| Onan - No AC - Continued [message #247174] |
Fri, 11 April 2014 20:52  |
Steve
Messages: 506
Registered: September 2013
Location: East Greenville, Pa
Karma: 1
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Last week I had a lengthy thread going for advice on my Onan that was not producing AC. Having read that again I have decided to start a new topic as most of the previous topic was around getting the Bridge Rectifier (BR) tested and replaced. Many thanks to all who helped.
At this point I am fairly certain that I have a good BR that is properly installed. I have measured the field coal resistance across the F-1 and F-2 BR wires and have the proper 38 ohms, and I am getting 2.9 volts of AC measured inside the coach at the electric panel. I understand that this low voltage is typical and is due to residual magnetism.
What to do next?
From reading Jim Miller's posts I understand that possible next steps might be to test the Reactor (although these seldom fail), or to possibly "excite" the coils with a battery?
I also read through my old threads and looked at dates. One fact I may have had wrong is that I don't think I actually used the electricity since last fall. I had run the engine several times over the winter but I did't actually check or use the electricity. I don't know if that changes any view on possibly needing to "excite" the coils.
Advice on next steps would be appreciated. Saturday is my opportunity to spend a little time with the GMC.
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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| Re: Onan - No AC - Continued [message #247196 is a reply to message #247174] |
Sat, 12 April 2014 06:28  |
Steve
Messages: 506
Registered: September 2013
Location: East Greenville, Pa
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A Haamilto,
I did order a brand new bridge rectifier so I now have a total of three that all test good. As the wires are clearly marked I am confident that I have installed them correctly. Two produce the same result in the Onan. The third will be tried in the Onan this morning.
I am now trying to determine everything I should or could do before removing the whole unit and opening it up for further testing.
Thanks for all of your replies. They are greatly appreciated.
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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| Re: Onan - No AC - Continued [message #247214 is a reply to message #247174] |
Sat, 12 April 2014 09:02 |
Steve
Messages: 506
Registered: September 2013
Location: East Greenville, Pa
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Maybe time to excite with a battery as Wild Bill suggested? It seems like I should be looking into the flywheel alternator? Could the low voltage coming into the BR be the issue?
1978 GMC Royal
Eastern Pennslyvania
1968 Chevrolet C20 396 Camper Special
1969 Chevrolet C20 Camper Special
1985 Buick Electra Park Avenue
1992 Camaro 25th Anniversary Heretage Edition Black
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| Re: [GMCnet] Onan - No AC - Continued [message #247217 is a reply to message #247214] |
Sat, 12 April 2014 09:30 |
Jim Miller
Messages: 501
Registered: March 2008
Karma: 10
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The flywheel alternator has absolutely nothing to do with the power-production end of your Onan.
It would not hurt to excite with a battery as another has suggested, just watch your polarities. F1=+, F2=- If you get a couple tens-of-volts AC out of the generator then you have proved the fields and the armature. I did not suggest this because you initially said you had good AC a week ago - now you say it was that year sometime - that is a different situation and was a critical piece of information.
Albeit unlikely, It is also possible that you have a bad armature or a brush problem. To prove this you need to pull the bell housing off the generator and go through the procedure described in the book using a 120V lamp and cheater cord. I'd not do that yet, lets try the external excitation with a battery first.
I've gone through the dissertation on self-excitation in earlier postings. In general: the INITIAL voltage that comes into the BR when you start the machine is due to nothing other than the residual magnetism in the field's metal plates. As the BR rectifies that voltage and applies it to the field coils then that excited magnetic field adds to the residual field - thus producing more generated AC - thus producing more rectified DC, thus producing more magnetic field and thus more AC, and so forth. Eventually the reactor and thyrector will clamp this excitation and that is how voltage regulation is achieved.
It would not hurt to look at the Onan service manual and read through the troubleshooting procedure and check off what you have done and then evaluate what you have not done. Posting on the GMCnet seems to always result in a cacophony of responses which only tend to cloud the solution rather than move you closer to it.
--Jim
On Apr 12, 2014, at 10:02 AM, Steve Adams wrote:
>
>
> Maybe time to excite with a battery as Wild Bill suggested? It seems like I should be looking into the flywheel alternator? Could the low voltage coming into the BR be the issue?
> --
> 1978 GMC Royal
> Eastern Pennslyvania
> 1968 Chevrolet C20 396 Camper Special
> 1969 Chevrolet C20 Camper Special
> 1985 Buick Electra Park Avenue
> 1992 Camaro 25th Anniversary Heretage Edition Black
>
> _______________________________________________
> GMCnet mailing list
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>
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan - No AC - Continued [message #247221 is a reply to message #247209] |
Sat, 12 April 2014 09:51 |
Jim Miller
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Registered: March 2008
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> I'm not sure what this tells me, other than I have some voltage in the places it should be. I am guessing that the AC volt input to the BR should be higher, which would in turn produce higher DC volts output. I will have to research and see what the AC volts into the bridge should be.
The AC voltage into the bridge (and naturally the rectified DC output from it) is controlled by the reactor and thyrector and is dependent on the load placed on the generator. These values are not specified in the manual however during my development of replacement field coils I applied various loads to a working Onan and came up with the following measurements of DC applied by the BR to the field:
AC out: 140V @ 0 A (0.0 kW): DC from bridge into field coils: 63VDC @ 1.8A
AC out: 130V @ 15A (2.0 kW): DC from bridge into field coils: 74VDC @ 2.1A
AC out: 125V @ 30A (3.8 kW): DC from bridge into field coils: 103VDC @ 2.9A
AC out: 125V @ 44A (5.5 kW): DC from bridge into field coils: 131VDC @ 3.7A
These measurements were taken with a field assembly that measured 38 Ohms and are guaranteed to be different on fields that have resistance other than 38 ohms. Perhaps some electronics purist will point out the R=E/I calculation on the field readings shown above indicates 35.3 ohms however that observation is not material to this discussion.
