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| About ready to kill the Onan....... [message #227429] |
Mon, 28 October 2013 17:42  |
Otterwan
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So I have an Onan that runs great but doesn't make AC. Pulled it and found the field coil was bad. Located a good field coil, installed it, checked the rectifier, the breaker, etc, hooked it up the coach and fired it up, no AC! What could I be missing?
The only thing that isn't hooked up is the pigtail that goes to the coach controls, but that shouldn't have any effect on AC output. Also, where do you measure AC output at the Onan? I've measured from both sides of the breaker to ground (the Onan case) and it only reads about 2.5 volts AC.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: About ready to kill the Onan....... [message #227433 is a reply to message #227429] |
Mon, 28 October 2013 18:11  |
A Hamilto
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| Otterwan wrote on Mon, 28 October 2013 17:42 |
So I have an Onan that runs great but doesn't make AC. Pulled it and found the field coil was bad. Located a good field coil, installed it, checked the rectifier, the breaker, etc, hooked it up the coach and fired it up, no AC! What could I be missing?
The only thing that isn't hooked up is the pigtail that goes to the coach controls, but that shouldn't have any effect on AC output. Also, where do you measure AC output at the Onan? I've measured from both sides of the breaker to ground (the Onan case) and it only reads about 2.5 volts AC.
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Shorted or open armature? Brushes not making contact with the armature? Connections to the brushes loose/corroded?
"North/South" orientation of a field winding reversed?
Jim Miller, where are you?
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| Re: [GMCnet] About ready to kill the Onan....... [message #227436 is a reply to message #227429] |
Mon, 28 October 2013 18:13 |
Jim Miller
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On Oct 28, 2013, at 6:42 PM, David Orders wrote:
> Located a good field coil, installed it, checked the rectifier, the breaker, etc, hooked it up the coach and fired it up, no AC!
> The only thing that isn't hooked up is the pigtail that goes to the coach controls, but that shouldn't have any effect on AC output. Also, where do you measure AC output at the Onan? I've measured from both sides of the breaker to ground (the Onan case) and it only reads about 2.5 volts AC.
Did you check the resistance of your field after you installed the replacement unit?
2-3 VAC on the output is the classic indication of a bad field. That 2.5VAC is seen because of the remanent magnetic field in the assembly.
Did you check the armature while you had it apart?
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227439 is a reply to message #227436] |
Mon, 28 October 2013 18:25 |
Otterwan
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Jim - 37.8 ohms across the field coil terminals, checked before and after installation. Armature reads as a short, or about half an ohm. Checked the rectifier according the manual instructions and it read about 6 ohms where it should and nothing were it should.
I know my control board is bad, as I am jumping 9 to 11 to start it, but as I understand it the board has nothing to do with AC output. Am I measure the AC output at the Onan correctly?
I guess the next step is to check the connections to the field coil and rectifier and make sure nothing came loose.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227441 is a reply to message #227439] |
Mon, 28 October 2013 18:31 |
Jim Miller
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On Oct 28, 2013, at 7:25 PM, David Orders wrote:
> Jim - 37.8 ohms across the field coil terminals, checked before and after installation.
Great.
> Armature reads as a short, or about half an ohm.
Measuring resistance of the armature using an ohmmeter is not a defined test; the manual describes the use of a 120V light bulb. I have personally seen an armature that tested very close to a short but was defective - it was shorted to the shaft. Onan recommended the light bulb test for a reason.
> Checked the rectifier according the manual instructions and it read about 6 ohms where it should and nothing were it should.
OK.
> I know my control board is bad, as I am jumping 9 to 11 to start it, but as I understand it the board has nothing to do with AC output.
That is correct; the control board has nothing to do with it.
> Am I measure the AC output at the Onan correctly?
Unless you have the leads loose at the generator you can just test inside the coach.
> I guess the next step is to check the connections to the field coil and rectifier and make sure nothing came loose.
Is it possible you have F1 and F2 reversed where they attach to the bridge?
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227443 is a reply to message #227441] |
Mon, 28 October 2013 18:49 |
Otterwan
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Jim - I did check from each armature ring to the shaft and there was no continuity.
