Burnt Resistor on Onan Board

I like that idea. I wonder what the output of the VR is. I'm guessing around 14 VDC which is similar to the 13.5 that I calculated before that we
should be seeing. We would see another .7 drop through the on board diode if we fed it in on pin 8.

That gives me another idea that I picked up from Colonel Ken at Bean Station last week on another project. He used two diodes out of a packaged full
wave bridge to give a 1.4 volt drop to slow down his wipers. If we were to insert a full wave bridge and only use 2 of them the drop might stay more
predictable.

I have changed the series resistor (R1) values in the past on Onan boards but their voltage drop varies with the change of current and relay coil
values. This solution would stay the same and could be adjusted by using 1 or 2 of the diodes out of the package.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Ken,

You raise an interesting point...

Does this resister ever burn on a unit that is still using the VR for the start battery. Maybe that part is clamping the flywheel alternator spike.

Matt
--
Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
Still Loving OE Rear Drum Brakes with Applied Control Arms
SE Michigan - Twixt A2 and Detroit
 
> Does this resister ever burn on a unit that is still using the VR for the start battery. Maybe that part is clamping the flywheel alternator spike.

I’ve speculated the same thing Matt.

BTW, those who mentioned the capacitor in this portion of the circuit… this circuit has a time constant characteristic that prevents the relay from latching too soon. In addition to latching power to the fuel solenoid, ignition and fuel pump - this relay also serves to DISengage the starter motor. If the cap weren’t there the relay could pick up too soon - and disengage the starter before the engine reached self-sustain speed.

During my development of “the fix” I experimented with various resistors in series with the relay coil. In each case I could not keep the relay coil current within specifications while at the same time keeping the time constant behavior correct. The 12V relay would kick in too soon - or too late - but never just right. Finally I changed to the 24V relay and temporarily put a pot in place of the resistor and experimented with various amounts of resistance until I got it right.

YMMV of course.

I would encourage those who are speculating on higher wattage resistors and other solutions to this problem to review the behavior I’ve described above and factor it into their design. There’s more to this circuit than meets the eye.

—Jim N8ECI


Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
I too have thought about that except I have had my VR disconnected for 10 or 15 years with no problem. I also have had an unmodified Dino board for
many years.

I measured George's AC output at BS and it was 31.5. When I get a chance I'll measure mine to see what it is. I'll also measure Jon's at the work
rally next week.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
All good ideas but lots of guessing and speculation without a picture of the waveform under start, run and shut down conditions. What if as the
flywheel slows the waveform clips and the square wave heating takes out the resistor? Again just guessing. Or with a sticky governor how much
overspeed happens when roof AC kicks on and RPM drops then overshoots. Or when roof AC kicks off and RPM overshoots before correcting and settling. I
have less belief this is it as this can really only be less than 10% unless the circuit design is so on the edge this puts it over.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
But wait, could it just be that the components are electrical in nature,
and are 40 years old, live in a vibration environment, are subjected to
water, tire dust, etc? Just saying.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

> All good ideas but lots of guessing and speculation without a picture of
> the waveform under start, run and shut down conditions. What if as the
> flywheel slows the waveform clips and the square wave heating takes out
> the resistor? Again just guessing. Or with a sticky governor how much
> overspeed happens when roof AC kicks on and RPM drops then overshoots. Or
> when roof AC kicks off and RPM overshoots before correcting and settling. I
> have less belief this is it as this can really only be less than 10%
> unless the circuit design is so on the edge this puts it over.
> --
> John Lebetski
> Woodstock, IL
> 77 Eleganza II
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
> But wait, could it just be that the components are electrical in nature, and are 40 years old, live in a vibration environment, are subjected to
> water, tire dust, etc?
> Just saying.
>
> Jim Hupy
> Salem, Or
> 78 GMC ROYALE 403
No. It happens on Dinosaur boards that are relatively new (2 - 10 years old).
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"Every day I become more convinced that I am the only person left on the planet that recognizes nonsense for what it is."
 
John/All,

The event for me happened while it was running. It was running fine and then just quit. Note that there was less then 5 hours of run time on this
board.

Jon
--
Jon Payne
76 Palm Beach
Westfield,IN
 
Jim, dust and dirt aren't a great problem because the original boards are conformal coated. Vibration does however take its toll, as does heat. The
caps dry out, and the original glass envelope diodes break in half. I find about half the boards I see heal up with replacement of all the caps and
diodes. Those which have looked at a shorted off - board lead sometimes need circuit runs replaced. Those which have been overfused (5 amps maximum
please) often have grevous damage to the printed wiring. Shorted leads being about the second or third failure mode after caps/diodes and failed
connections.

--johnny

--
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" - ol Andy, paraphrased
 
At the wor rally, I would like to start up your Onan with a jumper on it so it will run and then read the alternator output voltage.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
When I see a destroyed component from overcurrent, I look for a frayed wire or something touching which shouldn't. Ken's analysis indicates that the
resistor as supplied is more than sufficient for the task. Go looking for what's getting it out of its comfort zone. More than once I've seen
disconnected wires not properly insulated. Heatshrink and Ty-Raps are Your Friend when you disconnect something.

--johnny
--
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" - ol Andy, paraphrased
 
> At the wor rally, I would like to start up your Onan with a jumper on it so it will run and then read the alternator output voltage.

I suggest that while you are at it you take measurements of both the AC and DC component of the voltage across the relay coil. The AC component of the half-wave rectification results in additional heat dissipation in the relay coil that is not part of the coil’s published DC specifications. If you look at the pictures on my photo album you’ll see two relay coils removed from dinosaur boards that have their insulation melted to the point that what was originally 270 ohms became 15 ohms. All the P=IE figuring that is being done on the resistor’s power dissipation go out the window as the relay coil resistance starts dropping - especially by an order of magnitude.

Coil picture: http://www.jcmco.com/gallery/album30/aad

—73, Jim N8ECI


Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
 
>
> > At the wor rally, I would like to start up your Onan with a jumper on it so it will run and then read the alternator output voltage.
>
> I suggest that while you are at it you take measurements of both the AC and DC component of the voltage across the relay coil. The AC component
> of the half-wave rectification results in additional heat dissipation in the relay coil that is not part of the coil's published DC specifications.
> If you look at the pictures on my photo album you'll see two relay coils removed from dinosaur boards that have their insulation melted to the point
> that what was originally 270 ohms became 15 ohms. All the P=IE figuring that is being done on the resistor's power dissipation go out the window as
> the relay coil resistance starts dropping - especially by an order of magnitude.
>
> Coil picture: http://www.jcmco.com/gallery/album30/aad
>
> --73, Jim N8ECI
>
>
> Jim Miller
> 1977 Eleganza
> 1977 Royale
> Hamilton, OH

As an after thought I though about measuring the voltage across the relay coil on George's. Then it started raining and I never got around to it. If
we do not repair Jon's board, I will not be able to measure his. I'll see what I can do. I guess we could measure mine but I do not know what that
will tell us since mine has never failed.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
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