[AJD] Coil Testing

[John Nordhoff]

    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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