Onan 10HDKAG 61731x Generator Has Low Voltage

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Suddenly my generator output voltage dropped to around 80 VAC.

The nameplate tag has long since come off but with the help of the service manual the model number could be derived to be 10HDKAG 61731x. It is a 4-pole revolving field, brush-type, reconnectible, 10.0kW, single phase, 1.0 power factor, 60Hz unit. The reconnection is wired with T1-L1, T2-T3-L0, T4-L2. Which provides two legs at 120 volts each.The hour meter indicates 1,398.8 hrs.

Here are the things I've done so far in an attempt to figure out what is wrong.

- The wiring all looks good, tight and clean.

- I took the measurements on both sides of the main breaker mounted on the gen-set.

- The voltage regulator is part number 305-0875-04. I can adjust the regulator down to say 50 VAC per leg but no higher than the 80 VAC or so. I'm thinking the regulator is OK since it does adjust. At this point that is all I have to go by.

- I disconnected the regulator and applied 12 VDC to the field winding. The voltage measurement between L1-L2 was 105 VAC. Just within spec per the service manual. Both L1 & L2 were balanced when referenced to common at 52.5 VAC. When the regulator is connected it drives the field winding to -23 VDC. I don't know how much the field winding should be to get 120 VAC.

- I measured the frequency to be 61.5 Hz. Within spec.

- The power plant is a Kubota V1305-B diesel engine. The throttle was lock wired at the factory and painted over. It is clear that it has never been touched.

This unit is in a 1996 Monaco with the front generator slide out.

As difficult as it will be to climb under the motorhome to get at the back of the generator with the front slide out rod right smack in the middle of the work space I'm thinking of going for a brush set replacement. If possible I would like to replace the bearing since it is so old. I have no idea how the bearing comes off at this point.

Any thoughts about this problem and my plan for the brushes and bearing replacement?

Thanks for your valuable time.


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Hi Dale.

This is a link to some information on the Alternator style that I think you have.

Section 8 covers the alternator section of the generator.


I will see what I can find that covers the disassemble in better detail.

I did find this link that might get some information



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Thanks ****,

I downloaded the service manual and will compare what is has to say about what I've been doing so far.

The nameplate was great. It helped me understand what information I would need. I did find a serial number for the generator on one of the document that Monaco provided. I can see where I have the first part of the model number but not the second. Maybe by the time we get this problem solved I'll have that rest of that number reconstructed. If the weather clears a bit tomorrow I have some more tests I can run. It rained all day today so tomorrow I hope to climb back under there and see if I can remove the slide out bar. Then I can get the cover off of the generator.

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Thanks Rich,

I got all the manuals now for this generator.

I finally figured out a way to get the motorhome up in the air a bit so I could get to the back of the generator.

Here is what I have found so far:

Model: 10.0 HDKAG - 66046A
Serial: F953398965
Hours: 1,398.8

Dynamic Rotor / Stator Test
12 VDC applied to the F1/F2 exciter stator.
Q1 to common = 53.0 VAC (spec 62 VAC +/- 20 VAC)
L1 - L2 = 105 VAC (spec 125 VAC +/- 20 VAC)
L1 - L0 = 52.5 VAC (spec 62 VAC +/- 20 VAC)
L2 - L0 = 52.5 VAC (spec 62 VAC +/- 20 VAC)
Q60 - Q1 = 71.5 VAC (spec 75 VAC +/- 20 VAC)
S1 - S2 = 52.5 VAC (spec 62 VAC +/- 20 VAC)

Resistance between each slip ring and rotor shaft = OPEN ( spec > 100,000 K Ohms) (used a Fluke meter on the meg ohm scale)
Resistance between slip rings = 5.2 ohms. (spec 17.2 ohms)

This is as far as I have been able to get with resistance readings.

I was able to take the brush assembly out. I noticed one of the two brushes was stuck in the assembly housing. The slip ring associated with that brush was black while the other slip ring looked like normal copper showing. The brush assembly is soaking in an orange citric cleaner for 24 hours. Tomorrow the connectors will look brand new. I polished up the slip rings. I'll verify the brushes, which by the way look excellent, will move freely in the assembly.

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Sounds like you have a good handle on things.

I'm thinking that the carbon build up on the on slip ring and the fact that the one brush was stuck limited the voltage on the one side.

Resistance between slip rings = 5.2 ohms. (spec 17.2 ohms) Reading a little low for whatever reason, but I think I would install the brush when things are clean. Read the voltage with the coach still up, so you can get to it if you need to do more work.

The low reading maybe caused by a short in a winding, this could explain the carbon build up on the slip ring and the brush being stuck to the side of its holder. High current load on the circuit could have caused the issue.

You could see if there is a motor rewinding shop in your area that could rewind the coil if needed. That would mean removing the unit from the coach.

I can not remember if this model had a Belt driven water pump. If it does you might want to change it, as they recommend replacing at 1000 hrs. on the 7.5 and 8k size units.

Getting older has its good and bad side. Glad I'm still around, but must admit I really enjoyed the younger side. LOL

Work safe Rich.

P.S. When you master this one you will be the official source of information on this style Alternator setup !!!!

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After what seems like an eternity this baby is up and running.

The short story, the shorted rotor was the problem.

I ordered a new rotor assembly from Onan through a local RV dealer, $1,200 +,

After looking at the cost of a rewind, slip rings, bearing and shipping the new assembly looked like the best deal.

It also included a new fly wheel mounting plate and fan.

There was still the concern that the armature was not the whole problem but it turns out it was.

The new rotor measured 17.6 ohms. That was great to see when I unboxed it.

I have a suspicion the original rotor may have been defective from day one.

During my testing I turned the voltage regulator up one or two turns.

When it made no change in voltage I turned it down to see if the regulator was doing anything.

It was.

Fast forward to the new rotor install.

I got the generator set up on the garage floor so I could test it before wrestling it back into the motorhome.

Not knowing what I was going to see I turned the voltage regulator pot down 10 turns, thinking it would be at the minimum.

The time had come to start it up.

When it roared to life I was getting 136 volts.

I quickly turned it down several more turns till I had 118 volts. A good number.

I found it very strange the pot had to be turned down so many times.

My thought is the rotor had some windings shorted in the beginning and the regulator was cranked up to achieve 120 volts.

Then on the fateful day the voltage stopped at 80 is when more windings shorted and the regulator was limiting the current.

In addition to getting the generator back on line it is now clean as can be.

I intend to keep it clean. For weeks everything I touched has dirty diesel finger prints.

So, in the end, the testing outlined in the manual was dead on the money.



PS We plan to be in Gillette in June.

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