ONAN 20K GGDB Volatge too high (320VAC)

Cummins/Onan Air Cooled & Liquid Cooled standby generator troubleshooting and repair questions
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sparsons
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Hello,
I have been given an Onan 20K (Mod GGDB-3333277 Spec B) (ser B990861347) 7.5hrs LPG

The problem is that I see 300+VAC at the output and the field breaker keeps tripping. Attempted to adjust VR21, no joy can't get the voltage to go down. The RPM is set right and I have 60Hz, but no luck on any voltage adjustments. Tried adjustments on SX460 board, and some changes, but nothing close to 240VAC, the best I could get was 279VAC.

Purchased new SX460 and after replacing I get the same results. It is now beyond my knowledge. I have voltmeters and basic tools and enough experience to break stuff really well but not enough to call myself an expert, just experienced.

I saw in another post about sensing leads, checked wiring harnesses and didn't find anything loose, but the manual (928-0503D) didn't lead me to anything that looks right.

Any help would be a great relief as this is on a remote mountain and my only other choice is to pull it and take it for service, that took heavy equipment to get it up here.
kb8vul
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Is the unit a 12 wire generator? Meaning that it's capable of 3 phase 480 output and has been rewired for single phase operation?

I had one that was over current on the field and opening the breaker as well. But my issue was the AVR was not sensing any input voltage due to the AVR being damaged and would put out more and more field voltage trying to correct the fact it couldn't see ANY output voltage. Now you said you replaced the AVR so that would not be the issue. But, I would look at the input sense voltage on the AVR and see what it is seeing.

Did you get the unit with this problem or did it work correctly at one time and has developed this issue since you owned it? This goes back to the question about 3 phase 480 vs 208 3 phase or 240 single phase.
Assuming that since you are looking for it to run 240 volts, it's suppose to be wired single phase but please comment on what your expected output is.
Going on the assumption that you are looking for 240 single phase output. I would check the original data plate on the unit to see what the factory output was configured for originally. If it was 3 phase 208 or 480 and someone tried to switch it, had got it wrong, it would be doing odd things like this. And if the outputs were wired together wrong, it would be back feeding into its winding causing very high current in the windings and the field. Again, it's popping the field breaker so somethings just not right.

Please post pictured of unit and the data plates or at least the info on them. Model number type stuff so we can look at documentation on it.
sparsons
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Thanks for replying.

The generator is setup for 120/240 single phase. It worked at one time, but has been in storage for many years. Once we moved it to the site and set it up we discovered the problem. There are three CTs for the meters but only two are actually in use by cables.

Looking inside, it looks like I have two phase/hot wires and two neutrals. I am checking the voltage at the output breaker (2 Pole), and I am expecting 220/240 VAC.

I attached a picture showing the config and voltages the generator should be configured to provide.
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kb8vul
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Since you replaced the AVR and it still has the same issue. Try disconnecting the AVR and running the unit.
I would think that it will NOT produce any power at that point,, or it may put out some small voltage, but not it's rated output. That will prove it's not got some short in it.
The other thing I would be looking at would be the sense voltage and is it getting to the AVR. As mentioned before. If the AVR doesn't see any output it will try to push the field harder until it see's output.

Also, with bigger units, they self flash. Meaning that the battery voltage from the starter or some source is used to flash the field to get the generator up and producing power. How that is done on your unit is not known to me, but it could be done with a relay or similar switch that could be hung and causing the full field situation and the really high output you are seeing.

I will see if I can find any detailed info on the unit and get back to you.
Birken Vogt
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Is the voltage at the terminals of the AVR the same as the voltage you are measuring on the output of the generator?

Is the SX460 configured for 240 volt sensing but hooked to a 120 volt pair of wires or something crazy like that?
sparsons
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Had Rocky Mountain/Cummins come out and they said the alternator set is bad and would need to be rebuilt. Due to the age and location it was deemed a lost cause, so the free generator was going to cost too much. So got a new 25K inbound.

Thanks to all for the responses.
Birken Vogt
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Nonsense, a bad alternator would never make overvoltage, only undervoltage. Typical generator "troubleshooting".
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