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Onan 7.5JB Centrifugal switch HELP!


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  #1  
Old 10-28-2013, 11:33:19 PM
gov2mod gov2mod is offline
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Default Onan 7.5JB Centrifugal switch HELP!

I've got an Onan 7.5JB 3CR/8388AA generator with battery ignition. It was made in May 1977. When starting it, as it is cranking it tries to start but stops as soon as I let go of the start switch. If, when it's trying to start, I push the points shut on the centrifugal switch it will go ahead and start and will run fine until I shut it off. I understand the points on the centrifugal switch are supposed to shut at 900 rpm but I would guess the engine would already have to be running to reach 900 rpm. Any ideas on why I have this problem? Thanks for any help.
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Old 10-29-2013, 06:47:00 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

The points are adjustable. I believe the gap is 0.025" but someone will be along to correct that number if it is incorrect. (I'm working by memory as I dash out the door...)

John
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Old 10-29-2013, 07:03:19 AM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Quote:
Originally Posted by J. McNugget View Post
The points are adjustable. I believe the gap is 0.025" but someone will be along to correct that number if it is incorrect. (I'm working by memory as I dash out the door...)John
Close: .020. Like the points, be sure the contacts are clean. Has the plate assembly been removed lately, or did this just start out of "the blue"?
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Old 10-29-2013, 07:49:21 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Ran across this before. If you re-adjust point gap, and it works for a while but problem returns, I'd suggest you replace the soft plunger seal and the plunger that operate the point contact. I've had units where cleaning the plunger rod doesn't even help in the long run. Don't know why. But replacing them has always worked.
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Old 10-29-2013, 12:37:48 PM
gov2mod gov2mod is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

This is how the generator was when I bought it. The points are adjusted to the correct .020. If I push the points closed while cranking it starts and runs fine. I just what is supposed to pull these points closed so it can start.
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Old 10-29-2013, 07:27:25 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

On the back side of the centrifugal switch plate, there is a counter weight that rotates & causes a “sleeve” to move toward the front of the engine, allowing the nylon plunger that activates the switch to move back into the housing, thus closing the points.

If the nylon plunger is damaged or as Eric mentioned, it sticks, the plunger will not follow the “sleeve” back.

It sounds like you could use a parts manual, so you can see how it is put together. An e-mail address will get you this as well as service manuals if you would like.
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Old 10-29-2013, 07:54:31 PM
gov2mod gov2mod is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

It could just be gummed up from sitting for several years, I'll have to look at it. I've got a manual but it shows little of this switch. Email is gov1mod@aol.com Thanks for any help.
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Old 10-29-2013, 08:11:13 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

You should have a box full.
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Old 10-29-2013, 09:31:01 PM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Thanks, I really appreciate it.
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Old 10-30-2013, 06:34:04 PM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

I removed the switch, the diaphram and plunger, all looked great. The rubber was soft and pliable and the plunger didn't look worn or broken.
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Old 10-30-2013, 07:06:12 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Does the nylon plunger move back when the engine starts & comes up to speed?
You might try it with the rubber boot removed, so you can see the plunger better. It may leak a little oil, but should not be much.
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Old 10-30-2013, 08:44:44 PM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

I'll have to look in the next couple of days. I had surgery on my right hand last week and the stitches come out soon so I hope to get greasy again working on my 1001 projects.
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Old 08-12-2017, 11:47:29 PM
silverlake82 silverlake82 is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

I am new to this so forgive me if I post this wrong. I have the same issue with the centrifugal disconnect switch. Having difficulty finding a drawing or part #.
Also, is there an option to bypass this switch another way. Thanks.
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Old 08-12-2017, 11:56:23 PM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Hi SilverLake82 and welcome aboard! To get the fastest and most accurate responses, please start a new thread (topic listing) and include the full model and serial number along with the problems details (more details the better). Pictures are a great help too in many cases. You will have fast responses from the smart guys here (I'm only a smart azz ).

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Old 08-13-2017, 01:00:30 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Starting your Onan isn't like you would start a car. I was always told let off of the starter position as soon as the car engine fired up or "You'll RUIN the starter!"
On the generators with the centrifugal points, you have to hold the start button until the engine picks up speed and the points will cancel the starter. It is a hard thing to get used to but that seems the way they were designed. Having your engine tuned right so it starts and comes up to RPM will make it seem like you don't have to hold the start button so long.
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Old 08-13-2017, 07:46:08 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

If I am remembering correctly, your problem may not be with the centrifugal switch, but the ignition start relay inside the control box. What is the condition of your control box? Maybe just a dirty ignition relay contactor. Max, check me out on this, am I correct? I have had similar problem, that is, it would fire up and then die when the start switch was released. I merely sprayed contact cleaner on that relay and wha la good for a few more years.
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Old 08-13-2017, 08:17:18 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Leon, he says it stays running if he hold the switch closed.
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Old 08-13-2017, 10:26:23 AM
Leon N. Leon N. is offline
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Default Re: Onan 7.5JB Centrifugal switch HELP!

Handy hiker Yes that is because he is bypassing the start ignition relay. Once the plant reaches 900 rpm, the centrifugal switch closes and this starts off another chain of events, namely, it energizes the ignition relay which in turn disconnects the starting motor and the choke (if used). Then the operator releases the start switch which de energizes the start ignition relay and start solenoid.

So, I think he should study the Onan cycle diagram found on page 10 in the JB owners manual, 967-320 as well as the appropriate wiring diagram for his particular model. Good Luck.
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Old 08-13-2017, 11:16:15 AM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

As Leon posted. On starting the run system is bypassed. When the switch closes it becomes another system. Run relay in control box takes over.
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Old 08-13-2017, 02:16:17 PM
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Default Re: Onan 7.5JB Centrifugal switch HELP!

I guess i just don't understand how it could be something other then the points if closing the points manually make it run and not closing the points make it die as soom as the start button is released.
If I understand the system right, holding the start button pypasses the points and lets it start. Then when runningcthe points should close and continue to run and also disengage the starter. What am I missing?
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