| [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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