Overhaul of the Onan 12JC4R generator

I was fortunate enough to acquire this generator last fall.  It was new in 1969 and has unknown hours on it, but it appears in decent shape.  I am going to do a level two overhaul and install it as backup power for my house/shop.  The first order of business is a complete inspection.  I discovered a few problems; the starter didn’t crank, the distributor was loose, and the carburetor had some burned-out chunk of metal attached to it.

Onan 12JC4R generator
Onan 12JC4R generator

First, the starter:  These units use a Prestolite MEO3006 starter, which is common to several Chrysler products from the late ’60s and early ’70s.  This is obviously a replacement unit, as it is not “Onan Green.”  When I hooked a battery up and tried to turn the motor over, the start relay clicked but nothing else happened.  I dismounted the starter and removed the starter solenoid.  The interior of the starter motor looked in good condition, which points to the solenoid.  Sure enough, I removed the back of that unit and found two wires burned through and a large blackened area.  While I had the starter off, I hooked it up to a 12-volt battery and it worked fine.  A new starter costs $469.00, and a new solenoid cost $59.00.  I opted for the solenoid.

Onan 12JC4R burned out generator starter solenoid
Onan 12JC4R burned out generator starter solenoid

The next thing is the distributor.  I was checking the points and contemplating replacing the breaker points with an electronic ignition when I discovered the distributor could turn 1/8 of a turn in each direction, as when making timing adjustments.

Onan 12JC 4R distributor clamp
Onan 12JC 4R distributor clamp

I used a 3/8 box wrench and tighten up the clamp holding the distributor shaft.  It took several turns and makes me wonder why it was loose.  I will have to check the timing with a light once I get it running.  This also could be why the generator was not running when we took it out of service.

Onan 12JC 4R rotor and breaker points
Onan 12JC 4R rotor and breaker points

As for the points, they look brand new, as do the rotor and distributor cap.

Onan 12JC 4R generator spark plug, champion H8C
Onan 12JC 4R generator spark plug, champion H8C

The spark plugs look well used and the plug wires look original.

Finally, there was an electric choke mechanism on the carburetor which is completely unnecessary for a propane-fueled unit.  The choke plate itself was wired open.  The electric choke was burned open, so I removed the assembly.  I then spent some time at the local NAPA cross-referencing parts.  Here is a tune-up list:

NomenclatureOnan part (old)Onan part (new)Napa PartAlternate
Oil Filter122A185122-01931084Fram PH16
Distributor cap166B307166-0235AL91 
Ignition Coil**166B310166-0859-02701002PRX 405011
Plug wire #1167A1410167-1602701064 
Plug wire 2,3,4167A1409167-1602701063 
Spark Plug167-4167- Champ H8C***
Air Filter140B640140-19077-02241 
Starter191C324191-0324 Prestolite MEO3006
*Electronic ignition setN/A166-0825 Pertronics 1545**
**Ignition coil W/PRX 1545   PRX 405011

*Condenser and breaker points can be substituted for an electronic ignition kit, either Onan 166-0825 or Pertronics 1545 with Pertronics PRX 405011 coil.
**Pertronics electronic ignition must be used with Pertronics coil
***Champion RH8C plugs should be used with replacement wires without noise suppression plug boots.

This is for an Onan 12JC generator circa 1969 with a Studebaker engine. Other models/years may vary.  The other issue with this unit is there is no supervisory monitoring and control.  There is no oil pressure loss, overheating, or over-crank faults.  This is why the starter solenoid failed.  To remedy that situation, I started to design a better control circuit.  Then I looked around on the inner tubes and found somebody had already done this.  DynaGen makes the GSC400p which can monitor oil pressure, engine temperature, frequency, engine RPM, hours, voltage, and current.  It can fault for any out-of-tolerance condition, as programmed by the user.

Retrofit generator controller
Retrofit generator controller

I plan to install this in the original control box, leaving the original control circuit intact by using the remote start/stop connections.  I keep the original remote/start/stop switch and hand crank switch in place for use if the fancy controller fails.

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5 thoughts on “Overhaul of the Onan 12JC4R generator”

  1. Sounds like you have done your homework! I would stay with breaker point ignition as the Pertronics Hall effect ignitor gave me trouble even though less than 4 Amps was flowing through it with their supplied “Chink” 3 ohm coil. I finally gave up on their coils, and running any current through their Hall effect ignitor. I know the secret about the Chrysler solid-state ignition module that was introduced on their products for the 1973 model year, and adapted one to work with the Pertronic distributor Hall effect sensor. This with a Mallory high-performance extreme voltage coil, I believe I have the only Waukesha 180GKB 4 cylinder engine with a Pertronic distributor feeding a Chrysler electronic ignition module in the country! My generator used a Fairbanks-Morse magneto which went south, and I could not find a replacement anywhere. Those Studebaker 4 cylinder air-cooled engines are fairly tough, but change the oil, filter, air cleaner while you’re at it. The reason you lost the solenoid was probably over-cranking, which hopefully your new electronic controller will remedy.

  2. These are great generators. DO NOT use Champion plugs, go with the NKG equivalent. Replace the rotor cap and rotor. I would not recommend “upgrades” to the system because it is a very robust basic design. I have the unused transer switch for this generator if you are interested.

    Steve Sisk

    609 894 2289


  4. Hello Paul.
    I just read your entry on the internet. Is your plant still running ??
    Would you happen to have more part numbers for this generator ?? I am looking for the whole distributor.

    The plant I have seems to run only on propane, do you know if these plants could also be used on gasoline ??

    Any help would be greately appreciated.
    Best regards

  5. Can you still get a operation, main and parts manual on the onan RJC-3R/2122AC 12.5 kva.


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