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Catalina/Capri 25/250 Sailor's Forums

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Outboard Alternator question

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Scooter
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USA
228 Posts

Initially Posted - 06/29/2010 : 06:07:28

I have an 8hp Johnson (1989) outboard and I want to verify the alternator output. Last year the wires actually melted when it was hooked to the battery. It is possible it got hooked up in reverse polarity. (Long story... Don't ask.). I can't verify that because I removed things so quickly I will never know. The battery is fine.

I have since replaced the wires back to the motor block (Alternator) and throughout the boat but I am afraid to connect them without testing the output of the alternator first. I want to see if the alternator is toast. I also now have a switch on the breaker panel for the alternator so I can isolate it when the motor is not running.

Any thoughts on testing the alternator? Should I simply read just over 12 volts without a load(about 14vdc?)? Or must it have a load? I always thought it must have a load. I just don't want to use my primary boat battery. I could bring a car battery but before I go through all that I wanted to see if there's a way to test an alternator without a load?

If I worked more on cars then maybe I would know this?

Thoughts?

Scooter
C25 '89 SR/WK #5878 "Square Knot"
Vandemere NC.

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 06/29/2010 : 06:36:12

Definitely needs an initial power source and load, and the load cannot be removed once the alternator is powered up.

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delliottg
Former Mainsheet C250 Tech Editor

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4479 Posts

Response Posted - 06/29/2010 : 07:37:37

Here's a pretty good article on what can go wrong, how to diagnose it, and what you should & shouldn't do. http://www.aa1car.com/library/charging_checks.htm

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Voyager
Master Marine Consultant

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5379 Posts

Response Posted - 06/29/2010 : 20:22:38

Scooter,
Determining whether you've reversed the wires (+ and -) is easy. With a digital voltmeter and the engine off, select ohmmeter 1-10 Ohm setting, and place the red probe on the head (a metal part) and the black probe on the wire. If you read 1 or 0 Ohms, that cable is negative. If you read 50 or more ohms, or no connection, that cable is positive.

Get a piece black tape and wrap the negative lead, and get another color (red, yellow, orange) and wrap the positive lead. This way, later on, you won't have to guess.

If at some point in the past, you reversed the connections and cooked the wires, you probably also cooked the DC regulator and/or rectifier diodes.

Let's assume the alternator itself survived this trauma, and suppose the diodes and regulator failed in a way that would continue to supply current, there are two issues you'd have to check.

(1) Your regulator could be allowing 15 or more volts to flow into the battery. This will overcharge the battery and boil the electrolyte off, and you will smell rotten egg (H2S) smell.
(2) Your diodes failed in a connected manner. You will feed AC voltage to the battery. This will alternately charge and short out the battery, which is also not good.

If this is the case, you should replace the regulator and rectifier diodes. You will definitely need to consult the repair or shop manual to do that. If you feel comfortable doing it yourself, great, otherwise bring it to the shop and let them do it.

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Scooter
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228 Posts

Response Posted - 06/30/2010 : 04:39:01

Thanks for the responses. It's about what I thought. I have a car battery that needs replacing. It still works but is on it's way out. I will use that to test it. I will do the ohm test and also look for AC volts.

If it looks like the regulator is toast I will look to get it fixed but we are talking about moving up in boat size in the next year or two.

Square Knot may be for sale in the next year or two. I'll keep you all posted and we will post it on this forum first. It was this forum that lead us to this boat about 5-6 years ago.

Sorry for getting off subject but don't want to put up a flare yet.

Thanks again for the responses.

Edited by - on

aeckhart
Master Marine Consultant

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1709 Posts

Response Posted - 06/30/2010 : 10:34:34

My 1988 Evinrude 9.9 is a 4.5 amp alternator so it's probably not more than that.

I fried mine twice since I purchased the boat in 1989.

Another reason to buy a new motor. Most new ones that I've seen have 10-12 amp alternators.

