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Staff member
Because the following message didn't work, new users can no longer post on this board. READ ONLY!

Do Not attempt to start new threads on this FAQ board. New FAQ threads will be placed here once approved by a moderator. POST HERE: https://www.smokstak.com/forum/forumdisplay.php?f=1

OTHER Electrical Boards on SMOKSTAK: https://www.smokstak.com/forum/forumdisplay.php?f=107

Please put "Onan" in your thread titles. Smokstak is made up of many shared bulletin boards and this helps our readers to quickly identify the subject material at hand before clicking a thread.

Also, we are quickly and regularly indexed by Google. Abbreviations and the lack of keywords do nothing to help other users to find valuable information. At a minimum, include manufacturer name and your model number along with the prime idea behind the thread in your title.

Check the F.A.Q. on our main generator board too: http://www.smokstak.com/forum/showthread.php?t=47155

When was my ONAN built?

Onan GenSet Model Numbers

Do I have a Magneciter?

Fuel Consumption

Smokstak and Enginads site search at the bottom left of EACH page!


Jim Rankin

Onan serial numbers include a build date (MMYYxxxxxxx)as the first 3 or 4 numerals or in some cases a letter for the month (A=Jan, B=Feb etc)and then 2 digits for the year (MYYxxxxxx).

Older sets built before the late 1960's have a letter or a . for the decade, so for example 104.xxxxx would have been manufactured in October 1944. If machine was ever returned for repair etc, the . was stamped over with A. B was 1950's, C 1960's. 124Cxxxxxxx = Dec of "64

Different series of generators changed date coding systems at different times, but there seems to have been a change from letters for the decade to all numbers in the 1960's. The letters for the month seem to have appeared in the 1970's on some of the smaller sets.

Sometimes an engine block casting date can help confirm a build date

Voltage code 15 = a 12 lead reconnectable set. the ones I have seen seem to have been late 60's early 70's.

Engine type K during the 50's - 70's was International Harvester

Here's a link to a post where a magneciter is pictured and some good discussion about them from some experts.

Pictures are on post #8 of

Discussion near the last postings of http://www.smokstak.com/forum/showthread.php?t=36647&highlight=magneciter

From what I have seen on this board, Magneciters replaced the old manual regulator (rheostat) and electro-mechanical finger regulators probably in the 1950's and were replaced by electronic regulators (VR21 board for example)in the late 1960's


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Onan GenSet Model Numbers

a Short Primer for decoding Onan GenSet Model Numbers.

aa.a bbbb-cd/eeeeff An Example 12.5MDJC-3R/9523AA

aa.a is Power Output in Kw (rough approximation)
bbbb is Series of Engine and General Identifier
M=Marine, R=Radiator Cooled, Nothing=Air Cooled
D=Diesel, Nothing=Carbureted (Gasoline, Propane, or NG)
JC= J Series Engine , C=Number of Cylinders, A=1, B/E = 2, C/F = 4, a's, B's, and C's, have a 3.25" Bore, E's and F's have a 3.5" Bore
-c is Voltage Code -1 120 Vac only 1Ph, -2 240 Vac 1 Ph only, -3C is 120/240 Vac 1 Ph 4 wire, -3U 120/240 Volt, 1 Ph, 3 Wire, PF 1, -4 120/208 Vac 3Ph Y, -4X 480 Vac 3Ph Y, -5D 120/240 Vac Delta 3Ph w/L1-L2 C-Tapped for L0, -8 127/220 Vac 3 Phase 4 wire (Wye), -15 12 Lead Reconnectable, -18, 12 Lead Reconnectable
____There is some Speculation that -15=Normal Stack Size, and -18=Extended Stack Size. This is UNCONFIRMED
____-53 120/240Vac 50Hz, -518 12 lead 50Hz 12 Lead Reconnectable, -9x 347/600 Vac 3P Y, -99x is Special Order (Consult Data Plate)
d is the Starting Method R=Remote Electric Start, E=Electric Start M=Manual Start
/eeee is the Options Codes (Build Number) "Too many to Reference"
ff is the Spec Code These designate Changes in Specifications from
_____Original Factory Design specs. For J Series Gensets anything
_____with a double letter would be a YD Genend, anything with a
_____single letter is a Magniciter Genend, as the Factory upgraded
_____to the YD Genend a Spec Code AA.

If others have later or additional Information PM or eMail Me and I will update this FAQ.

Bruce in alaska:crazy: :crazy: :crazy:
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In Memory Of
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Re: Frequently Asked Questions on Onan Generators

We have started building a few Onan pages on our site to compliment the Kohler information.

