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Onan Generators

Onan JB Thermomagnetic Choke


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  #1  
Old 08-13-2017, 10:08:18 PM
JoeE. JoeE. is offline
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Default Onan JB Thermomagnetic Choke

Just got this unit. Hadn't run in a long time.

Mod. 7.52B, serial 0173550945

JB 3CR 8389T.

Moving it, one of the connections to the coil on the thermomagnetic choke got broken off.

I removed it and got it reattached and soldered the wire back to the lug.

I made a hasty drawing of the wire connections as they appeared before I removed the coil, but I find it was not detailed enough now.

As you can see by the drawing, there appear to be 4 connections on the body of the coil itself. After disassembly, I determine that only two of the connections actually go to the coil winding itself.
The other two have no wire connection to the coil itself and just connect whatever two wires you connect to it, to themselves. I have highlighted them green on the drawing. The actual coil connections are indicated by red lines.

Anyhow... I'm not sure I got the wires all back on the lugs they came off of.

One from the bimetal strip is labeled H and one G. None of the other wires that came off it are labeled. Two wires had covers over the female lugs and the others are bare lugs.

Neglected to take photos of the set, but can do that later.

I just can't envision the path the voltage takes in this at startup or else I may be able to figure it out myself.

Don't think the machine has low oil cutoff protection.... there is no pressure switch down at the oil filter housing.
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Old 08-13-2017, 10:20:26 PM
JohnnyC JohnnyC is offline
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Default Re: Onan JB thermomagnetic choke

Attached are 2 pictures that you forgot to take. I hope they help.

JohnnyC
New Jersey
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  #3  
Old 08-13-2017, 11:55:55 PM
JoeE. JoeE. is offline
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Default

Thanks.
That's the same animal. The two wires that go on the other lugs come from up in the control box, or whatever it's called.

Right now, the choke butterfly is seized in the housing and won't move, so I'm working on that... then I'll see how the entire thing works.

This thing needs some TLC... the external wiring has been cobbled together.. I'm just plodding along trying to decipher what they've done.

It was built into a mobile office .. like a mobile home... and powered some rooftop air conditioners.

I disturbed the adjusting linkage on the governor... not knowing what I was doing trying to get the vacuflo shroud off the front of the engine... have to get all that cleaned up and reassembled... hopefully I didn't mess up too much.

Anyhow... babysteps.
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Old 08-14-2017, 12:51:44 AM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan JB thermomagnetic choke

All the manuals you need are here:

http://www.twinslan.net/~n0nas/manuals/onan/

Your model is a 7.5JB 3CR/8389T

I'm pretty sure it was built January of 1973 from your posted serial #
I also believe it might be a magniciter generator end. If so there are precautions you need to take to make sure the engine is running @ 1,800 RPM or it could let out the magic smoke! But that's sounds like it's a little further down the road for you!

The smart guys are here to help!
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Old 08-14-2017, 01:07:12 AM
len k len k is online now
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Default Re: Onan JB thermomagnetic choke

When terminals on my choke broke out of the weak paper I silicone rubbered them back in place and physically secured them with tie wraps. I didn't put much silicone rubber on it since that would interfere with coil cooling, The 1.7 ohm coil draws a lot of amps (~ 5-7) while starter is cranking, so that's a high ~ 50-80 watts of power dissipation

Last edited by len k; 08-14-2017 at 11:16:20 AM.
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Old 08-14-2017, 05:08:26 PM
JoeE. JoeE. is offline
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Default Re: Onan JB thermomagnetic choke

Alrighty then.

Read thru some of the manuals I'd found online. Once in a while it'd say something like "produced after series P" or something like that.

I don't understand how a guy is to discern which production model letter series he may have,from the assortment of letters in the serial and model number.

In the control box atop the generator housing, it still has electrical relays and all the original stuff up there. On youtube, I've seen some that I take it that have more modern solid state equipment mounted up there.

Is that possible with this age of machine, or are only the newer versions of Onans capable of being fitted with that new stuff?

One other question.. . how come we (or maybe it's just me) can't "edit" my posts?
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Old 08-14-2017, 05:44:56 PM
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Default Re: Onan JB thermomagnetic choke

You have to be a member for 20 dollars to be able to edit
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Old 08-14-2017, 06:34:04 PM
grif grif is offline
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Default Re: Onan JB thermomagnetic choke

Quote:
Originally Posted by JoeE. View Post

I don't understand how a guy is to discern which production model letter series he may have,from the assortment of letters in the serial and model number.
That's why the list exists... and gives the smart guys breathing room from us rookies, and we get to glean their experience with a few nice comments and thanks as payment, with just a few keystrokes!!! I'd make a Ford/Onan comment about daily changes in production but somebody would take me to task ;-)

And it gives people like me affordable reading material.
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Old 08-14-2017, 06:52:43 PM
Max Thompson Max Thompson is offline
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Default Re: Onan JB thermomagnetic choke

Quote:
Originally Posted by JoeE. View Post
Once in a while it'd say something like "produced after series P" or something like that.

