| [GMCnet] Onan+Pertronix observations and experiments [message #337565] |
Tue, 02 October 2018 20:40  |
Jim Miller
Messages: 501
Registered: March 2008
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Over the past few months I have been working on alternative ignition designs for our Onan generators. During my tenure as a GMC owner I have suffered the failure of two Pertronix 1181 modules mounted in the blower housing scroll - one due to a fatigue failure of the module’s aluminum mounting tab due to wind buffeting - and the other due to a flywheel strike where the flywheel magnet somehow impacted the module’s face for at least several revolutions. These two events occurred with super-rigid mounting arrangements that I made myself - I was NOT using the C-channel mounting kit commonly found in the GMC community. Photo albums of these two:
http://www.jcmco.com/gallery/Pertronix and http://www.jcmco.com/gallery/PertronixModII
So I decided to experiment with something different…I mounted the module on the outside of the generator’s bellhousing and have it being triggered by a magnet embedded in an aluminum tonewheel:
http://www.jcmco.com/gallery/Pertronix-ModIII
This produced very good results - nearly identical to when the module is mounted inside the blower scroll and triggered by a flywheel magnet. However, during testing I made the observation that the ignition coil was running very hot and was uncomfortable to touch even after very short amounts of runtime. I determined that the Pertronix 1181 module that people have recommended for our Onans was keeping the coil energized for about 355 degrees of rotation of the crankshaft - and only dropped its output for a few milliseconds as the magnet went by the module at which time the spark plugs fired. This is contrasted with the 140 degrees per revolution which the OEM breaker points used as a coil dwell time. In other words, the Pertronix 1181 causes current to flow in the coil primary for almost three times as long as the original Onan design. No wonder the coil is getting so hot… once the magnetic field builds up in the core of the coil the remaining current flow produces nothing but heat. I wrote to Pertronix tech support for advice and they confirmed that the 1181 is not the right module for this application. Their words:
"You really should be using what we call a latching Ignitor module. This module takes two magnets to trigger the unit. One to turn it "ON" then one to turn it "OFF" to fire. This allows you to control the charge time of the coil. We use this module on all our applications that have less then 4 cylinders. This is so we don't overheat the coil and don't over heat the Ignitor module. … The part number I would recommend to modify would be part number: 1121”
At that point I bought an 1121 and modified my mount and magnet wheel for it: http://www.jcmco.com/gallery/PertronixModIV
The 1121 module has been in service on my Onan for 20 hours of runtime so far and has worked flawlessly. Just as with the 1181 module, the 1121 produces repeatable and precise spark timing but - unlike the 1181 - the 1121 produces no appreciable heating of the ignition coil. Having worked 35 years in an electronics career I know quite well that heat is the enemy of all electronic devices so I’m happy to have my coil running at nearly ambient temperature rather than around 140 degrees as when it was being fired by the 1181.
—Jim
Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan+Pertronix observations and experiments [message #337570 is a reply to message #337567] |
Tue, 02 October 2018 22:00  |
Jim Miller
Messages: 501
Registered: March 2008
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On Oct 2, 2018, at 10:16 PM, Bruce Hislop wrote:
> I wondered when I installed my Pertronics unit if it was ON for most of the flywheel revolution. I haven't yet had the time to hook my 'scope to it, thanks for clarifying this.
Hi Bruce,
Leave the ’scope on your lab bench and instead just put a simple bulb between +12 and the Pertronix trigger line and then rotate the engine by hand. The 1181 is an “Ignitor I” which is a simple on/off trigger. Other Pertronix modules have more brains in them and cannot be statically timed - but our 1181 is not one of those.
The 1181 was designed for an 8-cylinder application so there would have been a magnet coming by the module every 45 degrees of distributor rotation; this would result in a completely acceptable dwell time for an OEM replacement application. Using the 1181 on the Onan with a single flywheel magnet, on the other hand, makes for an eternity of charge time with the magnet coming by only every 360 degrees.
—Jim
Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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| Re: [GMCnet] Onan+Pertronix observations and experiments [message #337574 is a reply to message #337565] |
Wed, 03 October 2018 06:13 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
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Nice work, Jim!
A year or so after Lawrence Gaskins first presented the Pertronix mod, I
considered making a kit, so contacted Pertronix. The engineer with whom I
spoke was surprised that we weren't having dwell time problems with the
1181 module and suggested the one for VWs instead -- he didn't mention the
1121. Since I decided against undertaking the project, I never pursued it
further. It's great that you've done so; I'm sure everyone will have
better results with your design.
Ken H.
