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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan 7.5 JB-3CR running issue


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  #11  
Old 03-16-2017, 10:58:21 PM
David C David C is offline
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Default Re: 7.5 Onan JB-3CR running issue

If you just want to get it running, you can just run a hot wire from an on/off switch through the oil pressure switch (as long as it is normally open) then to the ignition coil (and resistor if coil requires on) and then to the points. Get a HD push button switch and run a wire to the "S" terminal on the starter solenoid for the start circuit. A very basic electrical system, but it does eliminate the capability of remote start. I know most of the guys here cringe when I mention this type of wiring, but it does allow you to use the generator until the time comes that you have rounded up all of the parts to return it to the original Onan design.

David C.
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  #12  
Old 03-16-2017, 11:22:34 PM
yellowlister yellowlister is offline
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Default Re: 7.5 Onan JB-3CR running issue

Pretty sure I have a centrifugal switch and may have the cover..
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Old 03-16-2017, 11:46:28 PM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Thanks guys, I sure appreciate all of the feedback.
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Old 03-17-2017, 09:20:53 AM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Will any 6V duel plug wire coil work for this unit? The onan coil is pricey and considering we have $250 into it now we are unsure what the budget for repairs will be. When it initially fired it ran for a few seconds before shutting down. What caused the ignition to lose power? and does anyone have an ignition schematic for this model. I'm not even sure what year the generator is. Can this be determined by the plate on the side of the cover? I do have the cover for the "Fan" side, but the cover over the carburetor and exhaust manifold is missing. can it be safely run without those without overheating?

I sure appreciate all of everyone's help.

Thanks!

---------- Post added at 09:20:53 AM ---------- Previous post was at 08:59:51 AM ----------

I should point out that when it fired it was using a coil off of an old miller welder with an onan engine. Spark was pretty weak though. Points were gapped to .020
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Old 03-17-2017, 09:40:43 AM
Ray Lynch Ray Lynch is offline
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Default Re: 7.5 Onan JB-3CR running issue

Built March 1977
It will overheat if the tins are not installed.
Ray
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  #16  
Old 03-17-2017, 09:47:23 AM
Kevin K Kevin K is online now
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Default Re: 7.5 Onan JB-3CR running issue

Quote:
Originally Posted by mheltunen View Post
Will any 6V duel plug wire coil work for this unit? The onan coil is pricey and considering we have $250 into it now we are unsure what the budget for repairs will be.
That should work fine. If you eliminate the 1.72 ohm ignition resistor, you should be able to use a 12 volt dual tower coil.

The age of your Onan is in the serial number. March (C) 1977 (77).

I found the production order for your generator. The wiring diagram number was difficult to read, but it looks like 612-4794.
Attached Files
File Type: pdf 75JB3CR12424AA.pdf (24.1 KB, 16 views)
File Type: pdf 612-4794.pdf (158.8 KB, 23 views)
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Old 03-17-2017, 09:54:29 AM
Leon N. Leon N. is offline
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Default Re: 7.5 Onan JB-3CR running issue

Mr. Heltunen: To answer some of your questions, such as finding the correct wiring diagram for your JB, we need one of the stack members to produce the factory build sheet for this particular machine. Also, what are your intentions or plans for this JB?

Yes, one can in some cases substitute non Onan parts and modify the wiring, etc, but looking at your pictures, this puppy need lots of attention and restoration of missing parts to return this machine to a reliably operating unit.

To answer some of your other questions, this JB was built in March of 1977. What caused the ignition to lose power?, Hmmm, hard to say since it appears the unit was modified and we do not know exactly what you have here. Yes, you do need the cover for the carburetor side and all the sheet metal enclosures to operate and provide adequate cooling.
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Old 03-17-2017, 10:00:16 AM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Our intentions for this unit was to provide power to a remote hunting cabin. I don't have any interest in a restoration just looking for a reliable and cost effective power source. I have a strong dislike for cheap Chinese gensets. Onans when properly maintained have always seemed like excellent units.
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  #19  
Old 03-17-2017, 12:52:01 PM
len k len k is offline
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Default Re: 7.5 Onan JB-3CR running issue

Can use modern 2 tower coils ( waste spark). Might need a resistor in series with primary if coil ohms are too low.

Here's some alternate ideas for using modern ignition moduals and coils

https://www.smokstak.com/forum/showthread.php?t=167037
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  #20  
Old 03-17-2017, 03:19:57 PM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Perhaps someone can clarify this for me as well. the coil on my model is a 6 volt 166C346. So i am assuming it is supposed to have a resistor on the wire feeding the positive side of this coil. Looking on my set I have been unable to locate one. There is a resistor on the negative side feeding the point. According to drawing 612-4794 the coil is fed from position #4 on terminal block 12. tracing it down on the drawing I see no indications of a resistor as well. I am not an electrician but usually I'm able to head scratch my way thru a schematic. The wires feeding centrifugal switch (wires #5 on TB12 and wire from B+ on tb11 have been cut and are hanging loose.

---------- Post added at 03:19:57 PM ---------- Previous post was at 03:17:13 PM ----------

I have also taken the time to remove any wiring modifications that were done and using the schematic I returned the wiring to their factory locations.
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