NH Sparky
 
I have a customers generator that the rpm was running away. Unit would continually hunt, with rpms going from just above idle to way above 3600.
I disconnected the stepper motor and could make the engine rpm steady, to the point where the controller wasn't faulting out on overvoltage.
The stepper motor resistance readings were all approx 11.6 ohms, although I could feel a distinct bad spot in the stepper arm travel when manually moved.
I installed a new stepper motor and the RPM hunting stopped. The issue now is the rpm's are way too high (90+Htz) but constant.
The output voltage was 240, so the ARV is working. Unit will run for 10 to 15 seconds before shutting down with a fault of [RPM sense loss].
I'm suspecting a bad controller, but feel I should be checking where the controller gets it's rpm sensing. I'm, at a loss as to what exactly test to determine if controller has what it is supposed to see.
The unit starts fine, will hunt for 2 to 3 seconds at a lower rpm then goes right to "very high rpm's" and stays there until the controller faults.
While it's running at the very high rpm, I can read 240v at the generator side in the main circuit breaker.
I'm mostly looking for anything else I should be checking before I order a new controller.
Generator model- 0059201 S/N 6902698 Controller- Nexus 0H6680D


 
Here's an update on the troubleshooting. After determining the controller gets its rpm sensing by counting ignition pulses, I went back and checked a few more things.
I installed two spark plug glow testers so I could see the spark. When i started the unit, rpm went to real high and stayed there, then controller faulted. I removed the stepper motor cam and restarted. I could control the rpm manually and it would run just fine for as long as I wanted, both spark plugs happily firing away.This would seem to prove the controller sees the pulses.
I reconnected the stepper motor and restarted the unit, everything worked perfectly, ran it for 20 minutes, no problems, Both voltage and rpm's were fine.
Shut down unit for ten minutes and restarted, right back to high rpm. I did notice that when the controller faults, around 4200 rpm, the far spark plug stops sparking, but the near plug sparks all the way down to 0 rpm? If I turn the unit off before it faults, both plugs spark all the way down to 0 rpm.
Is it possible that wire 18 is compromised somewhat and making intermittent contact, or is the controller bad and not making the conversion from pulses to rpm.
Thanks for your time
Dave
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Cobranut
Does the controller normally use the same wires from the coils to cut the ignition on shutdown as it does to sense rpm?
Perhaps one of those wires is intermittent open. 
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Birken Vogt
I think you got it mostly figured.  The controller senses RPM signal from the low voltage side of both coils which are internally connected together somewhere in the engine area and travel back to the controller on wire 18.  If one side goes open circuit then the controller is only going to see 1/2 the RPM signal as it is expecting and increase throttle.  I'm not clear on what is making it shut down but when it does, it grounds wire 18 which should cause both plugs to quit sparking. 

If one is still sparking then you have confirmation there is either an open wire between the coil and the junction point, or internally to the coil. 

It's up to you to figure out which it is. 

Without blowing up the generator in the process I might add.  This is bound to be rough on engine parts and the rotor.  Maybe it would be smartest to replace both the coil and the wire harness both, and not waste a whole machine in the process of testing.
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NH Sparky
Or, is it possible that one of the low voltage #18 wires is loose where the two splice together some where buried below the engine cowling.
Prior to starting the engine with the glow sensors attached, I moved the spark plug wires in and out through the cowling checking for possible plug wire damage where the wire exits the cowling (no damage noticed). when I next restarted the engine the second time, it worked perfectly.
I'm thinking a loose connection on the near coil, as this is the one that keeps sparking all the way to zero RPM after the controller faults.
The reason I'm thinking loose connection is because I got it to work perfectly once. If it were a bad coil, I don't believe it would have done that.
I think?
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Birken Vogt
Are you willing to risk engine or rotor damage for the price of a coil?  Surprised it hasn't already done something terrible the way you have described what has been happening.
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78buckshot
In order to get to the #18 harness you will have to remove the air cooling shroud from the front (right side) of the engine. It can be frustrating and a little painful for us old guys 'cause you will be on your knees for 3-5 hours. Almost all of the labor to replace the mags will be in removing the shroud, go prepared with the ignition magneto kit which will have both mags and the wire harness, replace the mags following the cut sheet included. You will need a brass feeler gage or thin business card to set the air gap between the flywheel and mags.
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NH Sparky
I think replacing both coils and the grounding wire makes the most sense. I have the individual part numbers for the three parts, but I don't see a part number for a "ignition magneto kit". I'll hunt around some more and see if I can find it.
Thanks for all the great insight. I will reply back with the results when I get the new parts installed
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Peddler
I don't think the 10 KW units coils are sold as a kit only the 1 liter air cooled.
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NH Sparky
Replaced the two coils and wire harness today. When I disassembled the air shroud, I discovered, as I expected, the low voltage wire on #2 coil was off it's tab. Since I already had the parts, I installed them. Adjusted valve lash, checked gas pressure and changed oil and filter and a new battery. Unit runs like a champ!
Thanks for all the help in diagnosing this problem, I could have easily gone off in the wrong direction if not for the great advice.
Dave
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78buckshot
Glad we could lend some help and to hear you got it running, thanks for the follow-up.
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