Ignition Coil Failure?? How to know???

Hello again. Can an ignition coil for a 74 850 Commando fail partially and just cause misfire at a certain RPM or is failure always catastrophic ??? What should the resistance of a coil be in ohms>>>What is the best way to test for a failed or failing coil??? Thanks Mark Cigainero

I have only had them fail totally but that can be temporary due to heat build up, any intermittant faults at certain revs have been elsewhere eg low tension circuit on points and battery voltage on boyers. Failed coils were always hotter than normal to touch and smelt burnt, the intermittant one ran for 100 yds then stopped completely and needed ignition turned off for 5 mins before I got another 100 yrds, but at least it got me home. You should be getting 3.5 ohms as a resistance.

Mark Cigainero said:

Hello again. Can an ignition coil for a 74 850 Commando fail partially and just cause misfire at a certain RPM or is failure always catastrophic ??? What should the resistance of a coil be in ohms>>>What is the best way to test for a failed or failing coil??? Thanks Mark Cigainero

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Boyer?

Stock Lucas 6V coils are 2 ohms each. Most common failures are catastrophic, but vibration at some particular RPM could certainly induce strange effects. The best troubleshooting is to substitute a known good coil. That's why we all have spares.

kommando said:

You should be getting 3.5 ohms as a resistance.

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The primary resistance of a standard 6V 17M6 coil, as fitted from 1971-on should be around 1.7-1.9 Ohms. A 12V coil would be 3.3-3.8 Ohms.

Thanks--I just ordered 2 new coils. The ones on my bike are over 35 years old anyway. I used to have a good spare but my buddy already swiped it for his Commando last year. What have I got to lose. I also ordered a new ignition switch. I will try it with new coils this weekend and get back with results. It is finally warming and drying out enough down here to test ride again. I tested my Pazon and it is getting a good 7mm spark on a gap tester like they said it should at the factory. They are adding a warning to never start the bike with a plug wire just pulled. It must BE GROUNDED. It could damage the Electronic ignition. If coils does not fix it I guess it is time for a new Pazon---Do,'t know what else to do now except throw $$$ at the damn thing. Anyway I appreciate the help--Mark Cigainero

Mark, just as a precaution - I have seen several after market ignition switches fail due to shoddy quality. Yes, this is more crap made in India for British "classic parts" distributors. Have a really good look at the unit you buy and probably worth speaking to your retailer about what he knows. I can tell you British Spares NZ have a bunch of them they won't sell and won' t re-stock due to the failure rate.

Mick

Thanks for the tip on the Ignition switch. I am geting it from MAP in Florida. They have a good reputation but regardless of that fact I still keep a jumper cheat sheat and the connectors to do it in my side cover just in case. Like you I am really sick and tired of shoddy merchandise. --Mark Cigainero

Mark Cigainero said:

Hello again. Can an ignition coil for a 74 850 Commando fail partially and just cause misfire at a certain RPM or is failure always catastrophic ??? What should the resistance of a coil be in ohms>>>What is the best way to test for a failed or failing coil??? Thanks Mark Cigainero

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Hi Mark,

Not sure why you suspect the coil, but the symptom of backfire at certain RPM can also be due to the low tension leads of a Boyer ignition. If you have a Boyer, the lead breaks inside it's insulation due to vibration and it's impossible to see. With the engine running pull on either wire. If the engine starts misfiring, you know what's causing it.