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Hank F.
Hank F., Technician
Category: Small Engine
Satisfied Customers: 14892
Experience:  Certified on Onan and Generac generators
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I have an Onan 5.5 HGJAB Spec C with about 450hrs from

Customer Question

Hello. I have an Onan 5.5 HGJAB Spec C with about 450hrs from model year ~2004. It sat for a few months recently but not terribly long IMO. I just bought the 5th wheel that it came with September 2014 so I've only ran it myself for a few minutes (maybe 10 mins) a few times to make sure it ran, and would operator the A/C etc.
Now I want to use it for real and I have tried many things but it always consistently shuts down within ~6 seconds of starting. The error code is 3-7 which I believe means faulty spark, however I've tried a number of things and none have changed my issue:
- replaced spark plugs and checked spark - all good.
- fresh fuel
- changed oil - haven't had a chance to change the oil filter but can't imagine that would be the issue...?
- cleaned and soaked carb.*
- checked fuel flow and it's strong while priming and while running (for the 6 seconds). I also tried bypassing the fuel power by holding it directly on the positive terminal so it would pump even when the engine wasn't turning over, and it didn't help (i.e. more fuel not needed from what I can tell).
- tried holding down the start button after the engine starts however doesn't make a difference from what I can tell. While doing this I sprayed carb cleaner directly into the carb and it didn't help keep it running either.
- choke seemed a little stiff when I removed the carb so I worked it back and forth a bit, also tried holding it open/closed manually while starting and neither helped (although it did seem to run a little longer into the ~6 seconds then when I closed the choke)
* I saw the other post for my same generator below and I'm not positive on the 3 tiny holes behind throttle plate since I'm not sure exactly where that is.
Lastly, I saw the following in a troubleshooting guide and I'm thinking this might be my problem except the oil pressure switch is at least $60 so didn't want to burn that money without being somewhat more sure (if that's possible!). I feel like it has to be something like this though since it fires up and quits so consistently every time - if it is was truly a fuel problem I was thinking it would run rough perhaps or start/stop with some inconsistency - perhaps that's not true either though?
Thanks for any help you can provide.
Q. Why does my generator start and then shut down as soon as the start switch is released?
A. Most models require both oil pressure and AC voltage from the generator to keep running. A lack of AC voltage can be caused by a variety of problems involving the control board, regulator, generator and wiring. A stuck oil pressure switch can also cause this symptom. The interaction between the various components of the system can be confusing during troubleshooting. Use the Troubleshooting Guide to isolate the problem.
Submitted: 2 years ago.
Category: Small Engine
Expert:  Hank F. replied 2 years ago.
My name is ***** ***** I am going to assist you with this.
The most common cause of a code 37 that I have seen is an issue with the drive system - either a broken drive belt, or a broken flywheel to pulley coupler.
Customer: replied 2 years ago.
Hi Hank. OK, before I dive into a major fix let me give you some more behavior I noticed...I thought the issue might be a faulty oil pressure switch so I borrowed a switch from my friends Onan that is the same as mine (well mine is Spec C and his is Spec B). What was weird is when I put his oil switch in it actually ran for about 2 full minutes (remember I had been getting ~6 seconds previously then it killed it with the 3-7 code) however after about the 2 minute mark it started to run rough for about 15 seconds and then quit - the fault code was 1-5. I tried restarting a few times but from that point forward it was back to the same quit after ~6 seconds and fault code 3-7. Then I figured it's not the oil pressure switch since it was back to the same old quit after ~6 seconds and fault code 3-7 repeatedly. I swapped the oil switch back to my original and again it ran for ~2 minutes then rough for 15 seconds then quit with fault code 1-5 again. After that of course I was back to the same old quit after ~6 seconds and fault code 3-7 repeatedly.So, I just wanted to let you know about this odd behavior in case it could narrow down the trouble shooting.Thanks for any tips or advice you can provide.
Customer: replied 2 years ago.
One more thing I should mention - when it did run for the 2 minutes twice it did seem to be at a high RPM both times...I'm not positive on that, but just going from the sounds of it, it sounded like it was running too high of an RPM to me.
Expert:  Hank F. replied 2 years ago.
An oil pressure switch will have nothing to do with a code 37.
But the symptoms you are describing point to a bad carburetor, and that should be giving you a code 36, not 37.
Customer: replied 2 years ago.
I've never received a code 36...Do you think a bad carb would still cause the shut down consistently after ~6 seconds (other than those 2 times it ran for ~2 minutes)? It won't run any longer even when I sprayed carb cleaner directly into the carb (air filter cover off of course) so that was making me think something electronic was killing it.Would it help if I uploaded a short video of the start / stop routine?
Expert:  Hank F. replied 2 years ago.
If carb cleaner will not keep it running, no, the carb is most likely not the issue.
It still goes back to that code 37.
6 seconds is about the right amount of time to determine if the belt is slipping.
But to test for that, the unit will need to go to an Onan shop, as a special tool is required to bypass the controller and keep the engine running while the generator is tested.