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Onan 7.5 JB-3CR running issue


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  #41  
Old 03-18-2017, 05:53:27 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

That's for a different year. On my original post I have the name tag and I believe it said mine is a 1977 model
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Old 03-18-2017, 10:23:00 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

The build sheet may have been used for several years for the same set configuration. The wiring diagrams were used for many years, you will sometimes see modification notes on them with dates far from the original drawing date.

If the model & spec. on the build sheet match your set, the wiring diagram listed on the sheet is correct.
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  #43  
Old 03-18-2017, 11:09:05 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Diagram #612-4794 matches my set. My question/problem is the coil looks to be fed from 1 lead (#4 from tb12). Wires #5 and the wire leading from B terminal to the centrifugal switch have been cut and the switch was removed. Looking at the diagram I assume #4 wire is full voltage (12v) for starting and then once started it switches to #5 terminal on tb12 but I don't understand the routing to the coil. In my mind by looking at the schematic once relay k12 closes and energizes the #5 wire how does it reach the positive terminal of the ignition coil?

---------- Post added at 11:09:05 PM ---------- Previous post was at 11:02:21 PM ----------

And what is the function of the centrifugal switch? I'm slowly working my way thru this set and becoming more familiar with its circuits. Anyone with replys don't be afraid to "dumb it down" for me. Again all help is appreciated
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  #44  
Old 03-19-2017, 12:01:52 AM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-3CR running issue

The centrifugal switch is nothing more than a mechanical RPM sensing device, (before the days electrical magnetic pick-ups). It is installed on these generators I believe as a safety device, it will not let the generator stay running till it reaches about 900RPM, if it doesn't see 900RPM once you let off the start switch the engine will just die. Once the engine is running up to 1,800RPM and you walk away, if the engine should faulter and drop below 900RPM and not totally die it will again shut the engine off.

The starting sequence for a "J" line engine whether a JB or a JC is that, you hold the start/stop switch in the start position till you hear it get up to speed, once the centrifugal switch senses 900RPM it will automatically turn on the run relay, latching in the fuel shut off solenoid, powers the ignition coil and kicks out the starter.
Unlike a car when you turn the key to start and you hear the engine go to idle and just let off the key and it stays running, on the Onan, if you were to just let off the start switch the second you hear the engine start it would just die because the centrifugal switch has not yet sensed that the engine has reached 900RPM.
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  #45  
Old 03-19-2017, 07:56:25 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

So if I'm understanding correctly the 2 leads leading to the centrifugal switch make contact (via the switch) once it obtains 900 rpms. So for a possible testing scenario I could put a simple switch inline on the two wires and manually switch it over. Again this is not a permanent "fix" just one to tide me over until I find a centrifugal switch and cover. I also may need a 6v coil. How does one test the coil to make certain it is indeed faulty?
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  #46  
Old 03-19-2017, 08:33:22 PM
fisherman81654 fisherman81654 is offline
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Default Re: Onan 7.5 JB-3CR running issue

Quote:
Originally Posted by mheltunen View Post
So if I'm understanding correctly the 2 leads leading to the centrifugal switch make contact (via the switch) once it obtains 900 rpms. So for a possible testing scenario I could put a simple switch inline on the two wires and manually switch it over. (Yes, but you have to remember to turn it off when you power down the unit.)









I also may need a 6v coil. How does one test the coil to make certain it is indeed faulty?
This should help http://gardentractorpullingtips.com/ignelect.htm
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  #47  
Old 03-19-2017, 09:11:29 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

To test my 6 volt JB coil I folded up paperclips to make a spark gap between the towers. Then wired up the coil, a 6 volt gelcell, and a points cap in series. Then briefly connected and disconnected a shorting wire across the cap.
Don't hold the bare primary wires....ZAP .
(Four alkaline D cells in series would work too if didn't have a gelcell.)

I got ~ 1.5 inch spark (~ 45,000 volts) , but since then I've been told you shouldn't make the gap that large as it stresses the coil insulation.

Primary resistance on my good coil was .9-1.0 ohms, manual has specs on secondary ohms.

