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Old 04-10-2021, 07:02 AM   #1
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Onan QD Generator Troubleshooting

I have a Onan QA 12500 generator with 300 hours which shuts down with a 36 fault code. Sometimes the generator will shutdown within minutes of being started and other times it will run for over an hour.

For the most part the generator will not start/stay running unless I first prime it by holding the stop button for about 10 seconds.

The generator RPM will typically run at 1800 +/- 5, except when a load (ie. HVAC) is added or removed at which time the RPM may rise/fall by as much as +/- 20 or 30. Sometimes the generator recovers from this RPM swing and other times it shuts down.

I have replaced the air and fuel filters. I have also tested the fuel pump per the service manual and successfully filled a jar with 1 pint of diesel fuel while holding the prime button for 1 minutes. This verifies that the fuel delivery system is okay.

The service manual has two more troubleshooting items. One is to test the governor actuator and the last one is to replace the control board. Before going through the effort of taking things apart to access and test the governor actuator as well as the expense of replacing these parts, I figured Iíd ask for words of wisdom. Thanks

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Old 04-10-2021, 07:18 AM   #2
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When you tested via priming did the fuel start coming out of the hose immediately or did it take a few. I have a 10k but I have no need to prime every time. It still sounds like a fuel delivery problem. May be there is a place up stream from the genny thatís sucking just a bit of air at times when the pump is running causing a vapor lock.

I donít think you should need to prime every time you start.
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Old 04-10-2021, 02:21 PM   #3
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I just went through a similar situation with my Onan 12,500 diesel generator on my Newmar Essex. Mine would also load up and the exhaust was dumping some black smoke. If I were comparing it to an older gas engine I would say it was running like the choke was partially closed. Obviously that isn’t the case but a comparison of how it was running. Sometimes it would run for a while and other times shut down after a few minutes also shortly after it got a good load on it like the AC it would shut down. What we found was I had a bad fuel injector so I replaced all three and it runs like a top. My generator had around 300 hours on it. Thus could possibly be you problem as well. The code mine was throwing said possible fuel filter or governor issue same as yours.
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Old 04-10-2021, 03:21 PM   #4
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Quote:
Originally Posted by Piros1 View Post
The code mine was throwing said possible fuel filter or governor issue same as yours.
Do you recall the exact fault code? The 36 fault code seems to be a catch all saying that the engine shut off and it wasn't called to do that. The fault code troubleshooting section does't say anything about possible bad injectors, so I'm curious what led you down that path. Was there a test for the injectors? Did you take it to an Onan service center or repair it yourself? Did you run through the other troubleshooting steps first (bad actuator and bad control board)?

Going through the troubleshooting list, the governor actuator was next on the list. Since it controls the amount of fuel for the engine in order to maintain the correct RPMs I think I can see how it could cause the symptoms I'm seeing. Having not worked on a generator and/or diesel engine before, I' pretty open to ideas and learning.

I do have some black residue on the driveway where the exhaust pipe exits. I figured it was just due to the engine running rough when trying to start it without priming first. As you stated, it does seem like the equivalent to a chock issue.
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Old 04-10-2021, 03:32 PM   #5
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Had same issue a year ago. Tough to get to, but unplug the multiplex cable where it connects to the main control card. Clean contacts on both the cable connector and the card. Firmly reseat the cable, reassemble any parts removed for access, try test run...

Had to do mine over 100 hours ago, no problems since!
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Old 04-10-2021, 09:35 PM   #6
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If all else fails, try to get the ginny running under full load for about 30 minutes. If it starts out blowing black smoke it may possibly be a soot build-up. I also had a problem with mine shutting down which I finally had to replace the water temp sensor. I don't remember the code it was throwing.
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Old 04-10-2021, 11:51 PM   #7
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Quote:
Originally Posted by flyboy013 View Post
Do you recall the exact fault code? The 36 fault code seems to be a catch all saying that the engine shut off and it wasn't called to do that. The fault code troubleshooting section does't say anything about possible bad injectors, so I'm curious what led you down that path. Was there a test for the injectors? Did you take it to an Onan service center or repair it yourself? Did you run through the other troubleshooting steps first (bad actuator and bad control board)?

Going through the troubleshooting list, the governor actuator was next on the list. Since it controls the amount of fuel for the engine in order to maintain the correct RPMs I think I can see how it could cause the symptoms I'm seeing. Having not worked on a generator and/or diesel engine before, I' pretty open to ideas and learning.

