Low Voltage, Onan

D

dave lowry

New member
Jul 7, 1998
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When our new A/C started tripping the breaker whenever the compressor came
on. The fan by itself seemed to work fine. We eventually discovered that
our Onan's output voltage was low. Some testing indicated no load voltage
of only 107 volts at 2780 RPM. Even when RPM was increased to 3000, the
no-load output only increased to 112 volts.

When I reported this to my locan Onan repair guy, he said the problem was
likely in the generator itself, and he would need to open it up to check
brushes etc.

I hate being at the mercy of the $70 an hour experets. Does this sound
reasonble?

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
Dave,
Are you talking about a real Onan brand or some other brand? Our Onan runs
at 1800 RPM. How are you measuring RPM? The ignition system on the Onan
fires both spark plugs every time so if you measure it from a spark plug
wire, you may want your meter to read 3600 RPM. I'd get a second opinion.
Bob McLaughlin
- -----Original Message-----
From: owner-gmcmotorhome
[mailto:owner-gmcmotorhome]On Behalf Of Dave Lowry
Sent: Wednesday, August 02, 2000 8:33 PM
To: gmcmotorhome
Subject: GMC: Low Voltage, Onan

When our new A/C started tripping the breaker whenever the compressor came
on. The fan by itself seemed to work fine. We eventually discovered that
our Onan's output voltage was low. Some testing indicated no load voltage
of only 107 volts at 2780 RPM. Even when RPM was increased to 3000, the
no-load output only increased to 112 volts.

When I reported this to my locan Onan repair guy, he said the problem was
likely in the generator itself, and he would need to open it up to check
brushes etc.

I hate being at the mercy of the $70 an hour experets. Does this sound
reasonble?

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
OOPS! I must appologize for the errant post. Please subtract 1000 from the
RPM numbers I posted.

For those who are still willing to consider my plight, I will humbly accept
your suggestions.

But then what's a thousand rpm among friends?

Thanks, Dave

>Dave,
>Are you talking about a real Onan brand or some other brand? Our Onan runs
>at 1800 RPM. How are you measuring RPM? The ignition system on the Onan
>fires both spark plugs every time so if you measure it from a spark plug
>wire, you may want your meter to read 3600 RPM. I'd get a second opinion.
>Bob McLaughlin
>-----Original Message-----
>From: owner-gmcmotorhome
>[mailto:owner-gmcmotorhome]On Behalf Of Dave Lowry
>Sent: Wednesday, August 02, 2000 8:33 PM
>To: gmcmotorhome
>Subject: GMC: Low Voltage, Onan
>
>
>When our new A/C started tripping the breaker whenever the compressor came
>on. The fan by itself seemed to work fine. We eventually discovered that
>our Onan's output voltage was low. Some testing indicated no load voltage
>of only 107 volts at 2780 RPM. Even when RPM was increased to 3000, the
>no-load output only increased to 112 volts.
>
>When I reported this to my locan Onan repair guy, he said the problem was
>likely in the generator itself, and he would need to open it up to check
>brushes etc.
>
>I hate being at the mercy of the $70 an hour experets. Does this sound
>reasonble?
>
>Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
Dave,
I'd try it at 2000 rpm for a while. There is a lot of information on the
net. Try http://www.california.com/~eagle/list.html as a start. It is
very unusual to have low voltage. It usually works or is dead. Radio shack
may have a cheap (make that low cost) frequency meter and that could tell
you a lot i.e. are both voltage and frequency low?
Good luck and let us know what happens.
Bob McLaughlin
 
>Dave,
>I'd try it at 2000 rpm for a while. There is a lot of information on the
>net. Try http://www.california.com/~eagle/list.html as a start. It is
>very unusual to have low voltage. It usually works or is dead. Radio shack
>may have a cheap (make that low cost) frequency meter and that could tell
>you a lot i.e. are both voltage and frequency low?
>Good luck and let us know what happens.
>Bob McLaughlin

Trouble is, even at 2000, a light load dropped voltage to 105 and it still
wouldn't run the new A/C.

I thought frequency and voltage fluctuated together.

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
Hi Dave

I can see a few things that may cause low voltage
1 - bad brushes
2 - bad reactor
3 - bad bridge rectifier
4 - low residual magnetism in the field

1 - I think everyone will understand how bad brushes would cause a problem.
I would think this would affect full load voltage more than no load voltage
but I am just guessing at that.

2 - The generator voltage is regulated by the reactor. The reactor feeds
current to the field winding in proportion to the current being drawn from
the generator. (Actually it is a fixed current plus some load dependent
added current to compensate for the internal resistance of the generator.)
This current is rectified by the bridge rectifier before being fed to the
field winding. I would guess a bad reactor would result in low or no
voltage.

