[AJD] Coil Testing
John Nordhoff
jmn50 at msn.com
Thu Apr 27 08:25:25 PDT 2006
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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