Onan trouble shooting

Ken Shaffer

Ken Shaffer

Member
Sep 6, 2017
99
2
8
My generator has not been used for at least eight years. I am now trying to get it working. I have been able to get it started and it runs fine but I
do not have any AC power inside. Shore power cord is connected to generator plug And switch on top of Onan is on. Any advise on where to start?

6K power drawer.

Ken Shaffer
73 canyon lands

--
Ken Shaffer
Marion Iowa
73 Canyon Lands, 455, Manny Tranny, Custom interior
 
Take a voltmeter and check to see if you have voltage to both sides of the top switch. Report back as to what you find and we’ll give more instructions.

Emery Stora

>
> My generator has not been used for at least eight years. I am now trying to get it working. I have been able to get it started and it runs fine but I
> do not have any AC power inside. Shore power cord is connected to generator plug And switch on top of Onan is on. Any advise on where to start?
>
> 6K power drawer.
>
> Ken Shaffer
> 73 canyon lands
>
> --
> Ken Shaffer
> Marion Iowa
> 73 Canyon Lands, 455, Manny Tranny, Custom interior
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Did you check your ac breakers to make sure they are on?

Sully
Bellevue wa

On Mon, May 27, 2019 at 4:25 PM Emery Stora via Gmclist <

> Take a voltmeter and check to see if you have voltage to both sides of the
> top switch. Report back as to what you find and we’ll give more
> instructions.
>
> Emery Stora
>
> > On May 27, 2019, at 4:53 PM, Ken Shaffer via Gmclist <

> >
> > My generator has not been used for at least eight years. I am now trying
> to get it working. I have been able to get it started and it runs fine but I
> > do not have any AC power inside. Shore power cord is connected to
> generator plug And switch on top of Onan is on. Any advise on where to
> start?
> >
> > 6K power drawer.
> >
> > Ken Shaffer
> > 73 canyon lands
> >
> > --
> > Ken Shaffer
> > Marion Iowa
> > 73 Canyon Lands, 455, Manny Tranny, Custom interior
> >
> >
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> My generator has not been used for at least eight years. I am now trying to get it working. I have been able to get it started and it runs fine but I do not have any AC power inside. Shore power cord is connected to generator plug And switch on top of Onan is on. Any advise on where to start?

Our Onans - being a self-exciting design - depend on residual magnetic field in the stator in order to begin the excitation process as the armature starts to turn during engine start. Over time that residual field decays away and given a long enough interval it will no longer be strong enough to start the excitation process and the classic result is “no AC”. Eight years is easily long enough for this to take place.

Check http://www.gmcws.org/Tech/dsimmons/onan_demo/ http://www.gmcws.org/Tech/dsimmons/onan_demo/ and see the section called “Loss of Residual Magnetism”.

Do not try anything else until you go through the “Loss of Residual Magnetism” procedure described above. MAKE SURE you label the + and - wires as you take them off the bridge rectifier and MAKE SURE they go back onto the same terminals after you carry out the flashing procedure.

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
What Mr. Miller said. Report back.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
I have 3volts reading on left side of breaker.
I energized the wires that the BR plugs into ... nothing.

Ken
--
Ken Shaffer
Marion Iowa
73 Canyon Lands, 455, Manny Tranny, Custom interior
 
Make sure the meter is set fo ac bolts. What was the reading on the right side?

Emery Stora

>
> I have 3volts reading on left side of breaker.
> I energized the wires that the BR plugs into ... nothing.
>
> Ken
> --
> Ken Shaffer
> Marion Iowa
> 73 Canyon Lands, 455, Manny Tranny, Custom interior
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
How about voltage on the right side of the breaker?

> I have 3volts reading on left side of breaker.
> I energized the wires that the BR plugs into ... nothing.
>
> Ken

--
1977 Kingsley 455 as stock as it gets except lots of Ragusa parts
 
I couldn’t get anything on the right side. But I didn’t try real hard. I
will re-check that tomorrow. I did pull off the BR and tried to test it. I
was only able to get 2.7 ohms with two terminals and nothing with a
combination of other terminals. My multi meter is not high-quality and I am
pretty inexperienced as well, so I will re-check Everything you have asked
with another tool tomorrow.

On Tue, May 28, 2019 at 8:22 PM Mike Hamm via Gmclist <

> How about voltage on the right side of the breaker?
>

> > I have 3volts reading on left side of breaker.
> > I energized the wires that the BR plugs into ... nothing.
> >
> > Ken
>
>
> --
> 1977 Kingsley 455 as stock as it gets except lots of Ragusa parts
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
The 3VAC reading is the classic indication of “no excitation”. There are only a few things that can cause this:

1. loss of residual magnetism in the field (thus the flashing procedure already described).
2. defective bridge rectifier
3. defective field assembly
4. defective compounding reactor (unlikely)

While you have the bridge out, use your meter to check resistance between the F1 and F2 wires that previously attached to the bridge’s + and - terminals. You should read around 38 ohms.

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
You don’t want to read ohms. Put the meter on ac volts. You should get 110 to 120 volts on each side of the switch when it’s on.

