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Old 13-01-2025, 07:01   #1
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Alternator voltage had dropped

Good morning everyone,

I need help to resolve an electrical anomaly.

Yesterday I swapped out the lift pump on my Onan MDKBL generator (original pump was leaking). Since installing the aftermarket replacement, I note the alternator isn’t re-charging the battery like it used to. When I run the generator my fully charged battery drops slowly to 12.45v and holds right there. ��

I measure 12.35 at the alternator.

Historically (consistently for 5 years) my generator’s alternator charged only to 13.1v. I don’t know if this was by design, but it worked, so I didn’t change anything. Well, 4 years ago I did pay to have the alternator “serviced”, but nothing changed, and I’m not sure there was any “service” done to the alternator.

But… with the simple swap of a lift pump my battery loses charge when the generator is running.

Thoughts?

Could it be the aftermarket lift pump (that seems identical in every way, shape, size, voltage, PSI, etc) is drawing more power than the alternator is producing?

What else would cause the voltage to drop to 12.45 and hold there?
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Old 13-01-2025, 07:59   #2
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Re: Alternator voltage had dropped

I would be thinking it's a battery problem more likely. A "fully charged" battery has a voltage higher than 12.45. What happens with batter voltage when not charging?
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Old 13-01-2025, 08:14   #3
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Re: Alternator voltage had dropped

Quote:
Originally Posted by sailorboy1 View Post
I would be thinking it's a battery problem more likely. A "fully charged" battery has a voltage higher than 12.45. What happens with batter voltage when not charging?
Thank you for helping. I can charge the starter battery using my Yanmar drive engine, because they share the start battery. When I run the Yanmar, its alternator charges the battery, at about 13.6v, and when it’s done and the battery has rested for 12 hours, the battery measures 12.93v. So, it seems the start battery is fine. It should be good, as it’s a strong battery, a big Odyssey Extreme, about 5 years old, showing no other symptoms of age.

The issue is only present when the generator is running.
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Old 13-01-2025, 10:34   #4
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Re: Alternator voltage had dropped

I seriously doubt the problem is the new lift pump. The voltages you measured (less than the battery at the alternator output tell me there is nothing coming out of the alternator.

Make sure the alternator is turning and the excitation wire is hooked up. If those are OK, have the alternator tested at a shop.
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Old 13-01-2025, 11:19   #5
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Re: Alternator voltage had dropped

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Originally Posted by donradcliffe View Post
I seriously doubt the problem is the new lift pump. The voltages you measured (less than the battery at the alternator output tell me there is nothing coming out of the alternator.

Make sure the alternator is turning and the excitation wire is hooked up. If those are OK, have the alternator tested at a shop.
Yes… thank you for helping.

I have just measured amps with my clamp meter (I guessed the biggest wire from the alternator is the power output wire) and it measure Zero amps. So yes, the voltage I measure there is probably coming from the battery, with nothing coming from the alternator.

I’ll check the exciter wire (once I figure out which one that is)… I assume it should have battery power to it.

Thanks!
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Old 13-01-2025, 11:21   #6
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Re: Alternator voltage had dropped

Interesting, according to the Onan service manual, the alternator ground and voltage regulation are both tied to the fuel pump circuit.

Click image for larger version

Name:	Onan Alternator.PNG
Views:	16
Size:	84.1 KB
ID:	297695

So, I would start looking at all the connections on that circuit, and looking for something broken, corroded, loose.
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Old 13-01-2025, 11:57   #7
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Re: Alternator voltage had dropped

is the alternator sense wire fuse blown?
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Old 13-01-2025, 11:59   #8
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Re: Alternator voltage had dropped

Quote:
Originally Posted by PippaB View Post
Interesting, according to the Onan service manual, the alternator ground and voltage regulation are both tied to the fuel pump circuit.

Attachment 297695

So, I would start looking at all the connections on that circuit, and looking for something broken, corroded, loose.
Oh wow!!!

That’s most surely the issue! I’ll check those wires!!

Thank you so much for finding this info!

Cheers!
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Old 13-01-2025, 12:10   #9
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Re: Alternator voltage had dropped

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Originally Posted by sailorboy1 View Post
is the alternator sense wire fuse blown?
Checking all wires now. Will advise. Cheers!
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Old 13-01-2025, 12:45   #10
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Re: Alternator voltage had dropped

Quote:
Originally Posted by PippaB View Post
Interesting, according to the Onan service manual, the alternator ground and voltage regulation are both tied to the fuel pump circuit.

Attachment 297695

So, I would start looking at all the connections on that circuit, and looking for something broken, corroded, loose.

THIS WAS THE ISSUE! Thank you much for taking the time, and knowledge, to lookup the schematic and spot the relationship!

I found the wire from the alternator had parted from a crimp connector that also had a wire to the lift pump. Different lengths of wire to one crimp connector, and it’s going to fail eventually, especially when my hands get in there.

I lift a rum to you Sir.

Randall
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