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Old 08-22-2015, 08:20 PM   #1
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ONAN 5500, 450 hrs. When stared voltage @120V, then starts to drop decreasing to 112V. Voltage then varies between the two readings, and there is no load. The generator motor will also start to fluctuate rev up and down a little. When the coach was purchased 2 years ago we had the unit completely serviced, all fluids, filters, belts etc. At that time the facility that did the service advised that at approximately 500 hours should have a complete service and valve adjustment. I have no reason to doubt the direction of the repair facility, and they are a Workhorse certified as well. Any direction, comments or input would be appreciate.

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Old 08-22-2015, 08:30 PM   #2
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Is the generator throwing any codes??? There should be a procedure to recover codes, check your manual.

Recommended service is 50 hours, how many hours have you put on it since serviced last. First thing I would do is change oil, check/clean/replace air filter, replace fuel filter.

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Old 08-23-2015, 11:04 AM   #3
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X2, like JJ said check for codes. Also remember a genset has to be exercised. I'd run it for awhile without a load, then start loading it down. After a hour or so, start removing the load and let it cool down before shutting it off. I had to do the same after letting it sit al winter, now it runs great. Just my 2 cents.
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Old 08-23-2015, 11:38 AM   #4
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There probably are no codes - the 112-120v range is well within electrical standards. The Onan would shut down if the voltage was outside of the acceptable range, and set a code if it did. If it doesn't shut down, then no code.

It is normal for the voltage to drop a bit under 120 with no load. 112 is a bit more than I might expect, though. And the fluctuating rpms suggests an engine fuel feed problem as well. It is not uncommon to have the carburetor get gummed up with fuel residue if the genset is not used regularly (at least monthly, under load). Some carb cleaner, e.g. Seafoam, may help.

Do you exercise your genset every month, per the owner manual? 30-60 minutes under moderate load? If not, you should.

What happens when you run the genset and have an a/c on to give it some load? Does it smoothly increase speed and produce a steady voltage in the 115-120v range? If not, what happens?

The Onan service advice is very conservative. I doubt if you need a major overhaul yet. Maybe not ever. 500 hours is not a lot of usage for a genset.
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Old 08-23-2015, 11:56 AM   #5
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I solved my problem with Sea Foam

I had the same problem on another motor home after sitting for 6 months without the Gen being exercised. I put a can of Sea Foam in the gas tank and ran the generator for a couple of hours. It cleaned the fuel line and carb. Ran much better. the generator needs to be exercised at least once a month unless prepared for storage.
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Old 08-23-2015, 03:26 PM   #6
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The only other thing it could be is the voltage regulator could be going bad. I had a Microlite 4000 on my first coach that had the same problem and it was the regulator. It was erratic for a while then wouldn't stay running at all. Just a thought.

Craig & Carolyn Roberts Apple Valley, CA
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Old 08-24-2015, 05:20 AM   #7
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Thanks for the feedback. I do not have more than 20 hours on the generator, have started it weekly but have placed it under load. I have ordered oil filter, fuel filter, air filter and will replace the spark plug.

Will place under load and follow your directions. Thanks to all.

Thanks again

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