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Onan MDJE 7.5 1978. Possble Burnt Windings


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  #1  
Old 03-08-2018, 07:37:30 PM
yachtgilana yachtgilana is offline
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Default Onan MDJE 7.5 1978. Possble Burnt Windings

Hello again.

My trusty gengen, got sick last night..

All was normal and I went to shower, my wife was near the control panel. She heard a hesitation, after about 20 minutes of normal running. She checked gauges, and carried on. Next, I heard a surge, like an overload, the governor opened up and she growled (the generator, not the wife) By the time I got out f the shower, Laura had shut it down.

I opened the engine room to find smoke...uh oh...but sometimes exhaust smoke does pour out of the inlet, I hoped.

This morning I started the trouble shooting according the to manual.

There was no excitation voltage, so I flashed the field, Immediately the excitation voltage shot up, and the last value I saw before removing the Multimeter leads was 37v AC. There was the same growl, the same smoke now coming out of the vent, immediate shut down.

My question is, there are 4 coils in there, the exciter stator, rotor, the gen stator and rotor.

Which one of those is most likely to burn?

I know I have to rewind it, and I will do it myself, but because of lack of access, I cant do the rest of the tests.

Over...
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Old 03-09-2018, 10:16:31 PM
yachtgilana yachtgilana is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

88 reads and no comments.

Has anyone ever seen a burned out Onan 7.5 or 6.0 ? If so, which are the most common windings to go?

Or something else?
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Old 03-09-2018, 10:33:32 PM
akirkland akirkland is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

Its impoosible to say which windings are the most likely to burn up. There is no choice but to get in there and look at each component. IF you're convinced that it has a burnt winding and you're willing and capable of repairing it yourself, then it sounds as though you must pull the gen and get to it.

However after rereading your original post, you applied dc voltage to the fields and the voltage came up slightly and the engine souded like it was under load. My guess, and at this point its only a guess, is the ac output windings are toast.
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Old 03-09-2018, 10:34:39 PM
Wayne 440 Wayne 440 is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

In my experience, there is no particularly common failure mode that produces smoke from the YD generator section. My respectfully submitted advice is that you take whatever steps required to overcome the "lack of access" issue, then follow the service manual procedure to determine where your fault is.
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Old 03-09-2018, 10:47:10 PM
yachtgilana yachtgilana is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

Thanks for that. The Gen is installed in a sailboat. I need to disconnect everything, place wooden bearers under it, then remove flywheel and ancillary equipment, then rotate it 180 degrees to see the end bell, and carry on with the steps. Normally not a problem except now, were at anchor, and if a swell kicks up, or we have to move in a hurry, we might have a lot of iron tumbling about in there.

So I was looking for some comments from people with experience, to try and narrow things down. It looks like it might be a rewind, and I am guessing its the Gen Stator, judging by the load it simulated.

Thanks Wayne, I will have to wait till I am on a dock, or on the hard next month.

"Unbridled Spirit" Love it.
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Old 03-09-2018, 10:56:04 PM
Wayne 440 Wayne 440 is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

Based on the fact that you had smoke and a loaded engine, that is high on the list of possibilities. Another member here just had what appears to be a stator failure in a JC.

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Old 03-09-2018, 11:41:08 PM
akirkland akirkland is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

Just so you know, rewinding a stator is a complex procedure. Forming coils, figuring out coil dimensions, wire size, wires in hand, slot insulation dimensioning, dipping , baking.
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Old 03-09-2018, 11:58:46 PM
zuhnc zuhnc is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

I acquired a 6MDJB a while back. Upon disassembly to repair the cracked block, new bearings, etc., I discovered that the stator was toast, due to water damage and burnt windings. Extensive web research revealed precious little about single-phase winding, either motor or generator; tons of stuff on 3-phase. After 2-3 months of research, I knew a little more about how the generator generates. Bit the bullet and started in on the rewind. The hardest part was "unwinding" the wire and measuring turns, span, wires-in-hand, and a multitude of other things. I finally developed a winding diagram. Obtained the proper wire for the main windings and battery charge winding. Took about two weeks, a couple of hours at a time, to rewind. All supplies bought on the web. When I was done with the engine and generator, I was able to produce full load, with no smoke. Materials were about $100.00 or so. I did not dip and bake; since it was NOT going to be a marine unit any more, I felt that that process was not needed. Rewinding yourself can be done, but it takes patience and care to get everything right. In your case, dipping and baking can be done by your motor shop, even if you do the rewind yourself. Good luck on the project. zuhnc
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Old 03-10-2018, 09:44:31 AM
akirkland akirkland is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

Z, although yours is not a marine unit, one of the many reasons to dip and bake the windings is to hold the coil wires tightly together. This helps prevent wires shorting together due to vibration.
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Old 03-10-2018, 02:12:32 PM
zuhnc zuhnc is offline
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Default Re: Onan MDJE 7.5 1978. Possble Burnt Windings

I do agree that dipping and baking would be very beneficial, regardless of the application; vacuum impregnation would be even more so. I feel confident in my rewind. I did use copious amounts of insulating varnish when installing the windings, and very tightly tied all the end turns together. Most marine units run for extended periods of time (got to keep the AC operational!), in horrendous moisture and heat conditions; those benefit most from the dipping and baking process. If a commercial shop were to do the rewind, I would insist on it; for personal use, it depends on how comfortable one is with the procedures one uses. I did use heavy duty insulation, packed the turns tightly, and used slot sticks, even though the original manufacture did not use them. Only time will tell how well mine holds up. If I have to rewind in a score of years, I will be satisfied with what I have done. If anyone wants information on my rewind, wire size, span, turns, etc, please send me a PM. zuhnc
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