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Onan 7.5 JB

Rupe53

Registered
First off... Merry Christmas to everyone.

Now that I have a few things to work with my next move will be to take a gander at that other cap for RF noise reduction. (wired to the + coil) and see if I can get my head into the control box area. The weather plays a big part here in Ct. Supposed to be in the 40s for the next week or so but I will have to dance with some rain showers, which is not always conducive to brain function when sorting out problems. Will keep you posted on my progress!

BTW, anyone have a clue as to the build date on this unit? (numbers in the first post)
 

Jim McIntyre

Subscriber
Last Subscription Date
07/10/2019
Not everyone is in agreement on the great Magnismoker vs. YD generator debate. Me? I'd take a YD any day, because it's easily repairable. At 30 years old, you might want to learn how to replace the YD voltage regulator in your set, should the need arise. (Which, admittedly is likely...)
 

Leon N.

Registered
Jim, judging from comments on the Stak I would say the magneciter has a greater MTBF (mean time between failure) than the YD design.
 

Combustor

Registered
Sounds like similar ign system to the old CCK's. Was given one of these with a dead coil, so replaced the sick condenser and found a pair of ballast type automotive coils, strapped one on each side and wired the primaries in series. Started and ran fine and the coils stayed cool. Looks a bit untidy but coils are easy to find and have run this old Lincoln welder quite a few hours with no drama. Regards,
Combustor.
 

Rupe53

Registered
Haven't been back to the job recently but did stumble on this while working in the parts room last week. Both have the same part number! Wondering if this change in production is the answer to my issue?

As you recall, the points with the white nylon (top) get hot enough to distort and close the gap. The points on the bottom have a different type of fibrous material and will probably take the heat better. What bothers me is I can't recall which ones I have been using over the years. To make matters worse is the fact that if you are on a job and made this change in the field you would likely not run the unit more than a few minutes and never discover there was an issue till a customer called back. (probably when the light are out)

Anyway, I need to make time to go over a few pointers made here on the board and then I will report back.onan points.JPG
 

Rupe53

Registered
Sounds like similar ign system to the old CCK's. Was given one of these with a dead coil, so replaced the sick condenser and found a pair of ballast type automotive coils, strapped one on each side and wired the primaries in series. Started and ran fine and the coils stayed cool. Looks a bit untidy but coils are easy to find and have run this old Lincoln welder quite a few hours with no drama. Regards,
Combustor.
Just a note to say I have run across this situation where parts are no longer available, like the coil for the early 7.5JB. The later coil has 2 high voltage towers but will not fit in the same space so I fashioned a strap and screwed the coil to the outside of the fan housing, then extended the primary wires, and used new OEM plug wires, which are just long enough. Had to trim the sheet metal and add some poly-loom to protect the wires when I put the outside cover back on. This was over 2 years ago at an off-grid cabin so I'm sure I would have heard something by now if the thing failed to run. This is what I may do with my problem child if I don't find anything else. The newer coil is 12 volt (instead of 6) but seems to have no issues running with reduced voltage. Spark is still blue and snappy.
 

Rupe53

Registered
So, had a brief break in the weather today. Rain subsided and temps headed close to 50 so got up close and personal with the old JB unit for an hour plus. Was in there long enough to change the points again (non plastic points this time) and do some tracing of wires. I found one of the diodes up near the pilot solenoid but with the cabinet in place all I could do was verify it's direction was correct. I could not find the other from my limited vantage point. There's just no room to see what you are doing with two hands and meter leads up there while the cabinet top is in place. Also went in with a meter and test light to find wire 10 has battery voltage going to the coil but doesn't cut back once the unit starts. At that point the wind had kicked up and the temp had dropped a good 10 degrees, plus some snow flurries. It was time to call it quits for the afternoon. Also talked with the old guy (well, 10 years older than me) who installed the updated relay kit. I'm starting to question the way he did the wiring so will have to dig up the cut sheet for the relay kit and spend another afternoon with the cabinet off when the weather breaks again.
 