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Tom Gauntt
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204 Posts

Response Posted - 07/01/2010 : 06:42:02

Scooter,

I'm not sure about the Johnson's, as I have a Mercury, but I think the basics are the same. Most of the engines of this vintage are two-strokes with a magneto rather than a true alternator. If your engine is still running, your magneto is probably okay. You don't say, but if your engine is electric start, I doubt you hooked it up reverse polarity, as the starter motor is DC and would spin backwards and the starter bendix would not engage. If you have a pull start and started it while the wires were crossed, you probably did what Bruce said... cooked the DC regulator/rectifier diodes. Here is a copy of the electrical schematic for an 8HP Johnson (I would copy the image location, but it's too big to fit on the forum page):

http://www.boats.net/images/diagrams/brp/IMAGES/432209/05.png

As you can tell, there are various electric components (rectifiers, power packs, coils, sensor assemblies, etc) that can be replaced, some more easily than others. Troubleshooting will also be challenging without a repair manual. Unless you really dig working on outboard electrical systems, have shop space and tools, have the repair manual, and have the time, I'd think about bringing it to a reputable mechanic for a diagnosis.

Edited by - on

Voyager
Master Marine Consultant

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5379 Posts

Response Posted - 07/01/2010 : 19:23:32

The schematic is pretty random looking without a key (what items the numbers describe), but number 11 looks like the regulator/rectifier diodes.
Tom, your guess that the alternator is really a magneto is very likely, looking at items 21 and 29. 21 and 29 probably feed the regulator/rectifier diodes.
I'm not sure what item 43 does?
Item 33 looks like the spark magneto that drives the spark coil number 46.
If I were going to guess, I'd check continuity on items 21 and 29. They should read less than 10 Ohms.
If I had the electrical schematic showing how all these gizmos were hooked up, or could sketch out a schematic from the wiring, that would be a good help.
If I could get the engine running with the alternator leads connected to a battery, I'd check the voltage with the engine off, then with the engine on. If I read 12.4-12.6 volts with the engine off, and read 13.5-14.4 with the engine running, I would conclude that the alternator was still good.

If the voltage read 15-16V, then I'd figure the regulator was blown. Also, if the voltage read 12.4-12.6 whether the engine were running or not, then I'd also figure the regulator was blown.

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Scooter
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228 Posts

Response Posted - 07/06/2010 : 05:07:38

Once again this forum is a wealth of information.

Thanks all for the info. It will be a couple of weeks before I can check all this.

I can't go sailing every day (yet).

FWIW I will give the forum an update if I find anything worth posting.

Edited by - on

John Russell
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3444 Posts

Response Posted - 07/06/2010 : 06:58:08

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Scooter
 ... It is possible it got hooked up in reverse polarity. (Long story... Don't ask.)... <hr height="1" noshade id="quote"></blockquote id="quote">Yes it is. Long story... Don't ask.

Let's just say thank god for effective fuses.

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Leon Sisson
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1893 Posts

Response Posted - 07/06/2010 : 22:31:26

I suggest owning a factory service manual for anything with an engine more expensive than a basic lawnmower. Small engine alternators seldom work the same way automotive ones do. Car alternators generally regulate output by controlling current to an exciter winding. Small engines (in the magneto ignition category) usually have permanent magnet alternators. If they even have voltage regulators, they frequently work by shorting excess output to ground, not by reducing output from the windings by controlling magnetic field strength. In almost any alternator charging system, reversing the leads on a hot battery will have a sudden adverse effect on the rectifier diodes. If they fail open, reduced or no charging. If they fail closed, not just AC to the battery, but maybe a drain on the battery, even with the engine off.

This suddenly toasted diodes problem is one real good reason to know what you're doing when connecting car jumper cables, and to not let anyone else 'help' you with that.

-- Leon Sisson

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DaveR
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2015 Posts

Response Posted - 07/07/2010 : 05:40:53

<blockquote id="quote">quote:<hr height="1" noshade id="quote">.........will have a sudden adverse effect on the rectifier diodes. If they fail open, reduced or no charging. If they fail closed, not just AC to the battery, but maybe..............<hr height="1" noshade id="quote"></blockquote id="quote">

Wow Leon, I guess I know where to go in the future for engine related electrical problems

Are you that versed with all things electrical?

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