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Old 07-20-2014, 08:13 PM   #1
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Diesel Onan Voltage Problem

Just bought a used Monico with a diesel Onan, Model 7.5 HDKAJ/11451D. It seems to run just fine but the voltage is not stable. With the roof a/c's on it will randomly drop down to 80-90 volts but does not trip offline or throw any codes.

I disconnected it from the coach this morning and connected it straight to several resistive and motor loads. Here are the steady state readings that I recorded.

115-117 VAC @ no load
111VAC @ 21 total amps
113 VAC@ 13 total amps

I also noticed that my clamp on amp meter and volt meter would momentarily drop out (very short transient). During my testing today I did not see any voltage below 109 VAC.

I have checked all the connections, ohm'd out the windings and verified all of the magnets were still bonded on. I am beginning to think that it is the inverter assembly but I thought I would post to see if anyone had any other ideas.

I would appreciate anyone's thoughts ....THANKS
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Old 07-21-2014, 05:21 AM   #2
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What were your HZ readings on all these differential load tests?
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Old 07-21-2014, 07:51 AM   #3
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Are any a/c compressors trying to start at the time the voltage dips? How long do the dips last? Are all electrical connections tight? Just a couple of things to check.
Good luck and safe travels!
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Old 07-21-2014, 08:32 AM   #4
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I had a similar issue on my 8kQD and took it to Cummins to find out that the governor needed to be adjusted. After the adjustment all is well.
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Old 07-22-2014, 08:52 AM   #5
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The numbers in your message appear to be within acceptable limits, but obviously 80-90 is not. Is that a brief transient? I don't see how sustained voltage that low would not cause either a breaker to trip (the amps go sky high at low voltage) or the a/c itself to labor and shut down. How did you determine that the voltage went that low?

In any case, the Onan itself is supposed to shut down and give a fault code if it senses low/high output voltage or frequency (Hz). It has a voltage regulator which in turn drives the engine governor.
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Old 07-22-2014, 10:59 PM   #6
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I recently replaced the start and run capacitors on my two roof ac units. My coach is a 99. It sure did improve the start up performance for both units. Very glad I chose to do both, as one ac was not starting properly. I think my genny likes it as well.

Food for thought...

Speaking of the Onan, I changed the oil/filter and fuel filter this past weekend. That fuel filter sure gave me a hassle. The rubber line going to the box filer is too fat to use a proper flare wrench and I had to use a slip joint plier with curved jaws to loosen it. A standard wrench just did not work and was rounding the soft brass fitting. Another thing, the rubber hose was just barely long enough to get the pliers on. Just one more inch of hose and that job would have been way easier...sheesh...
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Old 07-22-2014, 11:19 PM   #7
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Look here for the manual......check troubleshooting section.

http://www.cumminsonan.com/www/pdf/m.../981-0522B.pdf
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Old 07-23-2014, 08:00 AM   #8
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Originally Posted by KIX View Post
Look here for the manual......check troubleshooting section.

http://www.cumminsonan.com/www/pdf/m.../981-0522B.pdf
Muchos Grandios!
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Old 07-23-2014, 05:10 PM   #9
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I took some additional measurements yesterday. The freq is rock solid 59.7 Hz for all loads. At a 25 amp total load the voltage averaged 111 but with my fluke meter set to capture min/max voltages it would consistently record momentary drops to 90-97 volts with a steady load applied. If fact as I switched loads In and out the regulation was great.

Since it seemed to be working better I hooked it back up to the coach and turned on both a/c units. The fans and compressors kicked on with no issue then after a couple of minutes the voltage dropped into the 80's and the gen shut off. I checked for codes and it was reading 12 Which is governor or inverter issues. I am relatively sure it is the inverter/controller because engine rpm is staying constant. And the inverter is the most expensive part the best price I have found so far is $2200 for a refurbished unit. I can only imagine what onan wants for a new one.
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Old 07-24-2014, 05:18 AM   #10
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Make sure the controller/inverter heatsink/cooling fins aren't plugged up with mud or mice nests or something that the unit ingested such as a plastic bag, leaves, etc etc. If it's unable to cool itself it can get goofy.
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Old 07-24-2014, 06:10 AM   #11
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Quote:
Originally Posted by Carweld View Post
I am relatively sure it is the inverter/controller because engine rpm is staying constant. And the inverter is the most expensive part the best price I have found so far is $2200 for a refurbished unit. I can only imagine what onan wants for a new one.
We have the same model generator in our coach. The controller in ours quit about a year ago while we were in Florida. I believe the replacement was $1800 from Cummins in Ft. Myers. The frustrating part is that you know darn well the circuit board inside is probably the problem but you are stuck buying the huge controller. Installation was not hard but time consuming to get everything unbolted.

The difference with ours was that it would not run and gave us a fault code that narrowed it down to the controller.
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