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Old 06-29-2015, 06:46 PM   #1
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Onan generator voltage too high and low

Generator started acting funny over the weekend, need advice from you electrical gurus out there. It's a Cummins Onan QD 10kw, 4 years old, 650 hours. Runs great. Was running it nearly 24/7 over the weekend when I noticed the inverter panel did not indicate charging. Traced the problem to the generator voltage output. Here are my readings on the generator power output lines, right off the top of the genny:
L1 & gnd 120 volts
L1 & neutral 104
L2 & gnd 120
L2 & neut 145

Now all 4 of those should be 120 volts. Something is screwy. Think its my alternator gone bad? Or the voltage regulator? I think that is built in to alternator though.... Thanks!!!
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Old 06-30-2015, 09:26 AM   #2
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Sorry to see you haven't got any replies yet since I have experienced this same problem recently. In addition to the voltage fluctuations on both legs I have seen the frequency numbers shift. As of now the problem for me has disappeared and I have not found a solution. I searched the web and the only info I can get is to make sure that the filters are clean? Mine has been recently serviced and has 3000 hours with no signs of pervious problems in this area. A trip to Cummins might be in my future, too bad since I just got it out for a stuck thermostat. In your case a compete service might be needed including new brushes and cleaning the slip rings, something the average DIY'er doesn't have the tools for. A control board and or voyage regulator might also be the culprit. God luck with the repair and post back what you find. Are there any codes?
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Old 06-30-2015, 09:38 AM   #3
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No codes.

I'm pretty handy but without a step by step guide to replacing alternator or cleaning brushes I'm out of my league.

I just replaced oil, oil filter, air filter and fuel filter within the previous 20 hours of use.
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Old 06-30-2015, 10:15 AM   #4
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Maybe some help here Onan Generators - SmokStak These folks are Onan experts.
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Old 06-30-2015, 10:23 AM   #5
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Wilco, thanks!
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Old 06-30-2015, 09:04 PM   #6
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Electrician for 40 years. When the voltage is different between the legs (L1 & L2) it usually means the connection between the neutral and ground is not good. This causes the voltage to float depending on the load difference on each leg. The neutral and ground are connected somewhere to give a constant midpoint between the legs.
I know this doesn't give you a place to look, but that is the problem. Find it and get it fixed before you burn something up.

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Old 06-30-2015, 09:10 PM   #7
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Thanks Jim. I'm not sure where to start looking though. Could this be happening "downstream" from the AC wire block on the top of the generator? Or more likely between there and the alternator?
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Old 06-30-2015, 09:29 PM   #8
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It seems you have an open neutral probably in the transfer switch. Per this link:

Generator Ground-Neutral Bonding | No~Shock~Zone

The neutral and ground are connected together in the Pedestal when plugged in. When connected to the generator the transfer switch should have them connected together on the generator side.
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Old 06-30-2015, 09:45 PM   #9
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Quote:
Originally Posted by nothermark View Post
It seems you have an open neutral probably in the transfer switch. Per this link:

Generator Ground-Neutral Bonding | No~Shock~Zone

The neutral and ground are connected together in the Pedestal when plugged in. When connected to the generator the transfer switch should have them connected together on the generator side.
That's a little confusing. Everything had been working fine for 4 years prior. I tested the transfer switch by switching the generator wires to the shore power contact switch, in case the other side had burned up. Had same result. Switched wires back and a Honda generator powered the RV just fine for a day. Now its back home plugged into shore power. Doing all this I thought I eliminated the ATS as the problem.
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Old 06-30-2015, 10:19 PM   #10
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Sounds like neutral is floating.

It should be connected to safety ground in only one place.

That is usually source or meter connection but I have no clue here but some guesses but will not provide as it is your life...

Locate wiring diagram for gunny or mh to confirm exactly how the neutral is connected to ground then troubleshoot that.

I suspect a simple loose screw someplace but do not just tighten them all yet but continue measuring voltages and when faulty connection is found then fix and confirm repair.

Now you can do maintenance and check and tighten everything else.
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Old 07-01-2015, 08:07 AM   #11
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If you look at the link I posted neutral and ground are not connected together in the motor home by design. The connection takes place in the power pedestal when plugged in. This is the same as a branch panel in a house. When you use the Onan generator there is no pedestal to make the connection so the neutral and safety ground should be connected together on that side of the transfer switch.

If they are there will not be a big difference in voltage between the two. If they are not connected then anything can happen. I would look at the physical wiring around the generator and transfer switch. The specification is to mate them at the transfer switch as the generator does not have them connected and you do not want them connected when not on generator power. It will take a physical inspection to see where they are connected and what has come loose on that side of the wiring.

Moving the generator wiring to the plug side of the transfer switch or the output side of the transfer switch loses the connection between neutral and ground. It is designed and specified that way.

Not sure where the Honda comes into the picture.

I would trouble shoot with an ohmeter with all power off as well as the physical look for something loose or burned.
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Old 07-01-2015, 09:13 AM   #12
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So much for the idea that the generators have built-in protection against high voltage. So many of the discussions regarding the need for surge/voltage protectors indicate that there is no need to worry about generator over voltage.
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Old 07-01-2015, 09:19 AM   #13
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At least the inverter and ATS did it's job and did not let the bad juice through.
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Old 07-02-2015, 06:16 AM   #14
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The generator is working properly. Your wiring is broken.
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