On Tue, Oct 2, 2018 at 9:41 PM Jim Miller wrote:
> Over the past few months I have been working on alternative ignition
> designs for our Onan generators. During my tenure as a GMC owner I have
> suffered the failure of two Pertronix 1181 modules mounted in the blower
> housing scroll - one due to a fatigue failure of the module’s aluminum
> mounting tab due to wind buffeting - and the other due to a flywheel strike
> where the flywheel magnet somehow impacted the module’s face for at least
> several revolutions. These two events occurred with super-rigid mounting
> arrangements that I made myself - I was NOT using the C-channel mounting
> kit commonly found in the GMC community. Photo albums of these two:
>
> http://www.jcmco.com/gallery/Pertronix http://www.jcmco.com/gallery/Pertronix> and
> http://www.jcmco.com/gallery/PertronixModII http://www.jcmco.com/gallery/PertronixModII>
>
> So I decided to experiment with something different…I mounted the module
> on the outside of the generator’s bellhousing and have it being triggered
> by a magnet embedded in an aluminum tonewheel:
>
> http://www.jcmco.com/gallery/Pertronix-ModIII http://www.jcmco.com/gallery/Pertronix-ModIII>
>
> This produced very good results - nearly identical to when the module is
> mounted inside the blower scroll and triggered by a flywheel magnet.
> However, during testing I made the observation that the ignition coil was
> running very hot and was uncomfortable to touch even after very short
> amounts of runtime. I determined that the Pertronix 1181 module that people
> have recommended for our Onans was keeping the coil energized for about 355
> degrees of rotation of the crankshaft - and only dropped its output for a
> few milliseconds as the magnet went by the module at which time the spark
> plugs fired. This is contrasted with the 140 degrees per revolution which
> the OEM breaker points used as a coil dwell time. In other words, the
> Pertronix 1181 causes current to flow in the coil primary for almost three
> times as long as the original Onan design. No wonder the coil is getting so
> hot… once the magnetic field builds up in the core of the coil the
> remaining current flow produces nothing but heat. I wrote to Pertronix tech
> support for advice and they confirmed that the 1181 is not the right module
> for this application. Their words:
>
> "You really should be using what we call a latching Ignitor module. This
> module takes two magnets to trigger the unit. One to turn it "ON" then one
> to turn it "OFF" to fire. This allows you to control the charge time of the
> coil. We use this module on all our applications that have less then 4
> cylinders. This is so we don't overheat the coil and don't over heat the
> Ignitor module. … The part number I would recommend to modify would be part
> number: 1121”
>
> At that point I bought an 1121 and modified my mount and magnet wheel for
> it: http://www.jcmco.com/gallery/PertronixModIV http://www.jcmco.com/gallery/PertronixModIV>
>
> The 1121 module has been in service on my Onan for 20 hours of runtime so
> far and has worked flawlessly. Just as with the 1181 module, the 1121
> produces repeatable and precise spark timing but - unlike the 1181 - the
> 1121 produces no appreciable heating of the ignition coil. Having worked 35
> years in an electronics career I know quite well that heat is the enemy of
> all electronic devices so I’m happy to have my coil running at nearly
> ambient temperature rather than around 140 degrees as when it was being
> fired by the 1181.
>
> —Jim
>
> 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
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Onan+Pertronix observations and experiments [message #337575 is a reply to message #337571] |
Wed, 03 October 2018 06:20 |
Jim Miller
Messages: 501
Registered: March 2008
Karma: 10
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Senior Member |
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On Oct 2, 2018, at 11:58 PM, Steve Weinstock wrote:
> I recently installed a Pertronix ignition system (most likely an 1181) on my Onan 4K... looks like some rework is at hand. If I continue to use the C channel mounting method - I'll have to change the unit and then add a second magnet to the flywheel - correct ?? Will I have to relocate the existing flywheel magnet to a different vane ?? And how far away should the second magnet be ??
Hi Steve,
The 1121 is a bit smaller than the 1181 but has a wider, more substantial mounting bracket. I’ve never seen one of those C-Channel mounts in person so I can’t say if how it will fit up.
You would use the existing magnet location for the “turn off” magnet but you’ll need to add another one 140 degrees ahead for the “turn on” point. Note that the magnetic poles are different - the “turn on” magnet needs to have its North pole oriented toward the module and the “turn off” magnet has its South pole facing the module. You can determine which pole is which by hooking up the module on the bench with a 12V test lamp and swiping the magnets in front of it.
I never particularly liked the flywheel as a location for timing magnets due to the distortion of the magnetic field caused by the ferrous material in the flywheel itself - this distortion causes the pertronix to have to be dangerously close to the moving wheel in order to get reliable sensing. If I were going to use the flywheel vanes as magnet mounts I think it would be a good idea to cut down a little of the vane on each side of the magnet and then have the magnet stand proud a bit.
Before you go making changes to your working system you might want to wait for the GMCnet peer review of my original post to have taken place…just in case I’m missing something!
—Jim
Jim Miller
1977 Eleganza
1977 Royale
Hamilton, OH
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Jim Miller
1977 Eleganza II
1977 Royale
Hamilton, OH
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