Emery Stora
77 Kingsley
Frederick, CO

>
> I couldn’t get anything on the right side. But I didn’t try real hard. I
> will re-check that tomorrow. I did pull off the BR and tried to test it. I
> was only able to get 2.7 ohms with two terminals and nothing with a
> combination of other terminals. My multi meter is not high-quality and I am
> pretty inexperienced as well, so I will re-check Everything you have asked
> with another tool tomorrow.
>
> On Tue, May 28, 2019 at 8:22 PM Mike Hamm via Gmclist <

>
>> How about voltage on the right side of the breaker?
>>

>>> I have 3volts reading on left side of breaker.
>>> I energized the wires that the BR plugs into ... nothing.
>>>
>>> Ken
>>
>>
>> --
>> 1977 Kingsley 455 as stock as it gets except lots of Ragusa parts
>>
>> _______________________________________________
>> GMCnet mailing list
>> Unsubscribe or Change List Options:
>> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
Hi Emery,
He is referring to ohms when testing his bridge rectifier.
—Jim

>
> You don’t want to read ohms. Put the meter on ac volts. You should get 110 to 120 volts on each side of the switch when it’s on.
>
> Emery Stora
> 77 Kingsley
> Frederick, CO
>

>>
>> I couldn’t get anything on the right side. But I didn’t try real hard. I
>> will re-check that tomorrow. I did pull off the BR and tried to test it. I
>> was only able to get 2.7 ohms with two terminals and nothing with a
>> combination of other terminals. My multi meter is not high-quality and I am
>> pretty inexperienced as well, so I will re-check Everything you have asked
>> with another tool tomorrow.

Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
Correct, I did have my meter on AC when testing the breaker. I also
checked an outlet in my shop first to make sure all was reading correctly.
(got 120) Then I tested breaker and it read 3.

On Wed, May 29, 2019 at 7:24 AM Jim Miller via Gmclist <

> Hi Emery,
> He is referring to ohms when testing his bridge rectifier.
> —Jim
>
> > On May 29, 2019, at 8:01 AM, Emery Stora via Gmclist <

> >
> > You don’t want to read ohms. Put the meter on ac volts. You should get
> 110 to 120 volts on each side of the switch when it’s on.
> >
> > Emery Stora
> > 77 Kingsley
> > Frederick, CO
> >
> >> On May 28, 2019, at 10:12 PM, Ken Shaffer via Gmclist <

> >>
> >> I couldn’t get anything on the right side. But I didn’t try real hard. I
> >> will re-check that tomorrow. I did pull off the BR and tried to test
> it. I
> >> was only able to get 2.7 ohms with two terminals and nothing with a
> >> combination of other terminals. My multi meter is not high-quality and
> I am
> >> pretty inexperienced as well, so I will re-check Everything you have
> asked
> >> with another tool tomorrow.
>
>
> Jim Miller
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH
>
>
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
JIm, I checked the resistance between F1 and F2 wires and got 37.6.

> The 3VAC reading is the classic indication of “no excitation”. There are
> only a few things that can cause this:
>
> 1. loss of residual magnetism in the field (thus the flashing procedure
> already described).
> 2. defective bridge rectifier
> 3. defective field assembly
> 4. defective compounding reactor (unlikely)
>
> While you have the bridge out, use your meter to check resistance between
> the F1 and F2 wires that previously attached to the bridge’s + and -
> terminals. You should read around 38 ohms.
>
> Jim Miller
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH
>
>
>
>
 
> JIm, I checked the resistance between F1 and F2 wires and got 37.6.

Ken,

You got lucky on one.
Your next best hope is the bridge rectifier has gone out. (A very common issue)
I think that the Onan stuff tells how to check it. So do that next.
In simple terms, you should find a diode (that reads low one way and high the other) between each pair of pins.
When it doesn't check out, get a new one.

Good Luck

Matt

> > The 3VAC reading is the classic indication of "no excitation". There are
> > only a few things that can cause this:
> >
> > 1. loss of residual magnetism in the field (thus the flashing procedure
> > already described).
> > 2. defective bridge rectifier
> > 3. defective field assembly
> > 4. defective compounding reactor (unlikely)
> >
> > While you have the bridge out, use your meter to check resistance between
> > the F1 and F2 wires that previously attached to the bridge's + and -
> > terminals. You should read around 38 ohms.
> >
> > Jim Miller
> > 1977 Eleganza
> > 1977 Royale
> > Hamilton, OH

--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
You are right there. Do the flash procedure and you will probably fix it. Around 38 ohms says the field is good. Flash the thing and see what
happens. If flashing does not work, then the next thing is the checking / replacing the diode bridge.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Just wanted to give a final update of this Onan problem. Like usual, you guys were right on! The bridge rectifier was the culprit. Simple fix and
works as it should!
Thank you!!!

--
Ken Shaffer
Marion Iowa
73 Canyon Lands, 455, Manny Tranny, Custom interior
 
> Just wanted to give a final update of this Onan problem. Like usual, you guys were right on! The bridge rectifier was the culprit. Simple fix
> and works as it should!
> Thank you!!!

Ken,

Thank you for closing the loop on this.
We enjoy helping owners, particularly when we don't have to do anything but tap the keyboard.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Electronically Controlled Quiet Engine Cooling Fan
OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
A point for others. Onan used a bridge with a relatively low reverse voltage rating. When you buy them by the thousands there's a savings. When
you're replacing them one at a time, buy the 1KV version. It might be a dime more, and will survive much longer.

--johnny
--
Foolish Carriage, 76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
I forgive them all, save those who hurt the dogs. They must answer to me in hell
 
The lesson here is to test the bridge early if not first in the diagnostics and before flashing the field. Also meter the field Ohms before attempting
to flash. Just a more logical approach and saves waisted steps.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
You must log in or register to reply here.
Get your shine on at www.cruzingear.co
GMCMI Spring 2026 Rally

Share this page

Share this page
Top Bottom
Back