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Onan: 12.5 kw jc problems

The butcher

Hello all, new here and could use a little help on my onan 12.5kw jc genny. Can’t figure out if I need a new stator. I wasn’t getting any ac so I excited the generator but only get 140 on one leg a 5v on the other. Don’t have a megger to check insulation values but I did do a resistance check and I get .3-4 on both legs. I do have good continuity and it doesn’t seem to be shorted to ground anywhere. Any help would be greatly appreciated. FYI I don’t know much about the machine as I got it from someone who had passed away. There wasn’t a transfer switch on it so I’m concerned he could have back fed it.


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What are these "legs" you talk about? If it is a standard 4-wire JC, then you will have the leads labeled T-1, T-2, T-3, and T-4. If you examine your set, as you state you are only getting 140V out, it may be connected for 120V ONLY. If you disconnect all four wires, you should get 120V between T-1 and T2 , and 120V between T-3 and T-4. You should have nothing between either T1/T2 and T3/T4. If you don't get your 120V on each pair, that particular pair might be open. Check with an ohmmeter. At this point you are not concerned with resistance, only whether open or have continuity. Your set should be running at 1800 RPM for the voltage check, and stopped for the open test. For the stopped test, remove the penthouse on the control box and remove and isolate the two wires on terminals 1 and 2, on the bottom of the penthouse (top of control box with regulator). These two wires are connected to the T-1 and T-2 leads and supply reference voltage to the regulator. If either T1-T2, or T3-T4 pair are open, you need either a new stator or have your existing one rewound. zuhnc


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If you have a 12.5JC 3C it is a 4 wire 120/240 only generator. Follow zuhnc’s directions above and check your 120/240 voltage where the wires come out of the stator. This will eliminate breaker and wiring problems when trying to determine if the stator is bad. Picture shows where to check output voltage. Now if you have a different stator (12 wire, 3 phase, multi voltage) that’s another ballgame.