I tried to be careful but there is a chance I mixed up F1 and F2 as they are not marked on the field coil wires, only on the rectifier wires that I could see. Would it hurt anything to just switch them and see what happens?
I am using an original rectifier, the black plastic ones, as I have several and they tested good. I do also have a couple of the newer metal case ones I could try.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227447 is a reply to message #227443] |
Mon, 28 October 2013 18:55 |
Jim Miller
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On Oct 28, 2013, at 7:49 PM, David Orders wrote:
> I tried to be careful but there is a chance I mixed up F1 and F2 as they are not marked on the field coil wires, only on the rectifier wires that I could see. Would it hurt
> anything to just switch them and see what happens?
You can swap them, it will not hurt anything. Normally F1 goes to + and F2 to - however if someone has traded the pigtails then it would be reversed.
> I am using an original rectifier, the black plastic ones, as I have several and they tested good. I do also have a couple of the newer metal case ones I could try.
I am not a big fan of a static ohmmeter test of bridges as I have seen them fail in insidious ways...such as testing good with a meter but then falling over when meaningful amounts of current are passed through them. Or testing good at room temperature and then failing at slightly increased temps. Semiconductors can do funny things.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227448 is a reply to message #227447] |
Mon, 28 October 2013 19:02 |
Otterwan
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Thanks Jim. I'll check for bad connections first, then try switching the field coil wires. Cross your fingers!
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227449 is a reply to message #227448] |
Mon, 28 October 2013 19:08 |
Jim Miller
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On Oct 28, 2013, at 8:02 PM, David Orders wrote:
> Thanks Jim. I'll check for bad connections first, then try switching the field coil wires. Cross your fingers!
Double check your field resistance again while you have the wires loose. Just to make double sure. :)
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227452 is a reply to message #227449] |
Mon, 28 October 2013 19:16 |
Otterwan
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If something happened to the field coil during the 50 foot trip from the bench to the coach, the Onan's new name will be "anchor"!
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227538 is a reply to message #227528] |
Tue, 29 October 2013 07:16 |
Ken Henderson
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Larry,
I haven't mentioned that since I knew I'd seen it in the manual and figured
David knew of that possibility. Now, I tried to find a page reference.
Turns out, the entry in my manual is a marginal note. It says to flash
the field, using a jumper from +12 VDC to the F1 terminal on the bridge
rectifier. The jumper includes series 20 Ohm resistor and 6A diode
(cathode to F1). That's exactly what was often needed with the old
generators you mention -- except that we weren't so sophisticated about it
-- just momentarily touched a piece of haywire between the battery and the
field terminal. :-)
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL
www.gmcwipersetc.com
On Tue, Oct 29, 2013 at 4:08 AM, Larry Davick <ljdavick@comcast.net> wrote:
> Is there anything about the Onan that has to be “energized” or otherwise
> hooked up to power before it makes juice? Am I confusing this with the
> very old 6 volt generator in my friend’s Fraizier?
>
> Larry Davick
> A Mystery Machine
> 1976(ish) Palm Beach
> Fremont, CA
>
>
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] About ready to kill the Onan....... [message #227540 is a reply to message #227538] |
Tue, 29 October 2013 07:35 |
C Boyd
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I also had thought of the old days when we had to flash the field coils on Harley generators to polarize. This however is on a DC system and the Onan output is AC and should not have to in my thinking.
C. Boyd
76 Crestmont
East Tennessee
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| Re: [GMCnet] About ready to kill the Onan....... [message #227547 is a reply to message #227528] |
Tue, 29 October 2013 08:36 |
Jim Miller
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On Oct 29, 2013, at 4:08 AM, Larry Davick wrote:
> Is there anything about the Onan that has to be “energized” or otherwise hooked up to power before it makes juice
No. Remanent (or "residual") magnetism in the field will cause the armature to produce enough AC output to allow the excitation circuit to bootstrap itself up to full power.
I have seen the references to flashing the field with a battery and of the many Onans we've worked on at our 6 work rallies as well as all that we examined while developing a method of repairing the fields, not one of them has needed a re-flash using that procedure.
