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Old 23-06-2012, 07:25   #1
SG
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Mystery Alternator Problem

Our generator alternator doesn't charge the battery. We tested and it works fine on the bench. However, when we put it on the engine, it does nothing to charge.

It only has three leads: the battery hot and neutral, and the exciter. We wired the hot directly to the starter (just to make sure it wasn't the harness). We bypassed the harness' exciter input to " fool" it (even though there is voltage coming from that). The belt is turning the alternator.

My mechanic is mystified too!

Any ideas?
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Old 23-06-2012, 07:32   #2
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Re: Mystery alternator problem

This may be stating the obvious, but did you check the ground connections?

External or internal regulator?
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Old 23-06-2012, 09:13   #3
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Re: Mystery alternator problem

Do you have a diode battery isolator in the starter cable or is it a direct run to the battery?
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Old 23-06-2012, 09:20   #4
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Re: Mystery alternator problem

No isolator on the generator start battery (I have seperated both the engine start and generator batteries, from the house banks.

Also, the ground is working because we can start the generator.

We load tested the lines to and from the battery -- so it's not a question of capacity of the cables.

Any other ideas?
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Old 23-06-2012, 09:28   #5
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Re: Mystery alternator problem

The regulator is right on the alternator. It was replaced by the alternator shop -- apparently un-necessarily.

Last year, after 13 years, I had the alternator rebuilt. Our Westerbeke 7.6 KW generator had about 1400 plus hours on it.

When I noticed that I couldn't start the generator, we determined that it was beause the generator battery had been drawn down. By parallelling the engine start and geneartor batteries, I can start the generator (and recharge the generator start battery).

However, the Westerbeke uses an electric fuel pump which constantly draws power while the generator is running. So, eventually, without and alternator, the generator battery will draw down.

(I have a Mastervolt Inverter/Charger with trickle, or "echo", charge tap for the generator; however, the little automotive fuse for that "popped". We aren't quite sure if that is related or not; however, we can find no reason that it would be. The marine electricians that installed can't either.)
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Old 23-06-2012, 11:03   #6
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Re: Mystery alternator problem

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Also, the ground is working because we can start the generator.
Any other ideas?
That indicates your starter is grounded to the battery (-), but what about the alternator? Most often the alternator is grounded to the casing, which is grounded to the engine, which is grounded to the battery. Chance of bad ground is small there.

However, in the case of an isolated alternator, there would be a ground terminal that must be explicitly connected to something resolving to battery (-).

Isolated alternators are most often seen on engines in metal boats.

I dont know what is often used on generators. Could it be an isolated alternator?
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Old 23-06-2012, 20:36   #7
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Re: Mystery alternator problem

At this point I would take the alternator back to the repair shop have them specifically show you how to hook up the alternator bypassing all of the generators switchgear. Then all you have to do is mounted back to the generator hooked it up as shown by the repair shop. While at the repair shop asked them which wire needs to be switched on and off to activate and deactivate the alternator. Assuming it is a positive switch lead either find a wire on the generator that goes positive 12 V when the generator is running or install a oil pressure switch. That will solve all the hocus-pocus of the generator wiring and control circuits as far as the alternator is concern. Just my two cents, Mike.
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Old 23-06-2012, 21:17   #8
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Re: Mystery alternator problem

Thanks for the input, Mike.
The exciter pole on the back of the alternator is the wire to connect. It has 11.7 Volts of power when the generator is on. We took that exciter and connected it to another source independantly which had 12.8 volts. It didn't make any difference.

We took the power from the alternator and connected it directly to the same cable that comes directly from the battery (so that it we were sure that it would be going back to the battery without interuption.) That didn't make any difference.

The voltage regulatior (and rest of the alternator) worked without fail when we took it back to the shop and put it on the "rack".

I like the negative ground possiblity -- but if that doesn't work. Then I'm stumped.

Any other ideas?
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Old 23-06-2012, 22:06   #9
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Re: Mystery alternator problem

Not to sound rediculous here but do you have solar panels charging the same battery banks and were they charging at the same time?
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Old 23-06-2012, 22:07   #10
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Re: Mystery alternator problem

Sorry ... generator battery ... not housebank .. ignore last comment!
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Old 24-06-2012, 00:07   #11
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Re: Mystery alternator problem

Is the battery hot and neutral the same size terminals? If they are a neutral should go to negative battery. If the connection you are calling neutral is the same size terminal or wire as the exciter terminal or wire then it may not be a neutral in the sense of ground but the battery positive for the field of the alternator. Some alternators get their field current externally. That was my reasoning for going back to the alternator shop with the alternator in hand and having them show you exactly how to hook it up. If you decide to do that you might tell them to show you where in a ground wire to it just could be on the safe side.

Second choice; Call the alternator shop and ask them if the alternator is case ground. If they say yes then with a digital meter probe between the negative battery terminal and the alternator case and see if you get a voltage reading. Do you have the specific alternator brand and number or type? If you do that might help as we can know it’s exact wiring. A picture of the back of the alternator might also help. Again just my two cents, Mike.
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Old 24-06-2012, 08:17   #12
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Re: Mystery alternator problem

Thanks, Mike.
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