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Old 06-17-2021, 05:23 PM   #1
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Monaco Owners Club
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URGENT: Onan 12.5k Won't Stay Running / fuel pump issue

Hi All - need some help with an Onan 12.5kw Generator - trying to leave for a trip in 3 days but can't keep it running and kind of need it.

First: Rig is 2007 Beaver Marquis on Roadmaster S-Series
Second: Generator is a Kubota powered Onan 12.5 QD
Third: Code the Genset is spitting is 36
Fourth: Fuel is well past 1/2 tank. Added more anyways.
Fifth: Changed air, fuel filters and oil already

Here is the issue, if cold, it will start and run for an hour or so and dies. Won't start again - try priming - no go. Went in for a tune up to see if it was a filter problem. Started, ran for an hour or so and died.

Went back to the truck repair place and we hunted down the issue to fuel pump on the genset losing power after an hour or so. We gave the fuel pump 12v directly and the generator started right up.

Their best guess is a (1) bad fuel pump relay (heating up), but don't know where it is (not on the front/rear run boxes, assuming in the genset somewhere), (2) a bad control board (but can't figure out where it is either - help?) Assuming same as the relay? it is under the black panel on the top front side of the generator but not clear? Or (3) potentially a bad fuel pump that is drawing more current than it should.

I'm hoping that one of you wonderful folks on this forum knows these things better we do - we looked through the Onan Manual and the Beaver schematics and no real solution to a fuel pump relay or circuits (there is a diagram in the Onan book, but it doesn't tell me where the board would be or if there is a removable relay, etc).

Would love any advice on how to get closer to a solution here and what parts I might need to order or hunt down given that I have little time left before we get moving.

Thanks big time in advance. Will keep paying it forward on this forum.

Anand
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Old 06-17-2021, 05:50 PM   #2
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Had a issue with are generator on are coach the water temper sensor went bad causing that kind issue you are having
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Old 06-17-2021, 05:56 PM   #3
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Could potentially be a faulty code and apparently the only way to clear codes is to remove power. I'd go under and disconnect power for 30 mins and wire brush cable ends and mounting studs. Then reconnect. Hold down start button for 60 seconds and restart. See what happens. Worked on my 7500 QD.
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Old 06-17-2021, 05:58 PM   #4
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Quote:
Originally Posted by oh no View Post
Had a issue with are generator on are coach the water temper sensor went bad causing that kind issue you are having
Thanks, will look at that - I'd assume that would be a different code and wouldn't explain why the set would keep running when we put hot 12v on the pump - it should still stop, no? Or does a failed water-temp sensor get the control board to stop the fuel pump?
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Old 06-18-2021, 05:50 AM   #5
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Yes on the water sensor shutdown the motor .
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Old 06-18-2021, 07:51 AM   #6
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Control Board

Had the same issue on our 2009 Beaver. Around 2006 I was told Onan made a change to their control board so that the 12v supplied to the fuel pump was fed from the control board instead of through a relay. It would work for a while then quit. Was a heat issue with the board as I recall.


I had a tech replace the fuel pump - no change. Finally went to a Cummins dealer who found the tech bulletin and installed the updated control board to fix the problem. Might be worth looking into this through Cummins tech support.


If short on time you can just use the workaround already done - wire 12v to the fuel pump directly when you need to run the genset. I would be sure to disconnect the 'normal' fuel pump wire when running the temp 12v connection.
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Old 06-18-2021, 08:29 AM   #7
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Is there any chance you have gen start/stop enabled on your inverter? There could be a setting in the inverter that shuts the gen off after a certain state of charge is reached.
I had a devil of a time figuring this out on mine... dang near bought a new control board in desperation...
If the gen is shutting itself down due to some error condition, it should be logging a meaningful error message.
Edit: never mind. Just noticed the part where you said "run for an hour or so and dies. Won't start again - try priming - no go. ". If it was the inverter shutting it down, you would be able to restart it.
Cheers,
Walter
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Old 06-18-2021, 08:36 AM   #8
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The control board is a common $800 problem. It intermittently will stop supplying power to the fuel pump. Some folks simply wire a switch to 12 volts and the fuel pump turn it on when starting the generator. I have AGS Automatic Generator Start which I use frequently and couldn’t figure out a reliable way to run the fuel pump, so I bought the control board from Cummins. Problem solved.

Without AGS I would do the switched 12 volt workaround.
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Old 06-18-2021, 06:29 PM   #9
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Thanks / Solved!

Thank you for all of the responses - it may still be a overheating control board issue, but cleaned all of the pins on the connector to controller board and the 10 pin remote connector. Also replaced the fuel pump relay which was hot to the touch after 10 minutes of running. Not sure which of these worked but it ran like a champ after that.
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