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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan JB Thermomagnetic Choke


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  #31  
Old 08-18-2017, 08:18:50 PM
JoeE. JoeE. is offline
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Default Magneciter...

This is the up close shot of what I would call a circuit board on the magnaciter... It's got five diodes attached to the copper plate behind the black part of the housing, one large resistor there on the left side of the black part, then 4 more little diodes in the center of the black part connecting all four points there... I think those are diodes.
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  #32  
Old 08-18-2017, 08:26:46 PM
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Default Re: Onan JB Thermomagnetic Choke

The are the four output leads... It was set up with 1 & 3 and 2 & 4 connected....
One problem I see is that T2 was not grounded ....unless it was grounded outside of the generator housing... And I cut this thing loose from all the interior wiring of the motorhome it was in...
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  #33  
Old 08-18-2017, 08:29:37 PM
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Default Re: Onan JB Thermomagnetic Choke

As far as I know this thing ran until he quit using the motorhome... I used to notice he'd set out there in it parked with a generator running, using the two rooftop air conditioners... I never heard him say it wasn't performing like it was supposed to...
What exactly goes wrong if T2 is not grounded... Is there something on the Magnaciter circuit board that goes bad... Diodes or resistor or anything like that?
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  #34  
Old 08-18-2017, 08:37:20 PM
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Default Ground lugs

The bolt on the left is halfway loosened, so it had a wire connected to that ... And the regular ground lug has a wire that I've cut off there, also.
The large boat that's loose might have had a ground wire to the frame of the vehicle... I'm going to go out and look in the motorhome to see what I've got laying there that might tell me where that wire came from.
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  #35  
Old 08-18-2017, 09:11:40 PM
Leon N. Leon N. is offline
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Default Re: Ground lugs

OK. To address your questions and observations. You said T1 & T3 were tied together and T2 & T4 were also tied together. That means this generator was set up to output only 120 Volts but at 32 times 2 or 64 amperes. In your post # 34, that ground lug that is labeled "Ground" is should have T2 solidly connected to it. In fact you can tie both T2 & T4 to that lug. That is the 120 VAC return or chassis ground. Perhaps the former owner unknowingly grounded T2 through the motor home where the generator was bolted to the structure.

Bottom line, your JB like mine MUST have T2 tied to that ground lug shown in post 34.

For further emphasis of this see the wiring diagram printed on the inside of your JB control box cover and/or page 7 in 967-320 which I emailed to you a bit ago.

Why is this important you ask? Plants beginning Spec P ( yours is spec T) have a voltage tap at terminal 35 which allows automatic field flashing during plant cranking to assure voltage build up. The plant battery must be negative ground and generator output lead T2 must be tied to chassis ground like the 12 volt battery is. Otherwise, field flashing will become ineffective or it may damage the exciter. That is one or more of those diodes shown in your post #31 could fail.

Now those generator output wires as you show in post # 32 can be reconfigured to provide 120/240 volts merely by reconnecting them as shown on page 7 in 967-320 or as illustrated/printed on the inside of your JB control box cover. In the 120/240 configuration once again T2 MUST be grounded to that lug shown in post #34. In that arrangement T2 & T3 are tied together and serve as the 120/240 neutral/ground circuits. T1 and T4 are two hot circuits at 120 Volts and T1 to T4 are hot at 240 volts. In this arrangement you can only draw 32 amps continuous from either or both of the 120 volt circuits.

Hope this helps. Just make sure T2 is grounded before you attempt to crank your JB.

Last edited by Leon N.; 08-18-2017 at 09:14:07 PM. Reason: typo correction
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  #36  
Old 08-18-2017, 09:19:42 PM
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Default Re: Ground lugs

Gotcha....
I'd say this was cobbled up.

I'll check out the email...

From looking down in the control box... I'm gonna have to study the wiring diagrams to see what does what...

I've found no crispy or burnt wires anywhere.. no brittle insulation problems anywhere.... of which I am glad.

If you guys notice a sharp decline in the "Mud Dauber" wasp population... it's because I must have cleaned out about half the world population of them down inside that generator housing.
We don't have much problem with paper wasps, but the mud building type are hell.
Out in the country.. .they're really bad... but in town, not quite so much. Not sure why.
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  #37  
Old 08-18-2017, 09:39:53 PM
Leon N. Leon N. is offline
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Default Re: Ground lugs

I hope someone on the Stak can provide the factory build sheet for your plant. I think all they need is what you stated earlier: Mod. 7.52B, serial 0173550945
JB 3CR 8389T.

Would be curious to decode 8389? That ID's the optional equipment as it left the factory. The build sheet will list the control wiring diagram which may also be on your JB nameplate. I'll bet a copy can be found in 967-500.

Cheers
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  #38  
Old 08-18-2017, 10:06:17 PM
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Default Re: Ground lugs

It had remote start. There was a button up by the driver on the sidewall where your arm rests... would that be optional equipment?
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  #39  
Old 08-18-2017, 10:29:29 PM
Leon N. Leon N. is offline
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Default Re: Ground lugs

I doubt it. These plants are designed for remote start. All one has to do is run three wires to a on/off switch. But you never know, cuz there could be variations. Maybe someone else can chime in on that.
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  #40  
Old 08-19-2017, 06:58:40 AM
Leon N. Leon N. is offline
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Default Re: Ground lugs

To clarify my reference to a remote start/stop switch. See Figure 1-5, page 5 in 967-320. The remote start/stop switch is a single pole spring loaded center off style. I suspect that is what the motor home used or some variation ther of.
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