I do have some black residue on the driveway where the exhaust pipe exits. I figured it was just due to the engine running rough when trying to start it without priming first. As you stated, it does seem like the equivalent to a chock issue.
So we did it ourselves, I have a full time mechanic that works for me in my business and he is pretty good on Diesel engines. Before he looked at it I checked it out and changed the fuel filter, checked sir filter and intake tube for blockage so I was able to rule that out. That was the first step in the diagnostics and before took a look at it. I kept thinking it was fuel related. To answer your question it does not mention the injectors for the codes it was throwing. The same code as yours. My mechanic and I both thought it sounded as if it had a dead cylinder that is when he said we should pull the injectors and check them. It is a relatively easy job. The hardest part is taking the engine cover off the generator to access everything. We checked the belt while we were at it. This is pretty much a one man job that can be disassembled in about a half day or less and put back in the same time. When we pulled the injectors one was contaminated and felt was suspect. After installing the new injectors it seems to run fine.
I bought the coach used a year ago and it has ran bad since I owned it. Kind of a bad deal from the beginning. I got the screws on from the dealer. My fault for trusting them but that is a different long story for another time.
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Old 05-21-2021, 09:50 AM   #8
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Due to the continued issue with the generator cutting out with a fault code of 36, I decided to replace the fuel pump. I think (hope) the problem is now resolved.

When I had performed the fuel pump test documented in the service manual, the pump outputted exactly the required minimum amount of fuel which is 0.4 liters in 60 seconds. After watching some videos online of fuel pump testing, the output was much more fuel than the 0.4 liters (albeit it was a gasoline model). It made me thing that if 0.4 liters was the minimum, what should the normal amount be?

I decided to bite the bullet and replace the pump. After replacing the pump and priming the generator, the generator ran for 6 hours, but then cut off with a fault code of 36. This is MUCH longer than it was previously running (including this past weekend when I couldnít keep it running for more than a couple of minutes). I started the generator again this morning and it has been running for over 4.5 hours now. Iím going to let it run for a few more hours, but Iím hoping that the fuel pump has resolve the problem even though the original pump passed (barely) the service manual test.

As for the replacement pump, I decided that I didnít want to deal with the potential issues of an aftermarket pump, so I ordered it directly from Cummins for a bottom line price of $125. It arrived in two days. The parts manual calls for part number 149-2790. This part has been superseded by part number A047N931 which is what was originally installed. The even newer replacement part number is A064S967.


Iíve attached a graph of the generator RPMs. The graph has data points of every 2 seconds. There are only a couple of datapoint in which the RPMs where more than 1% away from the 1800 RPM target and each time the RPMs immediately return to the 1800 target.

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Old 05-21-2021, 10:45 AM   #9
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Did the fuel pump resolve your problem. Mine is running fine since the injectors were replaced.
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Old 05-21-2021, 10:56 AM   #10
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Quote:
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Did the fuel pump resolve your problem. Mine is running fine since the injectors were replaced.
It's now been running today for 5.5 hours with anywhere from 0 to 3 HVAC units running drawing between 3 and 45 total amps across both legs as the various systems cut on / off.

With the exception of the generator cutting off last night after 6 hours, I'd say the problem has been resolved. I'll probably have a better idea in a few weeks a I'll probably be running the AC units during my 16 hour drive southwest Florida. Fingers crossed.
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Old 06-03-2021, 05:11 PM   #11
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Problem not quite fixed. Even though the fuel pump tested out to be within limits (0.4 litters / minute) I replaced it anyway. While parked at home and at my current campsite it's been working fine. No issues to report other than the first time after replacing the fuel pump, the engine cut out after 6 hours, but ran fine thereafter.

BUT, while the coach is rolling down the road, the generator will not stay running. Fault code 36. I've tried running it multiple times during this trip. I had it running at a rest stop only to have it cut off a few minutes after we got moving again. I have a full tank of diesel so I don't see the fuel level being an issue. I did look at an ongoing thread where the owner's GAS generator would not stay running when the coach was traveling down the road, but the issue/suggestions seem to be gasoline related.

Any ideas of of where to look?
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Old 06-03-2021, 07:35 PM   #12
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Mine will cut out if there is a voltage drop maybe a bad ground or bad 12v to the unit.

Itís odd I would assume that the genny would produce its own power to stay running but on mine if I disconnect the 12v positive from my chassis battery it will die.
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Old 06-03-2021, 09:21 PM   #13
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Pull the 12v cables off the back , it resets the board. The governor is a false flag. It throws that code as it shuts down as the rpm drops. Reconnect the 12v after 10 minutes. If you shut it down under a load it screws with the board. I don't know why, but it does
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Old 06-03-2021, 11:11 PM   #14
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One main item to check on onan diesels is air in the fuel system, this usually shows up going down the road. Easy check that I do on all onan diesels. Remove the fuel return line from the rear of the genset, install a clear plastic line and place the other end in a fuel container, run genset, if you see bubbles going out the return line, the suction side fuel line has deteriorated, also while it is running, slide genset in and out, wiggle,shake fuel supply line. Again keep checking for bubbles in the return line.
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