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Onan 12.5JC-3CR, Questions, Just purchased


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  #1  
Old 03-18-2017, 03:18:08 PM
oakyone oakyone is offline
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Default Onan 12.5JC-3CR, Questions, Just purchased

I purchased this to use as back up power at home. I'm going to set up a breaker panel interlock with a 50A power inlet and SO cord to feed it. No fancy automatic transfer switch at this time. The unit runs and makes power. I just installed the PerTronics ignition - still could use new wires, cap, rotor and plugs. Runs good now. but has a little miss now and then. The control box doesn't have any visible mouse damage inside and all the wiring looks to be original. I did have to clear an old nest out from between the cylinders. It has the air shutters which open as it warms up. The unit has an all weather housing, which is nice, but hard to get at the spark plugs.

Model: 12.5JC-3CR/11417U
Serial: 0873683987


I believe this was manufactured in August of 1973??? Does this JC have the YD generator? I'd like to add the low oil pressure and high temperature shut downs if possible. I'm not super comfortable leaving a engine run for several hours without being able to watch for trouble. I know low oil pressure or overheating is VERY unlikely on a well maintained engine, but I'm a little crazy. Is there a set of pre-made spark plug wires that will fit, or do I have to get the universal ones?

I'm brand new to these Onans. Seems like an extremely well built unit, however - like an old tractor.

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Old 03-18-2017, 04:13:11 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

I guess I will retype this entire book since the first draft seems to have been swallowed up by my computer.

Welcome to The Stak, home of all things Onan and many Green Diseases infected folks.

Yes. Your set was built in 8/1973 to spec. U. It is a magneciter gen-end not a YD. Service manual 967-0500 and parts manual 967-0220 can be downloaded from this link for free, thanks to Doug & Mikes hard work. http://www.twinslan.net/~n0nas/manuals/onan/
The occasional miss may be cured by gapping the plugs to .035 if you have not already done this. Tune up parts listed below should be available from the local auto parts store.

Plugs; Champion H8
Points; Carquest 50-3507
Points; Napa CS709
Condenser; Carquest 50-1517
Condenser; Napa AL38
Coil; Carquest 52-2016
Distributor cap; Carquest 51-1509
Distributor cap; Napa AL91
Rotor; Napa AL32
Oil filter; Wix 51452
Oil filter; Napa 1084

I use the generic cut to fit spark plug wires. You can probably buy OEM wires if you want to mortgage your home to get them from Onan.

I think you will find a wiring diagram in the back of the service manual for a set with low oil pressure and high air temp. switches.

This is not the entire first draft, but. I will look after awhile for the build sheet & wiring diagram.

I found the build sheet and what I "think" is the correct diagram.
Attached Files
File Type: pdf 12.5 JC 3-18-17.pdf (25.1 KB, 40 views)
File Type: pdf 12.5 JC diagram 612-2658.pdf (121.2 KB, 42 views)

Last edited by Max Thompson; 03-18-2017 at 04:37:48 PM. Reason: Added build sheet & diagram
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Old 03-18-2017, 04:33:02 PM
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BigBlockChev BigBlockChev is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

It is a little hard to see but it appears that the top of the control box is flat which means it is a magneciter. If the generator end has 6 bolt bosses only 4 of which are used then it is a magneciter, if it has 4 bolt bosses then it is a YD gen end, the battery case obscures that detail in the first pic. The stock oil pressure switch would be located near the oil filter and have a single terminal, it opens on pressure build. It would be easier to add a two terminal, pressure to close oil pressure switch in series with the wires going to the centrifugal switch beside the oil filter, or in series with the power wire going to the coil in the same location. The stock high temp cutout switch is mounted in the discharge air stream at the rear of the machine below the exhaust manifold on the sheetmetal. You could wire a Klixon type switch in the same location in series with the coil to achieve the same effect.https://www.digikey.com/products/en/...=0&pageSize=25, sorry I cant give you a temp value for the air cooled machines , I mainly deal with the marine water cooled gensets , which use about a 250deg sensor. Since it appears that you don't have the factory options it would be more difficult to add the stock switches since the wiring etc is not there now. Cheers Dan
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Old 03-18-2017, 05:53:31 PM
Leon N. Leon N. is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Oops! The stock Onan High Temperature Cutoff Switch mounts on the exhaust manifold stud. The switch is connected in series with the control circuits and the ignition coil. It operates at 375 degrees plus or minus 10. It is designed for all the J line air cooled plants.