You can do the back-extrapolation and see what 12VDC applied directly across the field from a battery should give you in terms of AC on the output terminals. It'll certainly be more than the 2.0VAC you are seeing now IF the fields and armature are working properly.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Royale
Hamilton, OH
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| Re: Onan - No AC - Continued [message #247223 is a reply to message #247174] |
Sat, 12 April 2014 10:02 |
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WildBill
Messages: 232
Registered: January 2014
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You can also check field continuity first. Ohms should be low.
Remove the field wires from the regulator. Like I say ohms should be low,Then with engine running, no load hooked up connect pos and neg to the field wires while still disconnected from regulator and watch ac voltmeter on output. Ac input into rectifier and then dc output from rectifier. Apply voltage just long enough to verify voltages as full 14 volts to field could produce significantly more output voltage that 120 volts ac.
Posted while you were posting
[Updated on: Sat, 12 April 2014 10:06] Report message to a moderator |
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| Re: [GMCnet] Onan - No AC - Continued [message #247225 is a reply to message #247219] |
Sat, 12 April 2014 10:04 |
Jim Miller
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Registered: March 2008
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Shotgun troubleshooting might eventually find a solution but only by sheer chance would it happen in less time than methodical troubleshooting.
In the specific case of this Onan thread, some of the suggestions have been blatantly incorrect and serve only to move the solution further away rather than closer....and that approach is even less effective than shotgun troubleshooting. :(
On Apr 12, 2014, at 10:40 AM, Robert Mueller wrote:
> Jim,
>
> AKA shotgun troubleshooting! ;-)
>
> Regards,
> Rob M.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan - No AC - Continued [message #247231 is a reply to message #247223] |
Sat, 12 April 2014 10:10 |
Jim Miller
Messages: 501
Registered: March 2008
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On Apr 12, 2014, at 11:02 AM, Bill D wrote:
> You can also check field continuity first. Ohms should be low. Remove the field wires from the regulator. Like I say ohms should be low, probably 3 to 5 ohms.
He already did this and found the field resistance to be within specifications: 36-38 Ohms.
> Apply voltage just long enough to verify voltages as full 14 volts to field could produce significantly more output voltage that 120 volts ac.
That is true on many gensets but is certainly not true in the case of the Onan. Full-load excitation on the Onan is ~130VDC.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan - No AC - Continued [message #247233 is a reply to message #247232] |
Sat, 12 April 2014 10:27 |
Jim Miller
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Registered: March 2008
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On Apr 12, 2014, at 11:21 AM, A. wrote:
> I was really hoping it was the bridge rectifier. There isn't much else that doesn't involve major effort.
Applying external DC to the fields ("flashing" them) does not require major effort and solves the problem in some cases.
If it doesn't solve the problem it will at least provide a valuable data point that can help us rule out the armature and fields as the cause.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan - No AC - Continued [message #247234 is a reply to message #247230] |
Sat, 12 April 2014 10:44 |
Jim Miller
Messages: 501
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On Apr 12, 2014, at 11:12 AM, Bill D wrote:
> Wow! If I had to live with output ranges like that I would find a better regulator or update the onan with a more modern genset.
Nothing in the GMC RV gives a hoot about voltage variations across the range produced by a properly working Onan. We're not talking about grid-tied power production nor a 400Hz genset feeding a military radar set. We're talking about an RV application.
BTW: you can retrofit an electronic AVR to the Onan if you change the field coil configuration and that AVR will hold the output voltage spot-on; I have tried it. Why does my personal GMC still have the OEM VR system instead of an electronic one? One word: simplicity. The AVR has a lot of electronics in it which increases the possibility of failure. The OEM system is dirt simple (two coils and a semiconductor clamp) and is far less likely to fail. I'm happy to trade increased reliability for a few percent of variation in output voltage regulation.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan - No AC - Continued [message #247244 is a reply to message #247233] |
Sat, 12 April 2014 14:09 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
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| Jim Miller wrote on Sat, 12 April 2014 09:27 |
On Apr 12, 2014, at 11:21 AM, A. wrote:
> I was really hoping it was the bridge rectifier. There isn't much else that doesn't involve major effort.
Applying external DC to the fields ("flashing" them) does not require major effort and solves the problem in some cases.
If it doesn't solve the problem it will at least provide a valuable data point that can help us rule out the armature and fields as the cause.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Years ago my Onan quit charging and a quick reflash worked.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: Onan - No AC - Continued [message #247252 is a reply to message #247174] |
Sat, 12 April 2014 14:53 |
bobby5832708
Messages: 237
Registered: November 2006
Location: Winter Springs FL
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It's a long story, but a few years ago I found that the field is polarity sensitive. On my Onan, swapping the field wires on the + and - bridge terminals fixed it. If I remember correctly the output voltage was like 2 volts and swapping the field leads was the solution.
That was the closest my Onan came to being replaced. It still runs fine today.
Bob Heller
2017 Winnebago 29VE
Winter Springs FL
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