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Onan Generators Restoring, operating and maintaining vintage Onan generators.

Onan Generators

Onan 7.5JB-3CR/9804T - Welcome 'red' to the family


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  #51  
Old 03-17-2016, 01:55:55 AM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

So took a look at the head tonight. They're all coked up but nothing that can't be dealt with via the soda blaster and ultrasonic.

Noted a bit of rust on the face of intake valve on pto side, which matches up to the cylinder with the rust. Again, not too bad and the seats are clean & shiny.

https://www.dropbox.com/s/no2orco3bp...Oface.jpg?dl=0

https://www.dropbox.com/s/mmbu5bnr4v...erust.jpg?dl=0

Last edited by BStrutton; 03-17-2016 at 01:56:33 AM. Reason: can't freakin spell
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  #52  
Old 03-17-2016, 02:31:43 AM
Vanman Vanman is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Wow, cleaning up nicely so far. It must be noisy inside that enclosure. The generator is wearing a set of gun mufflers!! Nice to see that they are tan in color and not brown or black

Keith
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  #53  
Old 04-06-2016, 08:35:05 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

https://flic.kr/p/FWuQtn


Update: So I've got the control box out & Thur my relays from Allied should be here. Many of the contacts esp on the open frame relay were so bad off they disintegrate if you move em.

I want to make sure I am clear on this before I start. I've seen a lot of the fine work that Max has done but I haven't seen one that exactly matches my rig. Some seem to have 3 terminal strips instead of my 2 or don't have a solenoid pictured for example.. So my questions are:

1) Attached pic of my control box tray. Annotated relays with what I 'think' they are, nomenclature-wise according to the schematic. I'm I correct on these?

2) In pics of the conversions by Max where no solenoid is shown but two plug-able cube relays are shown, is the intent there that the 12v of the two is actually replacing the legacy solenoid-looking relay? If not, what does the 12v relay do?

3) Best method for getting those grossly rusted fasteners off the emergency relay with minimal risk to the relay since they seem pretty hard to come by?

4) How about those tiny screws that retain the switches on the panel. They surprised me. Not at all corroded like the others but won't seem to budge. I've got some penetrating oil soaking on them currently.

Finally thanks for the recommendations so far. I've got my 18 awg MTW, loom & soon relays ready to go to rewire that whole box. Hope to have the electronics ready to go for the weekend. Intend to hone the top of that bad cylinder in place Sat morning, put the head back on & try to fire it briefly just to check output. If it produces then I'll put together a plan to disassemble & refurb the beast..
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  #54  
Old 04-06-2016, 10:12:20 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Your control box is set up for remote start, that is the reason for the extra relay which you have marked as “emergency relay”. The LOP/OS emergency relay is the top one in the front of the control panel. The big black one below it I believe is the crank limiter. Unless you plan to use an auto start function as with a transfer switch, you can do away with the other relay & the crank limiter would not be necessary.

The 12-V cube relay in my box replaces the starter solenoid pictured in the lower right corner of your box.

I used an extra terminal strip on one of my first conversions to get some of the wires off the small screws on the relay base. I will try to pull your wiring diagram tomorrow & maybe offer some guidance, I may also have some pictures of another box without the extra terminal strip. It is way past my bed time tonight.
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Old 04-06-2016, 10:25:30 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Thanks Max! That makes so much more sense and now that you said that I can see the windings on the emergency relay which now I get is 'panel' mounted which I couldn't tell from the schematics.
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Old 04-07-2016, 08:01:54 AM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

I hope I have attached a picture of another box with only the 6 volt relay having been replaced & terminal strips being original.

I am having another fight with my computer, it has not worked to my satisfaction since it was built, sure wish I could "open it up" & see how it works like we do these old Onans. I can not pull the build sheet for this set to get the diagram #, though I am pretty sure it is the same as the one I use with the relay conversion pictures.

If you are in the "testing" phase, I would not worry about rewiring the box yet. I would "hot wire" the engine to start it. Battery + to the coil with an alligator clip or switch in the wire for quick disconnection if needed.
Another wire with a push button switch in line (you can just use a wire jumper but the push button is handy) to the starter shift solenoid to engage it & it should start. All other control wiring to the box should be disconnected or taped off during this process. You will have to keep an eye on your oil pressure since all safeties will be bypassed.
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Old 04-07-2016, 11:13:29 AM
nothingbutdarts nothingbutdarts is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Max, the picture is there and the control box is looking good!
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  #58  
Old 04-07-2016, 09:19:04 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Found an hour tonight to get out in the garage & do some soda blasting. A few pics of head / valves. The soda does a nice job, quickly. The one valve pic is top half blasted / bottom half not.
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  #59  
Old 04-10-2016, 07:07:52 PM
BStrutton BStrutton is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

Alright then.. How about an oil pressure question?

I've got the control box all apart right now but Max suggested I could hotwire it just to test start & making power. Taped off all connectors for testing.

I got the head & valves all cleaned up and put the head back on yesterday and adjusted valves.

Left the valve cover off so I could verify oil flow. New filter & fill.

Fuel off. No hotwire (yet) to coil. Just connect battery & jumper starter.

Unit is cranking over good. I think it makes sense that I should be able to verify oil pressure prior to introducing fuel and spark.

However, I am not seeing pressure on the gauge and I also am not getting oil flow up in the head.

I've cranked it 3-4 bursts of 15 seconds or so with breaks in between.

How much should one have to crank this to see pressure or (if gauge is bad) oil flowing in the head?

Also, given the rust that was living in the PTO cylinder before clean up, I was thinking of checking the compression to get an idea on the rings. But what would be a 'cold' value that a guy would like to see prior to getting this thing started?

Other thoughts? Trying to take my time & methodical approach given the state this thing arrived in. Not easy for me as I'm an impatient SOB

Thanks!
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Old 04-10-2016, 07:22:20 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5JB-3CR/9804T - Welcome 'red' to the family

You will usually see oil pressure starting to build in 5 seconds of cranking, not sure if it would make it to the valve covers in that time. Double check the gauge to be sure it is telling the truth. I would also squirt some oil into each cylinder before cranking too much.
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