The switch part number is 309 K 211. THe J-line air cooled plants are designed to have a discharged air temperature 140 degrees above ambient. If you can find a new HAT switch, I have the original Onan installation and performance data should you want a copy.
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Old 03-18-2017, 06:00:53 PM
TLB01 TLB01 is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Those "Hat" snap switches are a common part on pellet stoves and HVAC blowers. Cheap and plentiful.
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Old 03-18-2017, 07:43:34 PM
Mike57 Mike57 is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Leon, I'm curious about the 140 degree number. Were they saying a JB/JC were supposed to run exhaust air temps of ambient plus 140? I've run my JC loaded at mid 80's ambient and not had exhaust air go over 180 degrees (CHT was in the low 300's). I'm pretty sure if exhaust air ever hit 375, a JC motor would be toast by that time. I replaced my HAT with one that kicks at 250.
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Old 03-18-2017, 08:24:13 PM
Leon N. Leon N. is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Mike, the Onan tech told me the " maximum" rate of rise of the air temperature passing through the unit is 140 degrees above the ambient temperature. When you stop to think about it, that applies whether your machine is located in let's say, Texas or Alaska. Obviously this 140 degree figure will be affected by how the unit is installed. That is, the duct work, air flow restrictions etc. In my opinion and experience, the J line cooling systems were purposely over designed to account for such installation variables and swings in ambient temperatures.

In my installation, my JB ambient ranges from below zero to just over 100 degrees.

Be advised, the exhaust air never reaches 375 degrees. The figure of 375 applies to the Onan sensor which monitors the temperature of the exhaust manifold which is obviously much much hotter that the air flowing over it. I am sure Onan did not want false shut downs due to a hot day in Texas, for example. Also note, the Onan temperature controlled shutters restirct the exhaust cooling air and NOT the intake cooling air.
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Old 03-18-2017, 09:40:06 PM
oakyone oakyone is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Wow. Thanks everyone for responding so soon. I've already read a lot about these old Onans here and it's VERY helpful. I love old machines - new stuff usually isn't built to as high a standard.

375F seems reasonable (for overheat sensor) if it's mounted right to the exhaust manifold. I'm in western Oregon, so it rarely gets to 100F here and very rarely down to 0F or below.

Sounds like a two wire oil pressure switch should be pretty simple to install along with the overheat switch in the same circuit.

The unit has a solenoid valve between the two propane regulators. (Not sure why it has two, but it runs well.) The right hand switch on the control panel energizes this valve, but the set runs and starts fine weather it's on or off. Curious.

When I get a chance to work on it more, I'll update my progress. Any additional feedback on the exhaust manifold temp would be appreciated. Of course, 375F is WAY to hot for the air leaving the shutters.

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Old 03-18-2017, 09:46:50 PM
oakyone oakyone is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

One thing I forgot. It's my understanding the generator has built in protection in the control box. I want to protect the wire feeding out of the gen set, but do I have to worry about overload to the generator? I know a standard circuit breaker is very unlikely to trip, because the set can't hit the high inrush like main power.
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Old 03-18-2017, 09:56:19 PM
KPack KPack is offline
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Default Re: Onan 12.5JC-3CR, Questions, Just purchased

Make sure you have the right coil resister or non resister read the wiring schematic to see if yours has the resister or not made my Jc run better when I got the right one.
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