[AJD] Coil Testing

[Dave]

John thanks for your reply.

To be clear, I have the coil removed from the mag and on my bench. When
I messure the primary side of the coil, it reads 0.1 ohms, which is the
same reading I get when I short the leeds from the VOM. So the coil is
dead short. I would have expected to see a few ohms, you say 0.4 to 1.0
would be normal. This is what I wanted to know. I was sure the coil is
shot, but wanted someone to confirm for me, I think you have done so.

By the way electronics is my way of life, check out my web site and I
think you will see what I mean.

www.w9wrl.com

Thanks for you help
Dave


John Nordhoff wrote:

>    Dave, The points inside a Magento are usually closed, but depending 
> on where the mag is at in its rotation cycle (they stay open on the 
> high of a cam lobe for a brief period after firing), and an ohm meter 
> placed on the kill switch stud/terminal and to frame/case ground ought 
> to read somewhere around 0.4 to 1.0 ohms or so when the points are 
> open (place paper etc between them)  but near 0 when they close (if 
> they read much more then 0 with points closed, points may be burned or 
> pitted or carboned over or not closing fully) .
>
> NOTE AND CAUTION you need a relatively decent meter which has been 
> properly nulled out and reads/adjusts to ZERO with its leads shorted 
> to be able to measure such low ohms!!!!
>
>  Similar, if you measure the coils low voltage primary by removing 
> BOTH the condensor wire and any wires to the kill switch stud (to be 
> safe in case its shorted) from the points and measure between the 
> remaining coil wire and case/frame ground with the points open, of 
> course, it should be in the 0.4 to 1 ohm range. Thats the coils 
> primary winding youre left measuring with all else removed and the 
> points open.
>
>  Things which could cause the coil to always (regardless if points 
> open) be shorted out are 1)  The condensor is bad and shorted (remove 
> its lead n see if thats the short) 2) any wire to the kill switch stud 
> or the kill switch stud itslef is dead shorted cuz the plastic where 
> it passes through the Mags side is bad (remove any kill wire to points 
> n see if that cures the problem)
>
>  Remove the cap and insure any kill switch wire from the points to the 
> kill stud and/or the kill stud itself (where it passes through mags 
> case) isnt shorted out,,,,,,,,,,,,,,,,,insure the condensor or its 
> wire to the points isnt shorted out,,,,,,,,,,,(condensors can go bad 
> and become a dead short, they ought to read an open on a DC ohm 
> meter),,,,,,,,,,,, then measure the coils primary resistance with the 
> points open (thats the remaining wire to points other then condensor 
> or any kill wire),,,,,,,,,,,,,,and if good and continuous it ought to 
> read 0.4 to 1 ohms
>
>  NOTE I do NOT have the coils exact primary resistances, the 0.4 to 
> 1.0 ohms are approximations, Duane Larson likey has more exact values 
> but the above is still valid as a method to test for shorted wires or 
> shorted condensors etc. If the kill stud always reads 0 ohms to 
> frame/case ground EVEN WHEN POINTS ARE OPEN, I would suspect in order 
> a bad/shorted condensor,,,,,,,a shorted kill switch stud or wiring to 
> it,,,,,,,,,,,,, shorted wires to points (condensor or kill 
> wire),,,,,,,,,,,,points never are opening,,,,,,,,,,,,or a bad coil.
>
>  Most common causes of a "weak" spark when cranking a mag engine are" 
> Points are burned or pitted or carboned or mis gapped (set at 
> 0.015),,,,,,,,,if she makes a dull clunk when the spring trips versus 
> a loud sharp SNAP, the spring may be weak (if she starts n runs fine 
> if pullled but hard to start by cranking, look at the mags wind n trip 
> spring assembly) Of course, she MUST SNAP loud n sharp each 180 of 
> rotation when cranking, if not, try to loosen the mag a bit away from 
> the governor n see if that helps and it may need an extra gasket if 
> so,,,,,,,,,,,Condensor may be weak or the incorrect 
> value,,,,,,,,,,,,,,rotor magnetism may be weak and/or rotor 
> laminations or rotor are rusted over,,,,,,,,,,,,,,,, wires or cap or 
> rotor may be bad or carboned or cracked or theres moisture 
> present,,,,,,,,,one should use wire core wires on those mags.
>
>   There's also the start timing issues, if shes start timed too slow 
> shes harder to start, but if too fast, the flywheel might kick back. 
> Set it to start time SNAP at TDC or just a tad (1 to 3 degrees) 
> before. It ought to SNAP just as the Flywheels Left Hand Impulse  mark 
> passes by the flat mark on the tractors side located at 3 o clock. Low 
> compression makes em harder to start also, try removing the plugs and 
> squirt a lil oil n rotate to oil n seal up the rings n see if that 
> helps?? Then theres all the carb issues that can cause hard starting 
> remember
>
>
>  John T Nordhoff in Indiana
>
> ---- Original Message ----- From: "Dave" <deere at gwltd.com>
> To: <antique-johndeere at lists.antique-tractor.com>
> Sent: Thursday, April 27, 2006 7:36 AM
> Subject: [AJD] Coil Testing
>
>
>> I have a JD-B that will not start, spark seems to weak to none. 
>> Mostly none. This B has a Wico XH mag. The ground out stud is shorted 
>> to ground at all times. When I started to check it out, I found that 
>> the primary windings of the coil read dead short on my ohm meter. 
>> Seems to me that it should read a few ohms, and not dead short. Can 
>> anyone tell me what it should read?
>>
>> Dave
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