Doug Reed

Registered
Hi Rupe53.
How about a quick check with your voltmeter. Read the voltage from the ignition coil + (plus) terminal to engine frame or battery ground when the points are open, then again while they are closed. The engine must not be running for this, all we want it to read the voltage across the coil.

The issue is why do the points melt if the ballast resistor is doing its job to limit the current through the ignition coil? As Leon said, when the generator is running, the voltage at the coil + (plus) terminal will be 12 volts when the points are open and drop to 6 volts when the points are closed. If this is happening, the current in the ignition coil will be limited to 3 or 4 amps, which is normal.

But in your last post you mentioned "wire 10 has battery voltage going to the coil but doesn't cut back once the unit starts." The problem is your voltmeter might not see the difference because the voltage is changing too rapidly while the engine is running. That is why I suggest testing the voltage while the engine is stopped.

Just connect your voltmeter from the coil + (plus) side to engine or battery ground. Then manipulate the points until they open. Worst case, loosen the points so you can push them back and forth so you can see them open and close. Points open should read 12 volts (battery voltage), points closed should read 6 volts (about half battery voltage) if the ballast resistor is doing its job.

However, if the ignition coil + terminal reads 12 volts while the points are closed, that means wire 10 is probably connected wrong and is shorting out the ballast resistor. That means current is way beyond 3-4 amps and you have points problems. The way I remember it, wire 10 is only supposed to be connected to the battery while the START switch is engaged. As soon as the engine is running, wire 10 should be disconnected from battery voltage when the START switch is released. I was told this was done so the coil would get an extra "kick" during starting because the battery voltage was likely to be low.

You can run this same test by putting the voltmeter across the coil + and - connections but when points are open you will read zero volts across the coil. When the points are closed you will read 6 volts if the ballast resistor is working, or 12 volts if wire 10 is shorting out the ballast resistor. These readings are different because of where the meter is connected, i.e. reading across the coil only or reading across the coil AND points. The numbers are different but the test results are the same.

If you are lucky this will point to an error in wiring wire 10. You mentioned the previous owner installed a relay update kit. Maybe he moved a wire and got it wrong. It may be easier to test than it is to trace the wire and fix the connection, if that is the problem....

Good luck!

Doug.
 

Ray Lynch

Subscriber
Last Subscription Date
08/12/2019
So, had a brief break in the weather today. Rain subsided and temps headed close to 50 so got up close and personal with the old JB unit for an hour plus. Was in there long enough to change the points again (non plastic points this time) and do some tracing of wires. I found one of the diodes up near the pilot solenoid but with the cabinet in place all I could do was verify it's direction was correct. I could not find the other from my limited vantage point. There's just no room to see what you are doing with two hands and meter leads up there while the cabinet top is in place. Also went in with a meter and test light to find wire 10 has battery voltage going to the coil but doesn't cut back once the unit starts. At that point the wind had kicked up and the temp had dropped a good 10 degrees, plus some snow flurries. It was time to call it quits for the afternoon. Also talked with the old guy (well, 10 years older than me) who installed the updated relay kit. I'm starting to question the way he did the wiring so will have to dig up the cut sheet for the relay kit and spend another afternoon with the cabinet off when the weather breaks again.
As previously mentioned, check by testing the diode installed between relays K11 and K12. This diode prevents 12 volt backfeed thru the I terminal on K11 after the set starts.
Ray
 

Rupe53

Registered
Good advice, but if you have ever worked on one of these with the cabinet in place you can see why I had no easy access to the various testing points inside the control area. Snow and high winds play hell with concentration while trying to do this outdoors in February. This will have to wait for another day.

