[AJD] Coil Testing

Sun Apr 30 06:53:00 PDT 2006

Thanks, wow, great web site. As a retired Electrical Enginner I also love 
old radios.  have a Hallicrafters SX 110 and a Hammarlund HQ 110 plus a 
Crosley Super 11, Zenith 10S566, Grunow All Wave of my grandparents, 
Majextic TRF, Grunow 670 Tombstone etc etc

  I will look at the site more later.

  John Nordhoff aka  John T in Indiana jmn50 at msn.com
----- Original Message ----- 
From: "Dave" <deere at gwltd.com>
To: "Antique John Deere mailing list" 
<antique-johndeere at lists.antique-tractor.com>
Sent: Saturday, April 29, 2006 10:28 PM
Subject: Re: [AJD] Coil Testing

> 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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>>> http://www.antique-tractor.com/mailman/listinfo/antique-johndeere
>>
>>
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