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From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Onan Generator low operating voltage
Date: Fri, 10 Aug 2007 17:43:53 -0400
Message-ID: <>

On Fri, 10 Aug 2007 09:08:26 -0700, mrehmus <> wrote:

>The genset is indeed the model you assumed . . . with the A suffix.
>I've passed your first suggestion along and will do so with this
>latest thought.
>I remember the voltage being fairly steady at 120 VAC when I bought
>the MH so the transformer tap change, if required, would tell me that
>something has changed either in the excitation voltage supply or more
>fundamentally, in the field coils. The slip ring tarnish sounds
>possible although I've run the generator faithfully every month or
>more frequently and the MH has been on the road 1 week in June & July.
>I only noticed this when I started monitoring the performance of the
>MH electrical system because I installed an outboard autoformer and
>surge protector (just for shore power, not connected to the genset)
>and wanted to monitor the performance of the new gear.

Let me make a quick suggestion.  Since you say that the voltage WAS correct but isn't
now, you need to look for what changed.  First thing I'd do is put an ANALOG
voltmeter on the output and watch the needle for fluctuations.  You won't be able to
see those with a conventional DVM.  If you see them then either a wire is loose in
the field circuit (unlikely), a stuck brush or slip ring glaze is the problem.  Since
this thing has been mostly sitting around, I'd bet on a stuck brush that' just barely
making contact with the slip ring.

If an analog meter isn't handy then a low wattage (40-60) incandescent bulb will
substitute.  Watch for flicker.

If the voltage is steady but low then there is probably a winding problem, either in
the field voltage coil, the transformer or the field coil itself.  The fastest way I
know to detect this is to put each coil on a Baker Surge Tester (can be done in-situ
without disassembling the generator).  If there are shorted turns in any of the
coils, the Baker shows it instantly.  Guaranteed CW won't have one but most any
electric motor repair shop will.

The Surge tester works by hitting the coil with a fast rise high voltage pulse and
then examining the ring waveform.  A coil with a shorted turn shows a greatly damped

I suggest getting it out of the hands of CW before they do some damage to the thing.
Find an electric motor shop that knows generators.  That is, if you don't want to
take it to an Onan-authorized shop.


From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Onan Generator low operating voltage
Date: Fri, 10 Aug 2007 21:14:08 -0400
Message-ID: <>

On Fri, 10 Aug 2007 16:22:31 -0700, mrehmus <> wrote:

>Side note:  The Onan people said that that generator is a 'bad' one
>and I should replace it with a new unit because if it fails, it will
>likely take out all the 120 vac equipment when it goes. Hmmm. Old
>motorhome that cost me $14,000. $5,000+ for a new generator vs $3,000
>if I have to replace the AC units. I think if it fails I'll buy
>another motorhome
>Thanks Alan and John. You both contributed to my education and,
>indirectly, to the Camping World technician.

You're welcome.

The Onan guys was blowing you a line of BS.  While what he said is technically true -
certain failures can cause it to output too much voltage - the practical risk is
miniscule.  It's about like saying that if the pole transformer feeding your house
failed, it could run 15,000 volts into your house.  Technically true but when is the
last time you heard of that happening.  I should also note that all modern generators
except the inverter generators can do the same thing.  Even the so-called "capped

If it worries you then you can buy an over-voltage relay for under a hundred bux that
will isolate the generator if it over-volts.  For that matter, I could turn a GFI
unit into an over-voltage trip with just a couple of components.

What you ought to do is call him back and tell him that if you replace the generator
the replacement will be a Generac.  See if he still thinks it needs replacing.  Bet
he changes his tune.

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Onan Generator low operating voltage
Date: Fri, 10 Aug 2007 20:56:23 -0400
Message-ID: <>

On Fri, 10 Aug 2007 16:26:31 -0700, mrehmus <> wrote:

>I'd like to accummulate a list of good service shops around the US.
>Maybe that should be a separate location or group or whatever (or is
>there already such an animal?)

That's not possible simply because the quality is mainly with the individual
mechanics and not the shop.  Even good mechanics tend to be flighty, moving to where
they perceive the slightly better money to be.

The best mechanics are going to be the independents like Chris.  If the independent
has been in business awhile and has a track record then he has to be good.  Otherwise
he'd be starving.  Or working for CW.

At a shop, rough estimate of quality will be a combination of the flat rate labor
rate AND the percentage that they pay the mechanics.  A good shop will pay the
mechanic at least 50% of the flat rate and many times more.  The high pay tends to
attract the best mechanics.  At the same time you need to find out what the
"come-back penalty" is.  That is, what does it cost the mechanic when a repair comes
back and the fault is obviously his.  A good policy is that it costs him his entire
commission for the job.  Even better is full commission forfeiture plus he pays for
the rework.

I worked at a very good motorcycle dealership in high school and college and those
were the policies.  I used the same ones when I had a Datsun Z specialty shop.  I had
my pick of area mechanics :-)

Given the dollars at stake, I'd not hesitate to come right out and ask.  If a shop
won't tell you then perhaps it's time to find another shop.

A good list to compile if you want to spend the time, is a list of independent
mechanics and how long they've been in business.  Now THAT would be handy.


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