3 - If one leg of the bridge rectifier has failed then only half of the
current generated by the reactor will actually make it to the field winding
causing a low voltage. If both legs of the rectifier are bad no current
from the reactor will get to the field and you will get no voltage out.
These rectifiers are a weak point in the Onan. Usually both sides of the
bridge fail but it is possible for only one side to fail.

4 - The field current for the generator is self generated. Just like an old
car generator it relies on residual magnetism to start the generating
process. There are instructions ( I believe in the Onan section of the GMC
manual) on zapping the field to re-establish the residual magnetism. I
presume this is not a problem or you would not be getting 112V.

My first step would be to replace the bridge rectifier.

Good luck and let us know what you find.

Dave Mumert

- ----- Original Message -----
From: "Dave Lowry"
To:
Sent: Wednesday, August 02, 2000 8:45 PM
Subject: RE: GMC: Low Voltage, Onan

> OOPS! I must appologize for the errant post. Please subtract 1000 from
the
> RPM numbers I posted.
>
> For those who are still willing to consider my plight, I will humbly
accept
> your suggestions.
>
> But then what's a thousand rpm among friends?
>
> Thanks, Dave
>
> >Dave,
> >Are you talking about a real Onan brand or some other brand? Our Onan
runs
> >at 1800 RPM. How are you measuring RPM? The ignition system on the Onan
> >fires both spark plugs every time so if you measure it from a spark plug
> >wire, you may want your meter to read 3600 RPM. I'd get a second
opinion.
> >Bob McLaughlin
> >-----Original Message-----
> >From: owner-gmcmotorhome
> >[mailto:owner-gmcmotorhome]On Behalf Of Dave Lowry
> >Sent: Wednesday, August 02, 2000 8:33 PM
> >To: gmcmotorhome
> >Subject: GMC: Low Voltage, Onan
> >
> >
> >When our new A/C started tripping the breaker whenever the compressor
came
> >on. The fan by itself seemed to work fine. We eventually discovered that
> >our Onan's output voltage was low. Some testing indicated no load voltage
> >of only 107 volts at 2780 RPM. Even when RPM was increased to 3000, the
> >no-load output only increased to 112 volts.
> >
> >When I reported this to my locan Onan repair guy, he said the problem was
> >likely in the generator itself, and he would need to open it up to check
> >brushes etc.
> >
> >I hate being at the mercy of the $70 an hour experets. Does this sound
> >reasonble?
> >
> >Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
>
 
Hummmm
I have not looked at the RPM I just raise it until the voltage is
right.......I set it to 125 with no load. You might have to wait
until Duane gets back...

gene

>When our new A/C started tripping the breaker whenever the compressor came
>on. The fan by itself seemed to work fine. We eventually discovered that
>our Onan's output voltage was low. Some testing indicated no load voltage
>of only 107 volts at 2780 RPM. Even when RPM was increased to 3000, the
>no-load output only increased to 112 volts.
>
>When I reported this to my locan Onan repair guy, he said the problem was
>likely in the generator itself, and he would need to open it up to check
>brushes etc.
>
>I hate being at the mercy of the $70 an hour experets. Does this sound
>reasonble?
>
>Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA

Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/ http://www.california.com/~eagle/
 
Dave,
They should fluctuate together, so, if they are both low at 2000 rpm,
through away the rpm gage and up the rpm until you get 60 cycles and 115
volts. On the other hand if you have 107 volts and 65 cycles, you have an
electrical problem not mechanical. Right now we think the voltmeter is
correct because the circuit breaker pops under load. How has the accuracy
of the rpm gage been verified? Seems to me you need to know the frequency
to be sure the generator is running at the correct speed. HTH
Bob McLaughlin

- -----Original Message-----
From: owner-gmcmotorhome
[mailto:owner-gmcmotorhome]On Behalf Of Dave Lowry
Sent: Wednesday, August 02, 2000 11:18 PM
To: gmcmotorhome
Subject: RE: GMC: Low Voltage, Onan

>Dave,
>I'd try it at 2000 rpm for a while. There is a lot of information on the
>net. Try http://www.california.com/~eagle/list.html as a start. It is
>very unusual to have low voltage. It usually works or is dead. Radio
shack
>may have a cheap (make that low cost) frequency meter and that could tell
>you a lot i.e. are both voltage and frequency low?
>Good luck and let us know what happens.
>Bob McLaughlin

Trouble is, even at 2000, a light load dropped voltage to 105 and it still
wouldn't run the new A/C.

I thought frequency and voltage fluctuated together.

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
Dave, Bob & Gene;

Thanks for the input. The rpm was tested with a pretty good quality test
instrument, so I trust it. I've copied your responses and will share them
with the tech when I see him next week.

At least I can be confident that what I was told is possible.

Thanks,

Dave (advisor/navigator) & Dege (ALL else), '76 Royale, Santa Barbara, CA
 
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