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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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