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Onan 7.5 JB-3CR running issue


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  #31  
Old 03-17-2017, 10:25:19 PM
nothingbutdarts nothingbutdarts is online now
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Default Re: 7.5 Onan JB-3CR running issue

60's-70's MOPARs used the straight battery voltage to the coil during cranking like this JB does, then when you let off the ignition key the coil got fed with voltage thru a ballast resistor to lower the voltage.

Love the relays used in this set you posted a diaghram for MBB that David C has. This must be similar to what MAX does in his older JC sets!

Also interesting JohnnyC's 1993 JB does not have new style regular relays and David C's 1994 JB does have relays!

Sorry to kind of hijack this thread. Over and out!

Last edited by nothingbutdarts; 03-17-2017 at 10:51:28 PM. Reason: added info.
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  #32  
Old 03-17-2017, 11:01:44 PM
David C David C is offline
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Default Re: 7.5 Onan JB-3CR running issue

Here is a shot of the control box on the 1995 model JB. It is busy in there compared to a Contractor model.


David C.
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  #33  
Old 03-17-2017, 11:12:15 PM
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Default Re: Onan 7.5 JB-3CR running issue

Where did they move the starter solenoid to in the control box, do they use a relay in place of that also??
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  #34  
Old 03-18-2017, 05:59:09 AM
David C David C is offline
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Default Re: Onan 7.5 JB-3CR running issue

The relay is still there. It is in the top left corner of the picture.

David C.
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  #35  
Old 03-18-2017, 08:58:00 AM
Leon N. Leon N. is online now
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Default Re: Onan 7.5 JB-3CR running issue

OK I see what you are saying. I will say, those folks who have the earlier versions of the JB, say the mid sixties have a more simple and more reliable machine than what came later. Yes, the later designs if taken care of are probably just as good, but I may be old fashion, but I will stand behind my comments. There will never be anything ever again that will match the old JB magneciters in endurance, ruggedness and reliability. Yes, they could be made today, even better than before, but the market is not there.
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  #36  
Old 03-18-2017, 12:04:22 PM
len k len k is online now
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Default Re: Onan 7.5 JB-3CR running issue

I know the method of bypassing resistor makes for better spark during starting. I added that system to my 60's snow plow jeep.

If I wanted to add resistor bypass to 6V coil of a JB , I'ld use 2 resistors each 1/2 the ohms of original resistor. And only bypass one resistor during cranking. The reason being bat voltage may get down to 9-10 V during high amp load of cranking. With one resistor still in series coil likely will still only see 6V during cranking. Don't want to over stress it.
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  #37  
Old 03-18-2017, 04:37:03 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Ok after reading many of the helpful posts I still have some confusion. The two wires (which are cut on my unit) leading to the centrifugal clutch are used to power the ignition coil somehow? One of the wires does seem to have a ballast resistor inline .
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  #38  
Old 03-18-2017, 04:40:28 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

Previous owners hack job. The coil is fed from the K-12 relay after the start relay opens.

Do you have the wiring diagram?
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  #39  
Old 03-18-2017, 05:05:34 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

I've been using drawing 612-4794

Currently the only lead hooked to the ignition coil is lead #4.

I sure appreciate everyone's patience with me on this project. I've been learning quite a bit from all the helpful posts. At this point I will be disappointed if I'm unable to get it running. It's turned into a personal challenge.

Me Thompson, what you said makes a lot of sense to me now. Initially I was able to get it to fire and run for a couple of seconds before what seemed to me that the ignition circuit was broken. Looking back I can see it was clearly the case.

*mr Thompson. Sorry I'm using my phone and the buttons are far too small for my thumbs.
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  #40  
Old 03-18-2017, 05:10:17 PM
squidtrap squidtrap is offline
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Default Re: Onan 7.5 JB-3CR running issue

in post #30 MBB sent the wiring diagrams and build sheet. did not open them but i would figure thats what u have.
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