I don't understand how a guy is to discern which production model letter series he may have,from the assortment of letters in the serial and model number.

In the control box atop the generator housing, it still has electrical relays and all the original stuff up there. On youtube, I've seen some that I take it that have more modern solid state equipment mounted up there.
The last letter in the model number, in your case "T" is your spec. letter.

Your set has a magneciter to regulate voltage, Leon can expand more on that subject and you can read about it in one of the sickies at the top of this forum. The newer J sets have an electronic voltage regulator on top of the control box.
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Old 08-14-2017, 07:15:47 PM
len k len k is online now
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Default Re: Onan JB thermomagnetic choke

On some gens that came with an electronic voltage regulator on top of the control box owners have sometimes replaced them with a cheaper generic SX460 reg.
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Old 08-14-2017, 09:08:07 PM
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Default Re: Onan JB thermomagnetic choke

Be real careful with the choke mechanism because the parts are hard to find and when found, expensive.
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Old 08-14-2017, 10:01:56 PM
Leon N. Leon N. is offline
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Default Re: Onan JB thermomagnetic choke

Hello JoeE!

Don't believe Kpack. He must have been drinking something?

BTW! Make sure you comply with the generator output leads grounding scheme as printed on the inside of your control box cover. You have a gem here the likes of which will,never be built again. If taken care of it will outlast you.

A question please. Is your JB strictly gasoline-only or does it have a tri-fuel,carburetor?

WRT no oil pressure switch. Is there a pipe plug where the switch would go? If so, you could add one but you would have to add additional components inside the control box. A related question: Is there a hole probably covered with a plug on the right side of the front panel just below where the hand-crank toggle switch would be?

Do not try to start this puppy until you make sure that the generator lead T-2 is tied to the chassis per wiring diagram printed on the inside of the control box cover.

How about some pictures. Also, temporarily remove the generator end bell cover (two bolts)
and take a picture for us or at least indicate the condition of the Magneciter coils. Thanks!
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Old 08-15-2017, 02:38:28 AM
KPack KPack is offline
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Default Re: Onan JB thermomagnetic choke

Quote:
Originally Posted by Leon N. View Post
Hello JoeE!

Don't believe Kpack. He must have been drinking something?

BTW! Make sure you comply with the generator output leads grounding scheme as printed on the inside of your control box cover. You have a gem here the likes of which will,never be built again. If taken care of it will outlast you.

A question please. Is your JB strictly gasoline-only or does it have a tri-fuel,carburetor?

WRT no oil pressure switch. Is there a pipe plug where the switch would go? If so, you could add one but you would have to add additional components inside the control box. A related question: Is there a hole probably covered with a plug on the right side of the front panel just below where the hand-crank toggle switch would be?

Do not try to start this puppy until you make sure that the generator lead T-2 is tied to the chassis per wiring diagram printed on the inside of the control box cover.

How about some pictures. Also, temporarily remove the generator end bell cover (two bolts)
and take a picture for us or at least indicate the condition of the Magneciter coils. Thanks!
I thought you had to be a subscriber to edit your post.
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Old 08-15-2017, 09:09:19 AM
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Default Re: Onan JB thermomagnetic choke

Kpak is the new salesman for the site. You know how salesman are.

I think you just have to post a few times to edit.

Got your message Kpak. Where is the rain they promised.
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Old 08-15-2017, 09:32:02 AM
Wayne 440 Wayne 440 is online now
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Default Re: Onan JB thermomagnetic choke

Quote:
Originally Posted by KPack View Post
I thought you had to be a subscriber to edit your post.
As far as I know, you are correct.
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Old 08-15-2017, 10:58:46 AM
Jim Marcozzi Jim Marcozzi is offline
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Default Re: Onan JB thermomagnetic choke

What exactly does the coil do on these chokes? Most chokes I've seen just have the heater coil.
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Old 08-15-2017, 01:04:52 PM
len k len k is online now
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Default Re: Onan JB thermomagnetic choke

I think it closes the choke butterfly while cranking. Basically makes more vacuum behind choke butterfly so more gasoline gets sucked up dip tube from carb bowl. Making the air/fuel mix richer. Unusual design.
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Old 08-17-2017, 08:43:57 PM
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Here are four pictures.

Single fuel carburetor....
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Old 08-17-2017, 08:47:20 PM
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One of those shots shows the linkage for the governor system. Not familiar with the machine, I didn't understand what I was unscrewing when I took loose a rather long nut which protruded out the front of the fan housing when I was trying to get that assembly off.
The shot is looking inside the shroud.
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Old 08-17-2017, 08:50:22 PM
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Default Re: Onan JB Thermomagnetic Choke

Would be nice to have the edit feature available, that way I could go back and edit the text with the pictures to explain what I was taking pictures of. Would save a lot of unnecessary replies....
Doesn't make a lot of sense to have THAT particular feature unavailable to certain members. How about swap that for "Smilies are not available"
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