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Old 10-23-2016, 05:01 PM   #1
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Generator or A/C issue

Good evening,
This is for all of you smarter than me (lots of you, just ask the DW) Onan and A/C people. I had an earlier thread but the transfer switch repair has changed the issue some so I am starting a new thread.
I have a 2000 Monaco Windsor motorhome with an Onan 7.5HDKAJ11451F generator (62.5 amp output) (726.5 hours) and two DuoTherm13,500btu model 620315.321 airconditioners with 1530btu heat strips. Single four button DuoThrem comfortcontrol thermostat.
THE PROBLEM, when I turn on either a/c unit (nothing else at all running in the rig) the generator instantly dies (error code 3 sub cat 11) whenever Either a/c compressor even thinks about coming on. No breakers or fuses ever blow on the generator, rig or a/c units.
THE HEAD SCRATCHING PART, here are the tests that have been done:
1) the generator was just serviced. Oil changed with new filter, all levels checked, new fuel filter and air filter. Fuel tank is full of fresh diesel.
2) the generator will, at the same time, run both duotherms with heat strips on, the battery charger/inverter, the electric hot water heater and the microwave. Total amperage draw 54 amps (38 line one, 16 line two) with no problems. If I add the coffee pot (additional 12 amp load, 66 total amps) the generator quits
(As expected) with the same 3-11 error code.
3) on shore power, The front a/c draws 18amp max load when the a/c compressor kicks in and then draws 12amp while running. The rear A/C draws 13 running and 19 spike.
4) I installed OEM Duo Therm hard start kits with electrionic relays designed for these a/c units on both a/c's. that means the start capacitors are new and the ceramic PTAC are replaced with relays. The run capacitors test in spec.
5) both compressors test good. The scratch test shows no continuity to case from any terminal. There is continuity between all start/run/common terminals. The ohm test on the compressor are C-S 7.2Ohm C-R .7 Ohm S-R 7.9 OHM
6) on shore power both a/c units start and cool properly with no issues.
7) As stated before, the fan and heat strips on both units run without issue on the generator. It is only the cool mode that is an issue.

The generator operates properly with a 54 amp load. The A/c compressors test ok. Both a/c units work /cool properly on shore power. The max power amp draw on the a/c is not excessive and is well within the power produced by the generator using other loads. Both a/c units act exactly the same so if it is an issue it is the same issue on both units.

Any Thought or ideas????? Is there anything in the a/c circuit boards or thermostat circuit that could cause this issue?
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Old 10-23-2016, 08:09 PM   #2
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Buy a Kilowatt monitor and see if you are seeing any variation in the frequency and voltage the generator is providing as you are turning on the AC.

If there isn't any difference my feeling it that it still may be a generator issue as the shore power will pretty much have constant frequency with only slight variations in voltage.

Keep us posted on this interesting problem
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Old 10-23-2016, 09:02 PM   #3
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Have you tried changing the brushes and burnishing the cummutator? Arcing there will cause problems exacerbated by high current surges.
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Old 10-24-2016, 05:10 AM   #4
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I'm not near any of my tech manuals/etc to verify, but a quick google on my phone came up with "Controller Failure" for the 3-11 code, if that's any help.

Seems like the gen will work when tested with mostly resistive loads, but balks with the inductive compressors. Maybe the controller is sensing that?

Maybe a more-detailed google (or service manual) search would narrow it down further?
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Old 10-24-2016, 06:17 AM   #5
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Thanks, I did check the voltage. It does not fluctuate. I did not think to check the frequency. I will do that.
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Old 10-24-2016, 06:19 AM   #6
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Thanks Vsheets, I checked the brushes which looked good. I doubt it would hurt to change them. Thanks for the idea.
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Old 10-24-2016, 06:29 AM   #7
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Thanks RustyTools, the service manual does mention the controller assembly as a possibility for the 3-11 error code (Ac output short). But, since the generator runs fine with its breaker shut off, the manual states the generator is "probably" ok. Since I could load it to 54 amps with other loads I went looking for a short in the rig ac's.
I never gave the inductive vs resistive loads a thought. You might be on to something.
Is there anyone who can test the controller or should I just replace it? Any ideas on sources for a new or remaned one? Flightsystems?
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Old 10-24-2016, 06:51 AM   #8
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Ok, must be needing another cup of coffee. I am not sure what I checked through the housing that I thought was brushes.i don't think this unit has brushes.
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Old 10-24-2016, 08:38 AM   #9
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UPDATE: after several hours on the phone with various Onan tech support companies, including flight systems, the consensus is an issue with the inverter controller assembly. All of them say the boards are not available or repairable and the entire unit must be replaced for $2,000.
I located a company called Test Point in Forest hills MD (phone 410-420-7080) they state that they have repaired several of theese boards and will diagnose the board and repair it if possible. They will quote a repair cost after diagnosis.
This will take a awhile but, I will try to update this tread later.
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Old 10-24-2016, 03:44 PM   #10
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You said Line 1 was 38 Amps and when you turn on the coffee pot, adding 12 Amps the generator kicks off. If they are on the same leg, the total is 50 Amps. I believe the 50 Amp service means 50 Amps on each leg. It might just be an overload condition.
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Old 10-24-2016, 04:46 PM   #11
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Thanks, I agree that in that particular test I did overload the generator and killed it. But, it was carrying the 54 amp load without issue. However, With nothing else running it will not handle the 19 amp surge load put on by either air conditioner. Well under the full load and well below the 54 amps it will run using other loads. One ac is on line 1 and one on line 2. It will not run either a/c individually with everything else turned off.
By process of elimination it seems the issue is in the generator controller unit.
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