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Onan 7.5 JB-3CR running issue


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  #21  
Old 03-17-2017, 04:21:56 PM
David C David C is offline
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Default Re: 7.5 Onan JB-3CR running issue

The ignition system does not care where the resistor is at, since it appears from the schematic that your set is not wired up with a resistor bypass while cranking. I have 7.5JB that is using a coil that needs a resistor (has resistor bypass wiring) and it actually starts better with the resistor bypass disconnected.

David C.
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  #22  
Old 03-17-2017, 05:17:16 PM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

Ah ok. For some reason I was under the impression that it needed to be on the wire feeding the coil to drop the voltage down to 6 volts.
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  #23  
Old 03-17-2017, 05:58:31 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: 7.5 Onan JB-3CR running issue

Your voltage dropping resistor for the origional coil is in your picture #3 you have posted. that is wired in series with the coil and goes to the centrifugal switch.

You did ohms check to coil you have???
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  #24  
Old 03-17-2017, 06:22:49 PM
mheltunen mheltunen is offline
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Default Re: 7.5 Onan JB-3CR running issue

I did not check ohms on the original coil but it produced no spark and the casing was cracked and split. I had another coil from an old welder I wired in and it did produce weak spark. I did check the point for gap (.020) and cleaned them as well. The coil I tried I believe is weak as well because I was only able to get spark on one of the two cyclinders. I tried a couple different plug wires to rule out those.
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  #25  
Old 03-17-2017, 07:05:21 PM
len k len k is offline
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Default Re: 7.5 Onan JB-3CR running issue

Plastic coil case on mine was split too, coil ohmed out at 0.9-1.0 ohms. Originally it didn't make spark, but I cleaned the primary terminals and it did make spark. Need a good connection. Even brass terminals get mucky.
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  #26  
Old 03-17-2017, 07:57:17 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: 7.5 Onan JB-3CR running issue

Also the origional coil is a "waste spark" syst. in as both plugs need to be hooked up and the base of the plugs grounded for the coil to work.
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  #27  
Old 03-17-2017, 08:03:31 PM
len k len k is offline
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Default Re: 7.5 Onan JB-3CR running issue

Technically the plug bases just have to be connected to each other. Each end of coil secondary is connected to a coil tower, so current tries to get from tower to tower , not to ground like it does in old cars.

On my JB coil I got a ~ 1.5 inch long spark using paper clips stuck in each coil tower. But it's not recommended to have such a long gap , it stresses coil insulation.
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  #28  
Old 03-17-2017, 08:28:17 PM
Leon N. Leon N. is offline
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Default Re: 7.5 Onan JB-3CR running issue

Post 21 talks about a JB using resistor bypass wiring. Can anyone produce an Onan JB wiring diagram which shows ignition coil primary circuit "resistor bypass wiring"?
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  #29  
Old 03-17-2017, 08:57:08 PM
David C David C is offline
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Default Re: 7.5 Onan JB-3CR running issue

Quote:
Originally Posted by Leon N. View Post
Post 21 talks about a JB using resistor bypass wiring. Can anyone produce an Onan JB wiring diagram which shows ignition coil primary circuit "resistor bypass wiring"?
Leon,
If someone has a schematic for 1995 7.5JB, spec 68775E, it should show the bypass. The set I have does not look like anyone has hacked up the wiring.

David C.
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  #30  
Old 03-17-2017, 09:58:24 PM
MBB MBB is online now
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Default Re: 7.5 Onan JB-3CR running issue

Here is the build sheet and wiring diagrams
Attached Files
File Type: pdf 612-6642-1.pdf (226.7 KB, 20 views)
File Type: pdf 612-6642-2.pdf (216.1 KB, 14 views)
File Type: pdf 612-6642-3.pdf (158.6 KB, 15 views)
File Type: pdf 75JB68775E.pdf (33.6 KB, 20 views)
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  #31  
Old 03-17-2017, 10:25:19 PM
nothingbutdarts nothingbutdarts is offline
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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
nothingbutdarts nothingbutdarts is offline
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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 offline
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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 offline
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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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