiring locations to check? - where do I find a mechanical layout showing where the alternator is? Thanks for your help.

 

[Bosunsmate]

Re: No alternator output?

It could be your stator or rectifier, theres no alternator like a car as such.
The charge coil i think runs the engine on this one and the stator does the charging of the battery but im not 100% on that.
Either way if its not the rectifier thats faulty you will need to take the flywheel off and see if its the stator or charge coil the rectifier has its wires leading too.

The rectifier part number is 0581778
If you look that up you will probably come across a parts diagram for it. Iboats will most liekly sell it too.
Itd be good to have engine diagrams on this site

 

[Bosunsmate]

Re: No alternator output?

Take the wires that come off the rectifer and test the voltage coming to them from under the flywheel. Itl be VAC

 

[sloguy]

Re: No alternator output?

Thanks for the guidance, Bosunmate, but since there are 3 wires coming from the stator to the rectifier, I'm unsure how to connect my meter to check the VAC reading. I don't want to make an error that causes a short failure in the stator. That'll be big bucks to fix, whereas the rectifier is only about $30. The 3 wires are yellow, yellow with a large blue stripe, and yellow with a narrow blue(?) stripe.

 

[Bosunsmate]

Re: No alternator output?

Yes thatl be the stator that charges it not the charge coil.
See if you can find the correct place for the multimeter leads in here
CDI Electronics Practical Outboard Ignition Troubleshooting

 

[sloguy]

Re: No alternator output?

Thanks, again. Good info at that link, but the electrical guidance is all about spark/no spark and other running problems. Nothing about stator, rectifier, and battery charging. I'll wait for more specific guidance, then maybe have to take it to a service guy.....ugh. It may pay later to just spend $30+ and try it.

 

[boobie]

Re: No alternator output?

All the info you need on this can be found in an OEM Factory Service Manual.

 

[sloguy]

Re: No alternator output?

Ooops, Bosunmate, I did find some info for checking charging in your link that I can use. Will try some of it tomorrow.

 

[F_R]

Re: No alternator output?

Simply put, there should be a low resistance continuity between any two yellow leads from the stator. There should be no continuity between any of the yellow leads and ground. It is rare for one to go bad unless the coils are melted or something.

99% of the time the problem is a blown rectifier. I haven't followed the link mentioned, did it tell you how to test a rectifier? There are six diodes in that rectifier, each one requires two tests, forward and reverse polarity. That is twelve tests and it must pass all 12.

 

[sloguy]

Re: No alternator output?

Simply put, there should be a low resistance continuity between any two yellow leads from the stator. There should be no continuity between any of the yellow leads and ground. It is rare for one to go bad unless the coils are melted or something.

99% of the time the problem is a blown rectifier. I haven't followed the link mentioned, did it tell you how to test a rectifier? There are six diodes in that rectifier, each one requires two tests, forward and reverse polarity. That is twelve tests and it must pass all 12.

Nope. It didn't show/list 12 steps. A couple of running measurements and one bench test. Didn't sound too complete to me, but better than nothing. I'll follow that routine tomorrow.