As for checking the coil voltage without the engine running, it's not easily done because we are in the start / crank mode. I suppose I could remove more of the cabinet and disable the starter for the test but if I have to take the cabinet off I might as well retrace the control wiring up top first. (and also check those diodes with a meter)

From what I can tell, I have it narrowed down to the power switching between #4 (run feed) and #10 (crank feed) wires. Perhaps the guy who did the relay update got them crossed inside the control area where I can't see with the cabinet in place? Perhaps one of those diodes is no good? Either way I can't do further testing with the cabinet in place. The good news is I have another JB in a shed on the same property that I can use to compare notes. It's not in service and has not had the relay update so it's virgin territory. OTOH, it's surrounded with off-season storage so still have to wait for access. Winter in New England is like that.
 

len k

Subscriber
Last Subscription Date
12/12/2018
I'ld measure primary current thru spark coil while running. (See if it's shorted internally, and drawing more amps). And measure with ohmmeter that coil is not shorted to frame (ground), need to disconnect primary coil wires for this.

Also measure it's primary DC resistance, compare to spec.

Apples and oranges but......When I replace spark coil in my 60's Jeep yard snow plow I kept adding primary series resistors till spark length was too short (in my opinion) then reduced resistance a little. I used spark gap of ~ 3/4 inch for testing. Think while running I ended up with coil drawing average of ~ 2-3 ? amps at 12VDC ( analog DC amp meter).

Spark coil in my JB spec AA puts out a long spark ( has to jump 2 plug gaps) ,,,,,,,, IIRC mine will jump ~1.25 inch gap ( maybe 1.5 inch??) with factory series resistor and 12VDC bat
 
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Rupe53

Registered
I like the idea of doing an amp draw test on the coil and I have a couple of spares so I can compare. I know it's not shorted to ground, for sure. I will go back to that if I don't find other issues as mentioned earlier. (diode or relay wiring)

Will refresh my memory on this thread in a few weeks when I get some breathing room. I was supposed to retire today and there was too much to do so maybe I can retire tomorrow? My wife is also having a knee replacement on Wed so I wont have the flexibility to play with this unit for at least several weeks... and then there's the weather, if you follow my drift. For all I Know it will be Easter before I get time! (and decent weather at the same time)
 

Rupe53

Registered
Finally found the old points in question while cleaning out my work truck.

The top shot is what I posted a few weeks ago showing the difference between two sets of points made with various material, both OEM and the same part number. (go figure)

The lower shot is what failed and I recently took out. Notice how the nylon has melted enough for the spring tension to misalign the contacts, and also close up the gap while running.

Still wont get back to the job for a bit for further testing, but I figured some of you would be curious.
 

Attachments

Leon N.

Registered
Finally found the old points in question while cleaning out my work truck.

The top shot is what I posted a few weeks ago showing the difference between two sets of points made with various material, both OEM and the same part number. (go figure)

The lower shot is what failed and I recently took out. Notice how the nylon has melted enough for the spring tension to misalign the contacts, and also close up the gap while running.

Still wont get back to the job for a bit for further testing, but I figured some of you would be curious.
I do not understand why this guy just doesn’t get the correct wiring diagram and trace out the proper connectivity and components in his JB ignition system? If the coil’s primary is shorted then the voltage dropping resistor will probably over heat if the points are closed with the machine stopped. Something else must be amiss in this case? The ignition circuitry in these old machines is not very sophisticated, however, “every” component was selected for what could be multiple reasons. That so called dropping or ballast resistor is a good example.

Folks who purchase these old machines which have been previously tinkered with should first get the proper Onan data like the control wiring diagram and start from there.
 

Rupe53

Registered
I haven't gotten back to it because it's February in New England and it's under a deck in the dark with a full cabinet blocking access to the control head area. As I stated earlier in this thread, I have made multiple attempts to sneak in there as weather (and free time) permitted. Basically the cabinet needs to come off when I have enough time to sort it out plus put it back together in the same day. If the groundhog was correct, that may be fairly soon.

In the meanwhile, I refurbished a liquid cooled Onan 15Kw (Lister engine) for the guy's main house. When the weather breaks we'll be removing a 35 year old Generac (late 80s Mazda engine) and pouring a larger slab for the replacement.
 
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