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Old 11-07-2020, 17:44   #1
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Ugh, another alternator question 2GM20F Hitachi

2+ year lurker here and 2nd year boat owner with a love/hate relationship with the learning curve. I've searched and not found any threads that are exactly my situation. Anyway, we have a late 80's 2GM20F with a 35 amp Hitachi alternator. A couple weekends ago, the alternator failed while we were preparing for a 1 hour motor to another anchorage when I smelled smoke. The engine had been running at a fast idle for maybe 10 minutes. It was a little dramatic with smoke, burning plastic smell, etc. After I shut the engine down and the engine cooled, we discovered the alternator was totally seized. Jump ahead to the alternator shop where the diagnosis was something plastic perhaps getting sucked into the alt and melting for the smoke and smell, then seizing when it cooled? No other theories from the alternator guy (45 years experience, also a boater). The bearings hadn't failed. Anyway, the shop gave me a a rebuilt replacement and we were off. The strange thing is now the charge light is on all the time now. The boat is 30 years old, so I think I'm going to just unbundle and trace all wires and connections looking for any issues. Just strange that there was no light before the alternator was replaced. I would love any encouragement and troubleshooting tips.
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Old 11-07-2020, 18:16   #2
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Re: Ugh, another alternator question 2GM20F Hitachi

BTW, I forgot to mention that the alternator is charging.
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Old 12-07-2020, 08:45   #3
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Re: Ugh, another alternator question 2GM20F Hitachi

Look at Ferris Power products or give them a call. 508-743-9901.

They can supply an alternator and answer or help you with any sort of problem or fix.

Great service too.
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Old 12-07-2020, 08:48   #4
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Re: Ugh, another alternator question 2GM20F Hitachi

Oh, maybe the light bulb was faulty. Im not being smart, but sometimes the simplest things bite you in the rear. Murffy's law.
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Old 12-07-2020, 08:52   #5
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Re: Ugh, another alternator question 2GM20F Hitachi

Your battery switch wasn't "off" was it? ..isn't is it? That can kill an alternator.
Is the key switch defective not activating the alternator sensing?
There is a sensing wire from battery voltage that tells the internal alternator regulator to start up. Check that wire if you can find it.. did it corrode or melt?
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Old 12-07-2020, 11:02   #6
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Re: Ugh, another alternator question 2GM20F Hitachi

Ok update, the alternator isn't charging after all. Voltage on either bank doesn't rise with the engine running, so this issue isn't that unique after all. I'm getting the alt tested tomorrow.
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Old 12-07-2020, 11:30   #7
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Re: Ugh, another alternator question 2GM20F Hitachi

I don't know that particular alternator, but is there a small plug on the back that might be the sensing wire? As well as maybe other separate wires... Can you test that sensing wire for voltage, is the connector clean? It could be bridged like in this picture but it may be a separate wire...
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Old 13-07-2020, 12:39   #8
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Re: Ugh, another alternator question 2GM20F Hitachi

Yes, there is a small plug with two wires from the panel. Each delivers current with the key switch on, so it appears as though the alternator is getting current from the panel. So it's either not responding to it (blown diodes?), or there's a problem downstream. I wanted to take it to an alternator guy, but it seems there aren't many around any more, so I'll have to take it to a parts store for testing. I'll update when I get it back.
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Old 14-07-2020, 18:22   #9
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Re: Ugh, another alternator question 2GM20F Hitachi

OK, so this is on par with the time the pressure water system wasn't working and I bought a new pressure pump before checking the intake screen . Had I only started with page 137 in Nigel Calder's book, I could have saved myself a lot of headache. For newbies like me with this issue first advice is to buy (and read) the Nigel Calder's book. To test the an internally regulated alternator without removing it and trying to have a parts store test it (which I did and this alternator failed), start by draining the batteries a bit by turning on lights, pumps, radios, etc. for a few minutes. Next, turn the key switch on (engine off) and check the voltage on each battery bank. Next, with key still on (engine off), check voltage at the alternator, meter's red lead to BATT or positive terminal, black to E (Earth/Ground). Voltages should all be the same. Finally, start the engine and bring it up to 2500 RPM or so, let it run for a few minutes there, then retest at the alternator. If the voltage at the alternator is 1V or more higher than the starting voltage, then the alternator is working. This can also be accomplished with a clamp type meter around the red cable from the alternator, but I don't have one. Mine tested at 13.6V, 1.2 greater than the starting voltage on the batteries (which also read at 13.6 with the engine running). So I think I have determined this issue is with the charge light circuit as suggested above. When I turn the key on it is slower than the overheat light to come on and not as bright. Both clues.
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Old 14-07-2020, 19:38   #10
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Re: Ugh, another alternator question 2GM20F Hitachi

Quote:
Originally Posted by Michaelecushing View Post
....... So I think I have determined this issue is with the charge light circuit as suggested above. When I turn the key on it is slower than the overheat light to come on and not as bright. Both clues.
I don't exactly which Hitachi alternator you have on your 2GM20 but the attached drawing is what Yanmar used on the three terminal Hitachi alternators on the 1990+s 2GM20.

It should be self explanatory but if not, please ask for further guidance.

In essence, the voltage on the L terminal will be the same as the battery voltage when the alternator is working correctly. This ensures the charging lamp remains extinguished.

Perhaps the rebuilt unit has a different internal regulator circuit?

EDIT - I believe there is a draughtsman's error in the drawing but it doesn't affect the charge lamp circuit.
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