Correct Coil Resistance I have been chasing an intermittent stumbling of my engine when under load and have replaced or checked out all ignition components and am now to the coil. I got this diagram from some authoritative source and have always suggested it to others. Primary coil is in the range of the diagram. But when I just checked the resistance of my secondary coil in my stock 47 Ford coil that Skip Haney rebuilt I only get 6,130 ohms, not the 10-11,000 ohms per the diagram. Is that low enough to affect the spark or not? Is the coil bad and needs replaced or just marginal?
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Re: Correct Coil Resistance Call Skip.
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Re: Correct Coil Resistance Henry, I chased a miss or skip as you call it on the 33 for ages,turned out to be two loose valve guides and those two let oil into the cyl and fouled the plugs ,ran like a train while burning along at 50mph but slow and speed up and the miss was there.
Lawrie |
Re: Correct Coil Resistance So far I've replaced all of the spark plugs, the points, condenser, rotor, and coil resistor. All I can think of that's left is the coil so I'll call Skip tomorrow and see what he thinks. I'll also PM Bubbas Ignition since he hasn't commented to see what he thinks of the low resistance secondary coil.
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Re: Correct Coil Resistance Using the resistance of the coil Sec./Pri. will give you the approx. turns ratio. For both of the pictured examples it is about 13,500/1 . If Skips coil has a primary resistance of 0.5 Ohm you wind up with 12,300/1 . With 3 to 4 volt input, that's an approx. max. output of about 35,000V should be hot enough for a flat head.
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Re: Correct Coil Resistance Henry, Is this the same problem that you said leaving the hood open a bit while moving helped some?
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Right now I'm driving with my timing light taped to the cowl so I can see its flashes through the windshield while I drive. The pickup is on the high tension wire coming out of the coil. I mainly did this to confirm that it is a spark problem, not a fuel problem. Sure enough, when the engine stumbles so does the light flashes. http://i1059.photobucket.com/albums/...pskf7ol2oy.jpg |
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To be able to calculate output voltage you need the turns ration of the coil. |
Re: Correct Coil Resistance Sounds like a loose wire or connection unless it is heat related. Or a vacuum / carb issue on the advance. Are you running the original vacuum only advance distributor?
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It certainly acts like a loose wire so I bypassed all wiring from the battery to the coil with a jumper from the battery to the coil. Didn't help. |
Re: Correct Coil Resistance Check the spring loaded contact in the distributor if its a crab ,Ted
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Any '47 distributor (rabbit ears or crab) can have that problem. |
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Re: Correct Coil Resistance I think I figured out what you're talking about - the terminal that the wire from the coil connects to that has a spring loaded post that pushes against the points plate? If so, I can take that out and check it. I just removed that plate to replace my points and twisted the plate against that post to position it so hopefully it's OK. Distributor was built by Bubba a year or two ago.
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Re: Correct Coil Resistance Yes, it is the post for the coil wire.
The only reason I found the problem is that it finally quit totally and I found it was open circuit between the coil wire screw and the inside brass plate. Unscrew the post and then polish the end of the spring loaded pin with emery cloth. You can probably clean up the brass plate with emery cloth too without taking the distributor apart again. My distributor was serviced by Bubba too and he cleans and checks that connection. But for some reason it still failed. See more (and Jim L.'s response) at: https://www.fordbarn.com/forum/showth...52#post1152852 |
Re: Correct Coil Resistance Looking at your distributor photo again, it looks like that pin is way off to one side and hardly making contact. Perhaps the photo is of a different distributor?
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