Onan runs under load only

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MerrillAZ

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Oct 19, 2019
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Hi everyone, I?m hoping someone here can help me. I have a 2008 Shockwave 5th wheel with an Onan Microquiet 4000 gas generator. I can get it to run but it will only continue to run if I put it under load right away. If I plug in a space heater it runs fine. If I turn on the AC it runs fine. The problem comes when I turn off or unplug whatever is on. The generator immediately dies. In other words, it runs fine under power but will not run with nothing pulling juice. Any thoughts?? It?s driving me nuts! Thanks in advance!
 
Hi Merrill, welcome to The RV Forum!

It sounds like you have a plugged idle jet in the Onan's carburetor.  "Idle" doesn't mean low speed like in a car, but happens when the engine isn't under a load and the throttle closes.  When you put a load on the generator, the throttle opens and pulls gas through the main jet.

You may be able to turn the idle adjustment mixture screw on the carburetor and knock loose whatever's plugging the jet, but it's more likely you're looking at removing and disassembling the carburetor.
 
Thanks Lou, is the idle adjustment screw the altitude adjustment knob? The carb was just put on in February but it did sit too long between runs since then. My fault. I will look into the mixture screw, that makes sense.
 
I ran it again today with a small space heater plugged in. Ran great for 45 minutes. When the heater was turned off the generator went from running high rpm to shut down in one or two seconds. There was no trying to idle, just straight to off. It has never done this before and I've owned it since new in 2008. Still very confused!!
 
First thing is to check to see if a fault code is being produced following the shut down. You will need the service manual. The owners manual may offer enough info if you have that. Cummins/Onan 800-286-6467 will usually send a copy of the service manual to an email address. Have the model and serial numbers handy when you call.

There are several conditions that will shut down your Microquit 4000 electronically. If it is a computer shutdown, you should be able to find a code indicating why it shut the unit down. The Microquit 4000 will run differently under no load and could be initiating a shutdown by the computer.

It is possible that the Idle circuit in the carburetor is clogged or not adjusted properly but before making any adjustments on the idle circuit, look for a code and read up on the various sequences that result in a shutdown. The service manual offers a very good breakdown of the carburetor.

Good Luck

 
A shutdown in one or two seconds probably is not initiated by the computer and could very well be a carburetor issue but see if you can get a fault code.
 
I believe that is a over frequency fault. Generally that means the rpm of the generator is to high. The rpms of the generator will increase when switching from a load to no load. My guess is that you have a governor that needs adjusting to a lower nominal rpm.

I own a Microquiet 4000. 2001 spec J. My unit does not idle down and is always governed to a nominal 3600 rpms which generates the 60 cycles required for 120 volts ac.

I have never seen a 4000 idle down under no load but I am not familiar with everything that Onan did. My guess is that your unit is probably set up as mine is.

I did make a governor adjustment about 15 months ago. In my case the generator was dropping too low under load and the computer was shutting down due to low frequency. I made a slight adjustment by ear and it has been solid ever since. I also service my unit every 100 hours. Oil, plug, air filter. I find that about 100 hours, the spark plug needs replacing and it makes a very noticeable difference. I suspect the elevation adjustment on the carb is a little off. 

If you have any difficulty getting the service manual through Cummins, list your unit model and serial number here and you will probably find someone who has one for your unit and would send it over.
 
Thanks Henry, will the service manual show how to make a Governor adjustment? My generator is a Model No: 4KYFA26100K and is Spec: K. It is probably supposed to run consistently as yours does. I've never messed with the governor before but I'm sure it could be out of adjustment for one reason or another.
 
Get a Kill-A-Watt power monitoring meter from Amazon or most hardware stores. It will display the generator's voltage and frequency if you plug it into a generator fed outlet.  Use an extension cord to bring the meter back to the generator so you can watch it while making the adjustments.
 
I do not remember if my service manual went into any detail about adjusting the Governor. Later today I will have a look. The meter Lou suggests is a good idea. I didn't use one but did use a volt meter to keep an eye on the voltage which was better than nothing. My adjustment was fairly easy for me. My generator was shutting down under high load and probably close to 25 amps. I made a very small adjustment and it has been solid since. I believe the computer will shut down on high frequency at 70 cycles. You can do a little math to figure out the rpm adjustment. Its 60 cycles at 3600 rpms. Make small adjustments. I happened to get a little lucky and apparently guessed correctly which was not expected because of my old age hearing loss :))

You should also be sure the generator is running properly. Put a new plug in and be sure to gap it correctly, check the air cleaner for dirt and if you are not sure whether it is clean then replace it and dump the oil if it is time. Also make sure the altitude adjustment is set correctly for your elevation. It helps to keep things running clean. The other thing you will need to inspect is the linkage to the carb and the governor itself. Make sure everything is operating free and clean and lightly lubricated.
 
I just looked at my service manual. It does have a step by step guide to adjust the governor. It is fairly straight forward and on mine, a philips head crew.

Looking at my service manual it reads that it covers the following generators:

KY (Spec J-N)

KYD (Spec A-C)

Does your unit fall under those specs ?
 
Thanks again Lou and Henry. My generator does fall under the range you show, KY (Spec K). I'll get the manual and the monitoring meter and see if I can't get it dialed in. Oil, spark plug, air and fuel filters are all recent. The altitude adjustment is close as far as I can tell, right now I'm at about 650' and I change it to 7000'+ when I head to the mountains. I'll fiddle with that some too.
 
I do have a digital copy of the service manual that I could send to an email address. It is quite large of a file. Try to send me a personal message (PM) and include an email address and I will send the file over to you.
 
I have adjusted governors using an electric clock with a second hand plugged into the generator and another clock with a sweep second hand operating on battery or the home power system.  Just put a load on the generator and adjust speed till the clocks match speed.
 
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