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Allis Chalmers Discussion Forum

Ohms For a coil

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Author  [Modern View]

05-09-2010 08:51:13

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I have a problem with eating a lot of points on my D14. It was a 12V conversion.
My multimeter says it is 4.1 Is this a good or bad
Most one says it should be 3.3 So is this to high and a resister will fix it or just go with a new coil

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John T

05-10-2010 04:29:52

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 08:51:13  
Im pretty well in camp with Bob and Old on this one. Iffffffff that coils primary is really 4 ohms (low ohms and a cheap meter dont mix well), thats NOT the problem as Id expect it to be in the range of say 2.5 to 3.5 or so ohms if its really a "12 volt" coil. But 4 ohms isnt out of the ball park IN WHICH CASE THE POINTS WOULD ACTUALLY LAST LONGER as the Bobster notes since they then pass even less current. i.e. a 4 ohm coil would be easier on points then a 2 ohm as more ohms = less current flow.

Now if it were like 1.25 to 2 ohms then Id say theres a good chance its a 6 volt coil in which case the points are switching too much current causing their premature burning. If it was a 6 to 12 volt conversion then EITHER an external series voltage dropping (12 to 6) Ballast Resistor should have been added or else switch to a 12 volt coil

I agree with Bobs good idea that a bad or incorrect sized condensor is more likely the cause as such can cause premature points failure. Id try a new condensor and set of points n then see what happens. If the condesnor is sized properly to match the coil there should NOT be a big hole in one point versus a crater build up on the other.

let us know

John T

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05-09-2010 18:06:19

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 08:51:13  
4 ohms shouldnt burn points but some meters autorange so you need to make sure its 4 ohms and not milli or micro ohms

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05-09-2010 11:45:17

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 08:51:13  
Just out of curiosity, what is the voltage at the pos. side of the coil? It should not be 12 volts. There should be a resistor inline to the coil's pos. side so the voltage at the coil is 6 to 7 volts.

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05-09-2010 10:33:59

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 08:51:13  
Like Bob says the cheap home owned meters you get a crap shoot reading on. I have tried it on many coils and have yet to get 2 to read the same and I know the coils are good and a couple even new. Wrong or bad condenser as again Bob says etc.

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05-09-2010 09:10:43

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 08:51:13  
It's tough to get an REALLY accurate measurement of Ohms in such a low resistance with a consumer-grade meter.

That being said, if your coil's primary REALLY measures 4.1 Ohms, this will be better for the points than the 3.3 Ohm coil you THOUGHT would be better. MORE Ohms = less current = longer point life.

(HOW did you figure adding a resistor to the 4.1 Ohm coil would LOWER it's resistance towards the 3.3 Ohms you THOUGH would be idael... were you planning to put it "across" the coil's primary???)

A more accurate way is to measure CURRENT in the primary circuit. With the ignition switch "ON", engine stopped, points closed, typically you don't want to see much over 4 Amps of primary current. Much more will shorten point life and heat the coil, somewhat LESS current will make for a weak spark.

Another factor in point life is a mismatch of condenser and coil. Are the points simply burning, or does one contact point "pit", with a corresponding point of metal being deposited on the other contact point... that would be a sign of the condenser's capacity being mismatched to the specific coil you are using.

Tell us what the failed points look like.

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05-09-2010 11:31:29

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 Re: Ohms For a coil in reply to Bob, 05-09-2010 09:10:43  
They are just pitted bad. A little history I am Not a great mech. I inherited the tractor and I really don't have a go to guy in my area to ask That is why I rely on he forum

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05-09-2010 13:44:36

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 Re: Ohms For a coil in reply to gerks, 05-09-2010 11:31:29  
Sounds like you may have 12 volts on a 6 volt coil and that will cause your problem. Do you have a resister in line with the coil?? If not and you have an O'Reillys auto parts place close by get a part #VR-1 and put that in the wire from the switch to the coil and see if that helps

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Dick W.

05-09-2010 15:13:28

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 Re: Ohms For a coil in reply to old, 05-09-2010 13:44:36  
Easier yet, just buy a coil with a built in resistor. That way no outside resistor is needed.
I know AC Delco makes one that way.

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John T

05-10-2010 04:35:25

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 Re: Ohms For a coil in reply to Dick W., 05-09-2010 15:13:28  
I dont think most typical old tractor 12 volt coils actually have a built in stand alone resistor stuck somewhere in the can. Their increased primary resistance (3 versus 1.5 ohms) is obtained from either more coil wire length or else higher unit resistance wire. They did indeed make such coils and some had an internal compartment where the resistor was located and some had the tell tale seperation ring on the outside of the can, but I view that more on the older vintage automobiles then our say 40's to 60's vintage tractors.

John T

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05-09-2010 16:03:30

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 Re: Ohms For a coil in reply to Dick W., 05-09-2010 15:13:28  
Ture but a coil will cost around $20 and the VR-1 is around $5

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