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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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paulz
Member
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# Posted: 5 Dec 2025 07:10pm - Edited by: paulz
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Well the new regulator didn’t help. I can check the stator for open or short next, but I need to pull it off the deck and someplace I can see.
For the time being, I still have a couple spares. Several years ago a guy down the road came by “I’m moving, sold my place, want the backup genny?”it’s a Chinese KDE6500 diesel, electric start 110/220. Runs the log splitter no problem, as did the Onan.
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paulz
Member
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# Posted: 10 Dec 2025 01:31pm
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Hauled the dead lp Onan to the house for further troubleshooting. The diesel genny is doing fine, back to splitting wood*. It took several cranks to fire up from cold, there is no preheat on the key switch, nor glow plug I see. Just a cheapo I guess.
•this is about as big a chunk as the 110v splitter will do. Good enough, haven’t used the gas splitter in a long time.
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paulz
Member
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# Posted: 23 Mar 2026 02:15pm
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My dead Onan went by by. Kind of miss the lp power, no hauling gas or diesel around. I see new ‘dual fuel’ generators around $500. Anyone got one? How do they do on lp?
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darz5150
Member
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# Posted: 23 Mar 2026 04:08pm - Edited by: darz5150
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I've had one for a few years. Does fine on LP. Starts easy, burns clean. I only use gas for longer power outages.
Looks like I need to pressure wash it. Lol
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paulz
Member
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# Posted: 23 Mar 2026 05:06pm
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How do you switch, just turn the gas off and lp on? Does it use more on lp? I see it already has 12v out. Would need to be 13+ to charge batterys. Thanks!
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gcrank1
Member
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# Posted: 23 Mar 2026 05:41pm
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I believe there is 'less energy' in LP so it is not as much bang for the gal of fuel.
Ime dont use the 12v taps on gens for recharging batteries, it is SLOW/low amps and on some isn't well regulated. You are MUCH better off running a 120vac charger plugged into the gen!
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MtnDon
Member
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# Posted: 23 Mar 2026 06:12pm
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Propane has a out 25% less energy per gallon. Depending on the manufacturer this may affect the electrical energy output to be less on propane than gasoline. Read the manual & spec sheets carefully.
Regarding the 12 volt outlet on a generator. Generally it is best to think of the 12 volt output to be much the same as plugging into a cigar lighter socket. You run "stuff" but you do not try to charge a 12 volt battery by plugging into the 12 VDC.
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darz5150
Member
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# Posted: 23 Mar 2026 06:35pm - Edited by: darz5150
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Quoting: paulz How do you switch, just turn the gas off and lp on? Does it use more on lp? I see it already has 12v out. Would need to be 13+ to charge batterys. Thanks
Yes. You turn the petcock to gas or lp to switch.
Yes lp is less BTU s. That's why if it's just a short time I need to run it, I use lp. For major outages, I use gas.
The 12 volt out on my big Genny can have a high output. I did hook up a cheap solar charge controller before and It seemed to regulate it to a more stable output. That's how I could run it if I needed to tap the 12 volt cabin system into it, but I could just run a battery charger instead. I have measured the straight output as much as 18 volts or more, so if you hook something DC to it, make sure it can handle it.
I did try charging a lawn battery straight off it, and it did charge it, but I would bet if you leave it hooked up too long, it would probably boil the battery.
However, I have a 1000 watt sportsman Genny. On eco mode the 12 volt output is about 14/15. Not on eco mode it puts out 18 ish.
I have used it straight out to both help jump a battery, and on low, help charge/ top off a battery.
But on the big Genny I would just hook up a regular batt charger like crank said.
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darz5150
Member
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# Posted: 23 Mar 2026 10:50pm - Edited by: darz5150
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I dug out 3 manuals for different Gennys.
Sportsman 4000
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Steele 2000
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Sportsman 1000
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