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Allis Chalmers Discussion Forum
Show Parts for Model:

My Allis C Coils is showing a ground

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bisenberger

02-11-2018 19:25:34




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My mag coil is showing a ground when you put an Ohm meter on the lead wire and touch ground. It means my tractor won't start because the points are grounded.

Does that mean my coil is bad? I don't see much too change.




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bisenberger

02-12-2018 09:08:21




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-11-2018 19:25:34  
I disconnected all the wires from the points and checked each one to see which was giving me a ground. Only the coiled showed a ground to the case. It was a zero reading not just low resistance. Is there a way to check the coil otherwise?



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Dick L

02-12-2018 17:25:03




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-12-2018 09:08:21  
Any that I had that had a dead short stunk of burned wiring.



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Dick L

02-12-2018 14:58:27




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-12-2018 09:08:21  
If your checking from the case to the wire that fastens to the points and you get no reading the wound coil wire is open or broke somewhere. If your coil is one with the black paper covering the winding it is a common thing after this many years. I had a coil open on my one C last summer. I have yet to have one of the red molded covers open up in the winding. I am sure some have opened up for someone.

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Bob

02-12-2018 14:09:17




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-12-2018 09:08:21  
If your coil REALLY shows a "0" Ohms reading it's shorted. Simple as that.

But many Ohmmeters, if not high-quality/lab quality units are NOT real accurate at low resistance values.

And the expected value of about a half an Ohm would fit in that category, IMHO.



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teddy52food

02-12-2018 10:42:27




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-12-2018 09:08:21  
I have a coil & condenser tester that will tell if it is good or bad. I work on mags as a hobby & do it cheap if yours doesn't work.



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Dick L

02-12-2018 10:36:39




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-12-2018 09:08:21  
I check them with the magneto in a vise. With a good set of points set at .020. When you snap the impulse over the points will give a spark you can see between the points if the coil is good. That is not the bright spark you get at the plug wires. If the condenser is shorted out you will also not get a spark. This is a quick check that gives you positive knowledge of a good or bad coil. I place the magneto in a vise to check them as when snapping the impulse over while holding in my hand my reflexes tend to cause me to drop the magneto.

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Bob

02-11-2018 20:54:55




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-11-2018 19:25:34  
The mag coil primary is in parallel with the breaker points, and one end is grounded.

Even with the points "OPEN" the primary coil winding provides a ground path.

In other words, with the points "OPEN" measuring from the common (insulated) junction of the coil primary, the points, and the condenser, you will get a low resistance reading, the next thing to a "short" to ground.

If your Ohmmeter is accurate at low resistance, the reading likely will be -n the range of .4 to .8 Ohms.

What are you "seeing" with yours?

Secondary resistance is likely in the 4000 to 6000 Ohm range.

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old

02-11-2018 19:36:18




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 Re: My Allis C Coils is showing a ground in reply to bisenberger, 02-11-2018 19:25:34  
The way things work is the points ground the system and that in turn builds up the charge to make the spark. When the point open that is when the spark is let go so if your testing the coil with it in the system that means the points are closed and that the points are also good. If you slowly turn the engine over while hook up to the meter you should at least for a short time see and open but due to the impulse the meter may not keep up with the opening and closing of the points

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