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Dave(inMA) 
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 Topic: Magneto Points Theory Question - UPDATE 2
Posted: 05 Nov 2016 at 5:38pm |
I have a magneto for which there is zero resistance between ground and the contact where the wires to the coil and condenser attach to the points. This is the case whether the points are open or closed. I'm thinking this means that there's a short to ground elsewhere in the mag. Does this sound right?
Thanks for any input! Dave
Edited by Dave(inMA) - 15 Nov 2016 at 2:51pm
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Mrgoodwrench
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Posted: 05 Nov 2016 at 7:56pm |
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Sounds like a short to me
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There are 3 ways to do job GOOD, FAST, CHEAP. YOU MAY CHOOSE 2. If its FAST & CHEAP it won't be GOOD, if it's GOOD & CHEAP it won't be FAST, and if its GOOD & FAST it won't be CHEAP!!!!
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Dave Richards (WV)
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Posted: 05 Nov 2016 at 8:15pm |
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Remove the condenser and test again. Also check the insulator on the external grounding stud. Condensers usually open rather than short, but it can happen.
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Dave(inMA)
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Posted: 05 Nov 2016 at 9:11pm |
Thanks for the suggestions! I can check the condenser. The distributor compartment cover is made of non-conductive material so there is no insulator on the grounding stud.
I'll be away for a week so further testing will have to wait, I'm afraid. But all ideas are welcome!!
Dave
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steve(ill)
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Posted: 05 Nov 2016 at 10:56pm |
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Dave, double check that little kill screw going thru the case... most go thru the aluminum part and are insulated... the spring clip that you push down to kill the motor , shorts out to the aluminum case. ... the mount for the point, and the condenser are other good suggestions for grounding points.
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Like them all, but love the "B"s.
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Dave(inMA)
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Posted: 14 Nov 2016 at 11:48am |
Update. The black lead from the coil connects to the movable side of the points and the lead from the condenser. That connection point is isolated from ground.
I disconnected the lead from the coil and tested the resistance between the lead and ground. My VOM showed zero resistance indicating that the lead is grounded.
Does this indicate a bad coil?
Thanks!
Edited by Dave(inMA) - 14 Nov 2016 at 11:49am
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DougS
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Posted: 14 Nov 2016 at 2:46pm |
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Was your ohmmeter on the x1 scale and zeroed? A very low resistance would be normal and that is hard to read on many meters.
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Dave(inMA)
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Posted: 14 Nov 2016 at 3:02pm |
Doug, my VOM was zero'd, on the lowest scale and is digital. Perhaps the better question is to ask what the resistance should be between the black coil lead and ground?
Dave
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Gerald J.
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Posted: 14 Nov 2016 at 9:23pm |
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You are measuring the low voltage winding on the magneto coil, it probably is lower resistance than a typical ignition coil, so probably under an ohm or two. But it need not be that high to work.
Steve NJ ought to be able to answer the question of the resistance of that part of the magneto winding.
Gerald J.
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Matt (Jordan,MN)
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Posted: 14 Nov 2016 at 10:40pm |
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The resistance in the coil varies for different model or brands of mags. The Fairbanks fmj and fmx use part #r2477c coil. The primary winding ( test leads on point wire and center core bar) should be around 0.6-0.8 ohms. The secondary winding ( test leads on center core bar and higher output terminal) should be around 5500 ohms. What problems are you having with your mag?
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Tbone95
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Posted: 15 Nov 2016 at 6:30am |
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Doesn't seem to me you guys are answering the question....You're talking about the resistance of the coil, he's measuring coil to ground...??? I don't know, no expert here, maybe I'm the one misunderstanding. Just hate to see somebody get derailed troubleshooting on a misunderstanding.
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Gerald J.
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Posted: 15 Nov 2016 at 10:18am |
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He's measuring points to ground with the points open and that is measuring the magneto coil primary resistance.
Gerald J.
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Tbone95
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Posted: 15 Nov 2016 at 10:50am |
 Gerald J. wrote:
He's measuring points to ground with the points open and that is measuring the magneto coil primary resistance.
Gerald J.
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OK. I'm the one misunderstanding then. I'll shut up! 
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Dave(inMA)
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Posted: 15 Nov 2016 at 12:50pm |
Thanks, guys. I think Gerald is correct, though I have eliminated all parts except the coil (and its lead) from the test. If in fact the resistance of the primary coil is < 1 ohm, and if the test I'm describing indeed measures the resistance of the primary coil, then it's certainly possible that the "0.0" reading I'm getting is in fact something slightly greater than 0. I've spoken with Steve Barbato.....going to run another test on the mag this afternoon. Will report back.
Dave
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Dave(inMA)
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Posted: 15 Nov 2016 at 2:50pm |
Matt asked a good question. The problem I was having was that the engine wasn't firing and it appeared that the mag coil was shorted to ground, something I thought should not be the case in a good mag. Turns out my knowledge level needed work! Read on...
I spent an interesting couple of hours in the shop (and outside in the pouring rain) today: I checked the primary coil resistance to ground using a different VOM that provided a more sensitive scale....measured it at about 0.8 ohms. Then I spun the mag on the bench and saw a nice spark at the points every time they opened. SO....the mag appears to be okay (all this discussion because of VOM operator error) and the one very helpful bit of knowledge gained is that the primary coil is connected to ground and that its resistance is < 1 ohm.
I will look elsewhere for the cause of the engine not firing!
Thanks for all the input and help. Dave
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Gerald J.
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Posted: 15 Nov 2016 at 3:31pm |
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The magneto has multiple timings that have to agree to get fire to the desired plug. First the points have to close while the magnet is generating in the coil, then the points have to open before the magnet clears the coil and at the time in the crankshaft rotation appropriate for the speed. Generally just after top dead center while cranking and the impulse mechanism is kicking the magneto rotor faster than its being turned by the engine for part of a revolution, then 20 or 30 degrees before top dead center for ordinary operation. Unlike a distributor the magneto doesn't vary the advance for varying engine speed and that is something that hurts overall engine performance. Then the magneto distributor rotor that turns 1/4 the speed of the magneto drive shaft has to line up the rotor with the desired spark plug connection. That takes getting the two gears meshed correctly.
Gerald J.
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