I own a motorhome that has an ONAN 4.0 genny. It has worked fine for 10 years. It has 600hrs on it. I have maintained it flawlessly. Never had an issue until last Dec.

I was on a trip with my family when all the sudden the TV quit running and the clock on the micro went super dim. After a ton of tracing, testing and replacing parts, (brushes, V-regulator, circuit board) I have narrowed it down to one symtom. There are two pair of brushes . One side puts out 220V and the other puts out 50v I have called Onan and countless RV places. No one seems to know anything. With a meter on the the brushes they both put out 220V for about 3 seconds once fireing the genny. Then one holds the voltage and the other drops. I have swapped the brushes back and fourth, old ones new ones, and it's always on the one side. So it's not the brushes themselves.

I found this forum and thought I'd take a shot. Any of you guys have any ideas? Thanks in advance for your help.

 

Well, it could very well be a bad fuse. I have seen it happen before where a fuse goes bad, and turns its self into a resistor, and finally blow(could take upto a week or two).


If that is not the problem, which is not too likely, then i would say that it is the generator.

Good Luck!!!!

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remove all loads at the the one dropping then run gen, if the voltage holds then something is wrong in your motorhome circuit

 

You might look for bad solder connections or broken wires on the rotor. Centrifugal force can open a bad connection. Sometimes there are diodes there too. The low voltage side is probably the exciter winding. The first thing Onan will tell you is to replace the $250.00 circuit board which is only 2 terminal strips and 3 or 4 relays. What model Oh No generator do you have?

 

Thanks so far guys,
Definately not a fuse. Ran it unloaded, same result. Replaced the board already.We are now in the process of removing it from my RV so we can get behind the brushes. It is an Onan Emerald 4.0 BGE
Again, thanks.
If and when we get this solved, I'll let you all know.
FD

 

Not sure how likely it is, but it could be a problem with the (commutator?) copper face on which the brushes touch. Make sure it's not dirty or really scored. . . small score marks can be 'polished' out with some very fine abrasive paper (800 or 1000 grit is usually good) if you can get to it properly.

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Thanks Fingaz, yeah, we checked that too.

 

It could be a shorted turn on any of the windings; have you checked their impedance?

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Hero999,
We are going to dig into the back of th genny Tuesday as we were just way too busy today to me with my stuff. We will check that. Thanks.

 

Some generators can lose their residual magnetism and you have to “Flash the field” to restore it. If applicable, you should try this before tearing it apart. On some Onans you apply 12 volts DC across the F1 and F2 leads. Sometimes they want a fuse or light bulb in line or 6 volts. I did it to one Kohler that wanted 12 volts AC. If this applies to your generator, you need to find out EXACTLY what it wants and how to do it.

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What happens with the engine driving the generator, when the voltage drops? Does it sound like the mechanical load has increased or decreased? Do you get any vibration when the problem appears?

It sounds like you might have some kind of fault in the generator, which is being triggered by thermal expansion. It could be shorted windings, which would make it seem like the motor turning it suddenly has to work a lot harder, or some kind of open circuit (this could be in a lot of different places), which might make the engine act like the load has disappeared.

 

post your question to the many experienced RV owners here: http://www.rv.net/forums/index.cfm/f...s/forum/34.cfm

Plenty of them have Onan gensets and may be able to dial in to your specific problem.

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