Markot
Have 15kw air cooled generac. Started with flashing overspeed light. This model is older and does not read out a code.

Had a tech look at it. Said the ignition coils needed to be replaced. I bought the coil set and replaced per the instructions provided. I also removed the two rear rotor brushes, degreased and cleaned them and cleaned slip rings like new. Was a big job to pull apart. When finished it started right up, its outlet had 120v, no more overspeed light. I set exercise timer, it ran through cycle and shut itself down. I tested it a few more times and ran fine.

A week later, checked it,  green light is on saying system ready. Switched to manual start, and it ran for 5 sec and shut down with flashing overspeed light again. Reset got green light. Manual start and again shuts down with overspeed fault.
After all the work- I’m really happy with generator 🤨.

Mark
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78buckshot
A model and serial number will help us help you. At this point I assume the unit has a mechanical governor, you will need a multimeter that has frequency read capability. Frequency is in direct proportion to the speed of the engine. If the engine is running at 3600 RPM you will read 60hz. The governor can be adjusted to the correct speed. If the speed is correct and it still faults out then I would suspect the control board is not sensing accurately.
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Peddler
First of all the coils have absolutely nothing to do with a flashing overspeed light.  Flashing overspeed is "no AC output" and the brushes are your most likely cause but it can be other things as well.  The #4 wire gets a boost from the control board during cranking to excite the field.  If the slip rings are dirty it may not make enough contact to get the DC voltage to the rotor through the dirty slip rings.  Since you have already cleaned those your problem could be one of a few things.  Due to age the control board doesn't always boost the #4 wire at all or enough, this can be particularly bad when the unit starts very quickly.  The other possibility is the Voltage Regulator is failing and is not opening as easy as it should.  The easiest and most positive action you can take is to install a field boost kit, this essentially bypasses the control board and provides continuous boost to the rotor, very little down side (if the VR fails you could produce low voltage) but I would spend the money to replace the voltage regulator at the same time.  Ziller can help with the parts.
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Markot
Great- thanks for the insight. I will get those
parts from you. It’s actually my boss’s
generator and I don’t have to skimp
on costs. I’ve worked on cars, motorcycles
and small engines my whole life and the coil
diagnosis seemed ridiculous to me as well.
I had someone come out from a generac
advertised electrical company and that was
their repair proposal. I did figure it was an
electrical fault and didn’t want to waste time
guessing. The brush/slip ring issue was
a good start cause they were filthy, but beyond that
I just want issue resolved and I’ll order what
you recommend.

Thanks again
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deanp72
I had a similar issue with an older Guardian 7kw. It would shut off just after starting over and over again. I replaced the voltage regulator and it is running great. If you have access to another voltage regulator it is worth a try. I had to order 2 before I had a good one that would adjust down to the correct voltage. 
I am having a problem with another Guardian 7kw. Everytime we have a sudden temp change in the winter humidity (Lots of excess moisture) it will not start. I have found that if I take the air cleaner off and hold my hand over the intake as if to choke it (propane) it starts right up and will run fine and restart ok until the next big temp /humidity change (winter only). I have another 12kw that has done the same thing. 
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Markot
Thanks for the info-
It’s my boss’s generator that I am dealing
with- I will certainly get a new voltage regulator
along with whatever other suggestions I get.
It starts and runs fine except it shuts down with the
overspeed fault.

Once I replace voltage regulator, hopefully that
will fix it. If not I’ll have to replace engine,
generator, wiring, electronics, and finally the
cabinet. That will show it who’s boss.
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deanp72
If it starts and immediately shuts back off then it is a good bet that it is the voltage regulator. You may be able to put a voltage tester on the exit leads from the breaker and wants it starts it may show you what voltage is being produced before it shuts down. I have about 8 that I have installed and maintained but my knowledge only goes so far. My own 7kw was shutting down after starting and I eventually had several Generac dealer come out and charge me $3-400 dollars and didn't find the issue. Finally I called Ziller electric and their tech told me to check the voltage regulator and WOW.!!!! I bought a new regulator and replaced and adjusted the voltage and is running faithfully. I did however have to buy and send back 2 because they just would not adjust down the voltage correctly.
Finally I got a good one and was told they may have picked up some used ones on the shelf and sent to me by mistake. Good Luck and let me know if that fixed your problem. Note::There is a tiny screw to set the voltage correctly.
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Geoff Z
Finally I called Ziller electric and their tech told me to check the voltage regulator and WOW.!!!! I bought a new regulator and replaced and adjusted the voltage and is running faithfully. I did however have to buy and send back 2 because they just would not adjust down the voltage correctly.
Finally I got a good one and was told they may have picked up some used ones on the shelf and sent to me by mistake.

deanp72,
If our advice and/or forum have helped you we are very grateful for that. However I need to address your statements above. We have 2 cancelled orders from you in our system from 7/19/18. You are not in our accounting. We have never sold or shipped you anything. There is ZERO chance that anyone could ever receive a used part from Ziller Electric. We only carry brand new Generac OEM parts. We do not keep used parts. Our techs replace defective parts on our local generators here in Michigan and they are disposed of. We would never have told you we may have sent you used parts by mistake. It is simply not possible that could have ever happened. I'm happy to discuss this further with you personally if needed. You can reach me at 888-294-5537 between 8am and 4pm EST. Thank you, Geoff Z





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deanp72
Oh I am so sorry that it looked like I bought a defective part from Ziller.
I only got the tech tip from Ziller. You are a top notch company in my book. I have bought a 8KW from you so that should be in your books and your tech help me add the wire needed to use my old transfer switch. Anyway I got the regulators locally here. Ziller has always been a help and did not mean to imply that I got bad parts from Ziller.Please post this to explain.
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deanp72
I would like to clear something up from my last post. I did not mean to imply that I bought bad parts from Ziller. I ran down to our local dealer and he ran in and sold me what he had on his shelf. I tried it and it was bad and he took it back and explain that some of his technicians could have set a used part on the shelf. Ziller has always been fantastic with their products but to make my point they have always been there with technical support that is above all the rest so please I did not mean to imply anything bad and I am sorry that it looked like that. I will beat myself up for this.
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Geoff Z
Thank you for clearing that up. Thank you for also understanding my concern at seeing the post. No beating yourself up. We got to the bottom of this and that is that. Providing this forum and watching folks help each other causes us more joy than you could ever imagine. Sometimes the printed word doesn't convey what was really meant. I'm more guilty than anyone. I always try to proof read my comments before I hit post. However sometimes busy gets in the way. I then want to take it back later when I see it on the forum. 
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deanp72
So true I am always in a hurry to move on to what needs done next and should see how a comment could be interpreted. Anyway really want to convey how useful and what a great job you do. Happy New Year to you and all with hopes that 2020 will bring great prosperous to everyone.
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Markot
I wanted to follow up- my 15kw aircooled, with overspeed fault, is now fixed.
in addition to other efforts I posted earlier, the fix was replacing the voltage regulator as was recommended on here. I easily got it from Ziller and installed. I adjusted governor, and regulator pot to set voltage and frequency and it’s been fine. Thanks so much to the forum- it was right on the money. 
Now I hope I didn’t jinx it with this post 😬.

Markot
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Geoff Z
We are glad you found our forum useful. Great bunch of guys here! Your generator model line, although older, have been really good performers. Properly cared for it will continue on.
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deanp72
So glad that fixed it. As I stated I had the same problem and it was an easy fix.
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