With regard to excitation this is what I have learned from working on these machines:
1) a unit with a failed reactor, defective bridge or open field coil will produce around 2-3VAC; this results from passive excitation due to the remanent magnetism in the field's iron laminations. This is measured with NO LOAD on the output terminals.
2) a unit with a functional excitation system and good fields but with reversed F1 and F2 will produce 0 VAC. Why? As soon as the armature starts rotating the remanent magnetism starts producing AC output - which energizes the excitation system - which, due to the F1/F2 reversal produces the opposite flux from the coils which immediately cancels the remanent flux in the iron. Once that happens there is *no* magnetism remaining in the assembly - meaning *no* excitation - resulting in 0 VAC output. Once the F1/F2 situation is remedied the unit will work again as the cancelling flux is not enough to demagnetize the iron.
In the process of repairing the fields we check our coil polarity (North or South) by first using a compass to measure the polarity of the pole followed by energizing the coil on that pole with a 12V battery to make sure we match the remanent polarity.
It is possible that David has something wrong in his excitation system - reactor, thyrector, bridge rectifier or even a wiring error. Reversing F1 and F2 as I suggested is an easy thing to check before delving into more complicated troubleshooting.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227548 is a reply to message #227547] |
Tue, 29 October 2013 08:43 |
Jim Miller
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One additional thing for David to check is the wiring from the brush holders to the reactor. If one or more of these wires is disconnected or on the wrong brush then there won't be any active excitation and you'll get the usual 2-3VAC from the magnetism in the iron.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227611 is a reply to message #227548] |
Tue, 29 October 2013 14:53 |
Otterwan
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OK - Rechecked field coil resistance, still ~37 ohms
Double checked diode connections
Switched F1 and F2 leads
Tried a different (new style metal case) diode, checked it first according to the manual.
Switched F1 and F2 back
Still only getting ~2.7 volts AC between the breaker on the Onan and the Onan case.
Unless someone has any more ideas it looks like it's back to the workbench with it.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227617 is a reply to message #227611] |
Tue, 29 October 2013 15:32 |
Jim Miller
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On Oct 29, 2013, at 3:53 PM, David Orders wrote:
> Switched F1 and F2 leads
What sort of output did you get when you switched F1 and F2? And when you tried the different bridge rectifier?
Any chance the black or yellow wires on the brush holders got swapped around or disconnected during your field swap?
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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1977 Royale
Hamilton, OH
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| Re: [GMCnet] About ready to kill the Onan....... [message #227618 is a reply to message #227617] |
Tue, 29 October 2013 15:39 |
Otterwan
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| Jim Miller wrote on Tue, 29 October 2013 13:32 |
On Oct 29, 2013, at 3:53 PM, David Orders wrote:
> Switched F1 and F2 leads
What sort of output did you get when you switched F1 and F2? And when you tried the different bridge rectifier?
Any chance the black or yellow wires on the brush holders got swapped around or disconnected during your field swap?
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Exactly the same output regardless of rectifier or F1/F2 connection. Always about 2.7 VAC
I didn't remove the brushes when I took the bell housing off, just moved them out of the way.
I'm going to put it back on the bench. I can't think of anything else to try, other than maybe a different bell housing assembly.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] About ready to kill the Onan....... [message #227621 is a reply to message #227618] |
Tue, 29 October 2013 16:16 |
Jim Miller
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On Oct 29, 2013, at 4:39 PM, David Orders wrote:
> Exactly the same output regardless of rectifier or F1/F2 connection. Always about 2.7 VAC
> I didn't remove the brushes when I took the bell housing off, just moved them out of the way.
> I'm going to put it back on the bench. I can't think of anything else to try, other than maybe a different bell housing assembly.
Check all the wires to the brush holders to make sure none of them came loose during your bell housing work. I just put a bell housing on a unit last night and a wire came off a brush holder in the process.
If they are all still securely attached then start the engine and measure the AC voltage going to the AC terminals of the bridge. It might be upwards of 100 volts so don't get your fingers against anything.
--Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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Jim Miller
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1977 Royale
Hamilton, OH
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