Correct Coil Resistance

Old Henry

Old Henry

Well-known member
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?

ignition_coil2.gif
 
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
 
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.
 
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.
 
Terry said:
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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Thanks. Bubba also said it was OK. So, I'll put the coil back in and continue looking elsewhere. Sure hope I can fix it before my 5 day trip on the Pacific Coast Highway next week.
 
Henry, Is this the same problem that you said leaving the hood open a bit while moving helped some?
 
keith oh said:
Henry, Is this the same problem that you said leaving the hood open a bit while moving helped some?
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Not really. It is a very minor form of that same problem that has plagued me for many months. It never stops me dead like the other problem did, just "flickers" or "stumbles" with a slight loss of power with hard acceleration or hill climb. And the worst part, of course, is that it is intermittent, not always, just once in a while. That's what makes it hard to diagnose and fix. Fortunately, most of the time when I'm just going along at any speed with normal throttle it is either hardly noticeable or doesn't exist.

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.

DSC02442_zpskf7ol2oy.jpg
 
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Terry said:
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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Pri/sec resistance doesnt give you the secondary voltage.
To be able to calculate output voltage you need the turns ration of the coil.
 
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?
 
JSeery said:
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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Yes, stock distributor with centrifugal advance retarded by vacuum brake.

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.
 
FlatheadTed said:
Check the spring loaded contact in the distributor if its a crab ,Ted
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That spring loaded contact was causing problems for me. Took it out and cleaned both halves of the connection.

Any '47 distributor (rabbit ears or crab) can have that problem.
 
47COE said:
That spring loaded contact was causing problems for me. Took it out and cleaned both halves of the connection.

Any '47 distributor (rabbit ears or crab) can have that problem.
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FlatheadTed said:
Check the spring loaded contact in the distributor if its a crab ,Ted
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I guess I'm not sure what spring y'all are talking about. Is it the strip spring that pushes the contact points together that I just replaced with the contact points? Isn't that the only spring in my distributor other than the one in the vacuum brake. This is the kind of distributor I have:

Distributor2.jpg
 
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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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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:

http://www.fordbarn.com/forum/showthread.php?p=1152852#post1152852
 
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?
 
47COE said:
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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That is an old photo of my original distributor that was worn out, not the one Jim built for me. It isn't even all together. You won't find the retaining ring around the points plate. That was the one Jim got in exchange for the one he sent me.
 
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