I had to clean the "mucky" primary connections on brass terminals to get mine to work, bad connection.
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  #48  
Old 03-19-2017, 09:16:35 PM
squidtrap squidtrap is offline
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Default Re: Onan 7.5 JB-3CR running issue

starting on page 29 of manual 967-0500 it goes through the whole testing procedure. The coil should be between 7-10K but u should go through the whole section and do all the tests.
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  #49  
Old 03-20-2017, 09:13:04 AM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

It runs!! after borrowing a coil and adding a (temporary) single pole switch to the wires feeding the centrifugal switch I was able to get it running. the engine is searching a bit so I can see some fine adjustments will be needed. My next question looking forward to the AC portion is connections. I will attach some pictures as well. In the box where the AC output connections are terminated I see 4 wires leading into the box. 2 of the wires lead to a ground lug and the other 2 are wire nutted individually. I am assuming the the two leads with the wire nuts are the hot wires. Each lead I assume is 120V and doubling them would bring it to 240. At this time I only wish to use 120V. So to wire a simple outlet in would I use one of the 120 leads to the hot side of the outlet and the wires leading to the ground lug for the neutral? Is there any way to ground the outlet like in a conventional house circuit? Again I apologize for what may be a basic set of questions. I'm not an electrician by trade but find I can usually figure it out with research and questions.

Thanks!
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  #50  
Old 03-20-2017, 09:15:30 AM
mheltunen mheltunen is offline
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Photo Re: Onan 7.5 JB-3CR running issue

Attached are a couple of pictures of my AC output box and the control box as well.
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  #51  
Old 03-20-2017, 09:37:49 AM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-3CR running issue

Turn the cover around that you took off the outlet box and there should be a wiring diaghram showing the different voltage configurations.
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  #52  
Old 03-20-2017, 10:03:12 AM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Thanks, I'll take a look.
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  #53  
Old 03-20-2017, 11:29:13 AM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

No diagram to be found....😡
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  #54  
Old 03-20-2017, 12:11:38 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

You should find the "reconnection diagram" at the bottom of you electrical print, I think one of us posted it with your build sheet.

The "searching" you described is called "hunting" and this condition is covered in the governor adjustment section of the service manual. I would not do a lot of adjusting to the governor until you have run it for a while with a load on it.

Governor and carb. adjustments both need to be done under load (preferably full load) and one can affect the other. There are very detailed instructions for both in the manual.
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  #55  
Old 03-20-2017, 12:14:44 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

I hooked one of the wires (that had the wire nut) to the hot side of an outlet. The grounded wire I hooked to the other side and had power . I was adjusting the carb and it stalled.. I adjusted carb ( gas block) and re fired it. Now I have power to the outlet but only about 20v . Trouble light was very dim. I checked the connections in the control box and everything is set correctly. Any thoughts?
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  #56  
Old 03-20-2017, 12:57:16 PM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5 JB-3CR running issue

Do you think it is running at or near 1,800 RPM??
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  #57  
Old 03-20-2017, 01:42:15 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

I believe it is close although i did not tach it. I hooked up the voltage regulator to ac power via 1,2 pins on the back and measured voltage at the F1 and F2 pins and came up with about 30-40 volts so im sure its not functioning correctly. On the back side of the board I see some signs of heat (brown/black marks) and suspect the worst.
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  #58  
Old 03-20-2017, 01:54:22 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

here are some pics from the front and rear of the board:
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  #59  
Old 03-20-2017, 07:16:42 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

I wouldn't get excited about the discoloring on the back near those 2 power resistors. Each is rated ~ 5 watts, runs hot normally. It's a slightly darker than other board I've seen. But this type of discoloring is normal for old boards in many other applications. Back then they let the components run hot and board discolors from the heat. Doesn't necessarily mean they are bad.

With these regs usually problem is the 2 adjustment pots get dirty inside( bad). Can install reg, run gen , monitor 120 voltage leads, turn the pots a bit and see if voltage jumps arround.

Alternately you can test to see if genhead windings/diodes/ect are good. Disconnect reg and tape off the leads. Measure from F1 to F2 on genhead winding, should be around 12 ohms. If it is hook up a car bat across F1-F2 leads with gen running (only draws ~ 1 amp), I got ~ 140 VAC @ no-load out the 120 VAC leads when I did that.
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  #60  
Old 03-20-2017, 07:21:56 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

Can we assume you checked the resettable breaker for the VR (push button in top portion of the control box).

Did you have 120-V at the receptacle after you wired it up?
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