There is a 'decoding' page at:


Time is our enemy as usual, we have lots of data but little spare time to get it scanned and onto the site.

Most recently we have been doing European translations of our company website, and we also have more Kohler information part-finished.



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Re: Frequently Asked Questions on Onan Generators

Some of the questions I hear all the time are :

Question: when I push the start button my genset starts. when I release it the engine cuts off
Answer: Post you question in the Onan Forum, giving the complete Model and Serial Numbers
as there are MANY Onan Wiring Diagrams, for MANY different models, BUT our Resident SmartGuys
have the answers to get you going again.

Question: the generator sat a long time , I got the engine running but now will not produce electricity
Answer: Likely, your Genend needs "Flashing", and this procedure is different for each type genend.
Post you question in the Onan Forum, giving the complete Model and Serial Numbers, and one of
the Resident Smartguys can advise you on your particular unit.

Question: where can I get parts?
Answer: First place to look is your local Cummins/Onan Distributor's Parts Dept. then ask on the Onan forum.

Question: only 1 cylinder will fire
Answer: Usually a BAD Spark Plug, or Injector, for specifics, ost you question in the Onan Forum, giving the complete Model and Serial Numbers, and one of the Resident Smartguys can advise you on your particular unit.

Question: when I put a load on the set it does not speed up.
Answer: Most Gensets are Constant Speed type units. The Speed, (and Frequency) is fixed by the design of the Genend.
If your Governor isn't responding to added load, that usually is a Governor Problem. Post you question in the Onan Forum, giving the complete Model and Serial Numbers, and one of the Resident Smartguys can advise you on your particular unit.

Just a few FAQ I hear maybe some one could erase this & start a list :)
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Bill Geyer

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Re: Points for 6.5 NH-3CR

Ben Clark posted this info in another thread, and I thought it might be helpful here.
Standard Ignition part # S13-407, about $18.00

Jack Hottel

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Fuel Consumption

The fuel consumption question pops up from time to time. Here is the data for a 15.0JC at 60Hz, (1800 RPM):
The gasoline consumption in gallons per hour is:

Load: 0 1/4 2/4 3/4 4/4
GPH: 1.0 1.3 1.6 2.0 2.4

For natural gas with a heat value of 1000 Btu/cubic foot it will consume 255 cubic feet/hour at full load.

For propane with a heat value of 2500 Btu/cubic foot it will consume 110 cubic feet/hour at full load.

Maybe some others can chime in with other units.

Billy Shafer Comment:
Here's dry fuel consumption for one of the larger ones.

70kW with IH 549 V-8 engine manufactured in the 1960's and 1970's
full load-
Propane 2500 BTU/cubic ft. = 400 CF/hr (93.13MJ/cubic Meter = 11.33 cubic Meter/hr)

Natural gas 1000 btu/cubic foot = 1400 cubic feet/hr (37.25 MJ/cubic meter = 39.65 cubic meters/hr)

Onan tech Comment:
Rule of thumb for diesel fuel comsumption on diesel sets: .08 x KW= GPH, or .08GPH per KW.
when replacing onan points with oem onan points: onan oils them so they wont rust, esp on older stock. If you install new points and it wont run, wash off contacts with brake clean. best practice is too just do it before you put em in. This has messed me up before.
let it evaporate before starting unless you want your cap to fly into your neighbors yard.


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Magneciter Info

Hi All,

I have a thread going here with a bit of discussion, theory, diagrams and troubleshooting info for Magneciter gen sets. I kind of like them - miminal electronic 'stuff' to go bad. I can live with brushes - I've got two extra sets my son will probably inherit in 40 years...:)


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J series brush info

The part number for the brush holder is 212-1064. For the brushes it is 214-0059.

Thanks to Billy for this!


Staff member
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Onan FAQ....MonkeyWards/Onan Genends

Contributed by Eric........

To the best of my knowledge, yes.

Here's the list I can find, of all Montgomery Wards gensets with an Onan generator on them:

MX 21121A, MX 21122A, MX 21123A, MX 21124A, MX 21125A, MX 21125B, MX 21126A, MX 21127A, MX 21128A

MX 78601A, MX 78602A, MX 78603A, MX 78604A, MX 78605A, MX 78606A, MX 78607A, MX 78608A, MX 78609A, MX 78610A

MX27112A, MX27113A

MX27131A, MX27132A



In regards to the attachment, the cover says "Owners Guide and Parts List,", there is no "guide" in the books....just the parts portion.


Jim McIntyre

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Re: Onan posters - please be advised!

I agree. And it'd be extra nice if people would use meaningful thread titles. So, we'd avoid titles such as:

"Need Help!"
"What is it?"
"Onan Generator Question"
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