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paulz
Member
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# Posted: 25 Oct 2025 08:38am
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Yesterday my Onan quit putting out AC, had to switch generators to finish my work. Checked the usual things, GFI sockets, Onan circuit breakers, nothing. It has two lines and breakers, both out. Digging deeper, I pulled a breaker and jumpered around it, no help. Then pulled the brushes, look fine. Only other thing besides the alternator coils is the voltage regulator. Have to have that professionally checked apparently. Anyone been here?
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gcrank1
Member
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# Posted: 25 Oct 2025 07:42pm
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Other than the obvious things you have checked there has to be an output spec from the unit before it goes into the regulating unit. Check output there 1st.
I suspect the unit is an alternator that feeds a voltage regulator and some rectified into dc? If those dont get fed base power they wont work.
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Brettny
Member
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# Posted: 26 Oct 2025 08:29am
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Did you ohm out each coil?
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paulz
Member
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# Posted: 26 Oct 2025 12:26pm
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It has two slip rings with brushes. Im getting 22.7 ohms measured between the two.
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Brettny
Member
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# Posted: 27 Oct 2025 07:05am
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No shorts to ground? If not I would suspect the voltage regulator.
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paulz
Member
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# Posted: 27 Oct 2025 07:34am
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Yeah thats my thought. But a new regulator is 500 clams. Model is Marquis 7000. It did run out of fuel (lp) shortly before AC went away. Sputtered and could have pulled a wire loose but it restarted and i split wood for another 10 minutes before AC went away.
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groingo
Member
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# Posted: 31 Oct 2025 02:53pm
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paulz:
My cure for a fussy generator was to chuck it outside on to of a garbage pile where I left it for two years, then one day I wondered, so I grabbed it, blew it out (lots of fur needles in it) put a bit of gas in it and two pulls it fired up and has started 2nd pull ever since!
With that it got a complete cleanup and now sits at the foot of my bed ready to go (less gas) should I ever need it! Sometimes you have to show them who's boss!
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paulz
Member
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# Posted: 2 Nov 2025 09:46am
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Yeah I’ve waited a few days, fired it up hoping it had fixed itself! No luck. I have learned a bit about the complications of this thing. When you first press the electric start it sends 12v to the slip ring brushes to magnetize the field windings and start generating AC, then turns that off. So I need to check that.
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gcrank1
Member
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# Posted: 3 Nov 2025 09:42am
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Yeah, check the 12v energizing feed for delivery; iirc once one is charging it runs off itself so maybe the 12v feed is no longer needed so it is dropped off, maybe not.
If no energizing feed maybe hot wire jumper it to see if it starts charging?
Have you looked online for the 'troubleshooting guide' for the unit?
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paulz
Member
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# Posted: 3 Nov 2025 11:41am
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I have the online Onan service manual pdf, that’s how i got this far. Be back the cabin today to start it with DVM on the brushes.
https://onan.xmsi.net/965-0531B%20Onan%20BGM%20NHM%20Marquis%20series%20RV%20Genset%2 0Service%20manual%20(02-1991).pdf
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paulz
Member
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# Posted: 4 Nov 2025 09:19pm
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Quoting: gcrank1 Yeah, check the 12v energizing feed
Checked today, it’s getting that. I was wishing like groingo said and just leaving it alone for a few days it would fix itself. Not much else to test but a new $500 voltage regulator. Can buy a whole new HF genny for about that.
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DaveBell
Moderator
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# Posted: 5 Nov 2025 02:35pm - Edited by: DaveBell
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Look at James Condon on Youtube. In one of his latest videos he replaces a bad voltage regulator full wave rectifier with a cheap one from ebay.
Is there a part number on your rectifier? I know what they do from my Army SATCOM days but do not know if there are different sizes or capacities.
They do have directional polarity.
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paulz
Member
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# Posted: 5 Nov 2025 02:51pm
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Bought a used one on EBay this morning $75. It’s a gamble. It’s a complicated part, check out all those wires, not your usual plus minus charge terminals.
Price of dinner for two these days, worth a try.
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