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Onan 7.5 JB-3CR running issue


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  #81  
Old 04-21-2017, 12:03:30 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-3CR running issue

Moheltunen re your question in post 76. I occasionally have the same problem and I have to spray some contact cleaner on the ignition start relay a shown in your post 72.
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  #82  
Old 04-21-2017, 12:06:35 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

What do you use for contact cleaner?

I've heard good things about De-oxit, but haven't used it yet.
De-oxit makes 4 different types, one type is for metal contacts.
Relay likely has non-ferrous contacts (so likely DeoxIT® Gold G-Series)

Another, likley DeoxIT® Fader F-Series, is for volume controls , conductive plastic or wire wound types (JB/JC voltage reg pots),
http://store.caig.com/s.nl/it.I/id.9...&category=-117

Last edited by len k; 04-21-2017 at 12:39:27 PM.
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  #83  
Old 04-21-2017, 12:16:04 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Thanks Len. Your explanation makes a lot of sense to me . I will check out this circuit and contacts and report back. The intermittent starting to me seems very indicative of a poor contact.
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  #84  
Old 04-21-2017, 12:21:07 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

On my JB paper schematic I color code the coil and contacts of each relay with a different color highlighter. Makes it easier to see which contacts each relay coil controls.

Also I mark the schematic's constantly live +12V bat lines with red marker, and the fused constantly live +12V line with orange. Makes it a lot easier to see how power flows.

Good to print out a LARGE schematic, I use the "poster" feature of the print menu, so you can use a regular printer. Then cut and tape the pages into a large print , ~ 2ft x 3ft, that way you can READ it.

Last edited by len k; 04-21-2017 at 12:33:53 PM.
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  #85  
Old 04-21-2017, 01:18:07 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

I cleaned the contact points on k12 and followed with a electrical cleaner bath. It's clean and shiny now but no change on starting. Set stays running as long as the start switch is held. I disconnected the voltage regulator and taped off the leads and re tested and results were the same. I disconnected the low oil shutoff and still no change. The battle continues.......
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  #86  
Old 04-21-2017, 01:57:23 PM
Leon N. Leon N. is offline
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Default Re: Onan 7.5 JB-3CR running issue

I always use CRC QD Electronic Cleaner. It is an aerosol which I can point directly on the ignition relay. Trouble is, my JB control box is still dirty due to a previous mouse infestation and if the plant sits unused and weather conditions are just right the de-energized open contacts get dirty and that will prevent the ignition relay from operating correctly - sometimes.

Now in this gentleman's case he may have "other" problems like the centrifugal switch, or a wiring error or bad connection , etc, etc. He should trace the circuits out using the correct control wiring diagram for his machine.

He should also be very familiar with how the start/stop control circuits work. Maybe the attached will help explain this.
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  #87  
Old 04-21-2017, 02:18:21 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

Leon.... could try dragging some ~ 360-400 grit sandpaper over the contacts to remove surface corrosion from mouse pee. IF rough have to start with ~ 180

Some relay contact have a thin coating like gold , but that's more for "signal" level currents. Being a power relay I'm guessing contacts are solid (no coating) possibly solid sliver, so I'ld think they are safe to sandpaper. Silver is used in "high" current applications, of course high depends on contact size. There are about 6 common contact materials.

I hear many people in the audio community raving about how good de-oxit is for fixing noisy volume control pots in stereos, I have to buy some for mine. I'll try some of their metal contact cleaner for my stereo's audio level switches, may be good too.
.

Last edited by len k; 04-21-2017 at 02:30:56 PM.
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  #88  
Old 04-21-2017, 02:46:22 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

an possible idea... Since I switched over to an 12V onan replacement coil would the resistor R11 or R12 need to be removed? I have removed resistor RT1 between points and coil like instructions indicated. Currently on startup coil (I believe) gets 12V for initial start, once relay K12 closes (via centrifugal switch) is the voltage reduced to 6V?

---------- Post added at 02:46:22 PM ---------- Previous post was at 02:34:04 PM ----------

http://www.partsfortechs.com/asapcar...rs-p-1347.html. This is the replacement coil I used.
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  #89  
Old 04-21-2017, 02:48:40 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

I don't have your schematic, I'm looking at 612-4793 schematic. It's for 7.5-15kw -3CR/1AA which I would assume is same as your spec AA JB.

From my schematic you must leave R11 in the circuit.
R11 reduces voltage to coil of K12 relay , and allows stop switch to kill current to that relay without frying stop switch or centrifugal switch. K12 has ~ 6V coil.

My schematic does not show an R12

But in general, if your switching to a 12V spark coil, from the original 6V coil, I'ld only remove the resistor RT1 between the points and spark coil.

