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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 offline
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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, 22 views)
File Type: pdf 612-6642-2.pdf (216.1 KB, 17 views)
File Type: pdf 612-6642-3.pdf (158.6 KB, 17 views)
File Type: pdf 75JB68775E.pdf (33.6 KB, 21 views)
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