While the idea is used on cars from the early 70's , I have not seen an Onan that supplies 12V to a ~9V spark coil for starting then supplies it thru resistor for ~ 9V while running.
(idea is that during engine cranking bat voltage at spark coil may drop to only ~9V, so cars used a 9V coil to have full spark voltage while cranking)
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  #90  
Old 04-21-2017, 02:54:23 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Len, i'm using schematic 612-4794. To the best of my knowledge thats the one for my set.
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  #91  
Old 04-21-2017, 02:59:05 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

Just thought of what your R12 might be, your gen may have a low oil pressure switch, my spec AA doesn't. As I recall that circuit also used an additional resistor and circuit breaker delay for oil switch circuit.

In either case if your using a 12V spark coil instead of a 6V coil, I'ld only remove the resistor RT1 between points and spark coil. It's purpose is to drop the extra 6V when using a 6V spark coil. Sounds like Onan had a warehouse full of 6V coils left over from the 60's they needed to use up.

---------------

Ok, I found your 612-4794 schematic.
Don't touch R11 or R12, if you remove them you'll fry stuff.
Only remove RT1 IF your using a 12V spark coil.

Last edited by len k; 04-21-2017 at 04:04:47 PM.
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  #92  
Old 04-21-2017, 07:06:18 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

I don't see where you have checked to see if you are getting power to the coil of K-12 when the centrifugal switch closes. If you are getting power to K-12, is the coil pulling the relay closed? (note that this is a 6-V relay coil)

These problems are why I have been replacing the old open relays with a cube style plug in relay. It is a cheap, easy cure for many problems, even if they have not occurred yet.
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  #93  
Old 04-21-2017, 07:24:17 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

It gets stranger. I checked all the circuits for continuity as well as 12v and everything checked out good. Tried to start it again and same result. I then hooked 12v to the positive side of the coil and it ran perfect (as anticipated). I then I hooked the jumper and for grins tried to start it again and voila .....it started perfect. I tried a couple more times and no problem , she fired right up. I'm confused but decided to move on . I then checked the hz to set the governer and it's reading 300 hz. So I checked the other leg and it showed around 118 hz. Now I know this is not possible because if so I'm sure the engine would have been in pieces on the floor (blown engine). Any thoughts on this or has someone seen this before?

---------- Post added at 07:24:17 PM ---------- Previous post was at 07:23:14 PM ----------

*unhooked the jumper.
My thumbs are far to big for typing on my phone
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  #94  
Old 04-21-2017, 07:34:10 PM
len k len k is offline
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Default Re: Onan 7.5 JB-3CR running issue

When testing for 12V I'ld use an ice pick test light. Voltmeters draw so little current, I've had one show full 12V thru a green HEAVILY corroded car connector that wouldn't even light a tiny dash light. You may have intermittent dirty connections. Mouse pee in control box should do that.

300 hz doesn't sound real, would be 9000 rpm, 5X normal.
What are you using for a hz meter?

118 hz........ 118 volts maybe???
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  #95  
Old 04-21-2017, 07:44:10 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Attached is a photo of the meter I used. Same meter when checking hz at an outlet in house reads 60 hz.
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  #96  
Old 04-21-2017, 07:53:22 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

During this testing I'm checking each leg separately. I checked the other leg and it reads 59 hz. I then adjusted the governor to 62.5 hz (no load) then checked voltage and it read 120.9 volts. The high hz reading is odd to me but seems to be running fine now and generating power.
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  #97  
Old 04-21-2017, 09:11:51 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 7.5 JB-3CR running issue

Quote:
Originally Posted by mheltunen View Post
I then hooked 12v to the positive side of the coil and it ran perfect (as anticipated).
When doing this you are bypassing the K-12 relay's function (as well as the safety shutdowns)
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  #98  
Old 04-21-2017, 09:22:46 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

K12 was only bypassed for one test fire (5 seconds). I then restarted the set without the jumper and all wiring in the factory positions and it fired right up and ran perfect. The testing of each leg was done while relay k12 was operational. Currently everything works perfect and set runs excellent. Initially I had spark plugs gapped to .020 and it seemed to miss occasionally so I re gapped them to .035 and the set ran very smooth with no vibration at all. Currently all that is left for me to do is find a door for the front and get a stand built and it's ready to put into operation.
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  #99  
Old 04-21-2017, 09:27:44 PM
Ray Lynch Ray Lynch is offline
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Default Re: Onan 7.5 JB-3CR running issue

mheltunen
Have you installed new points, plugs and condenser or are you using the originals that came when you got the set?
Ray
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  #100  
Old 04-21-2017, 09:33:23 PM
mheltunen mheltunen is offline
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Default Re: Onan 7.5 JB-3CR running issue

Ray , to the best of my knowledge the points and condenser are original. I did set both the pints and the switch to .020 but I am seeing some arcing at the points while the set is running.

---------- Post added at 09:33:23 PM ---------- Previous post was at 09:32:21 PM ----------

-new plugs though
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