I have a craftsman lawn tractor with an 18HP Kohler twin, model M18S.
Twice now it has failed to start due to no spark. By the time I fiddle
with ignition switch and safety switch to see if something is not making
contact, it mysteriously starts. I would say in my experience with other
engines the spark intensity looks weak.
My question is about the ignition unit. Does anyone know if these are
prone to this type of intermittent failure as they get older (13 years)?
Is there some way to diagnose this other than stick a screwdriver in
the sparkplug wire to see if there's a visible spark? If there's a
better forum that specializes in small engine repair please let me
know, but I saw a fair number of posts on mower engines on this group.
Ignition units fail.. There is a conductive pencil with a bulb in it
that when touched to the wire will light on spark. Modules usualy fail
temporarily with heat , the engine running. Then wont refire. Or just
But if after you mess with the saftey and key switch start there. Seat
safteys fail easily , your key switch may be bad.
Coils produce the spark voltage not the module A bad coil may visibly
arc or have bad insulation. If your spark is yellow it could be a Plug
, Plug wire , Coil , or dirty - corroded 12v or ground. Start with the
safteys and new plug and plug wire.
Thanks for all the thoughts. I have the schematic: both the PTO safety
and the clutch safety are also in the starter circuit. Since it cranks
fine that means it can't be those safety switches. That leaves the seat
switch which needs to be open for spark, so I just removed the connector
from the switch which guarantees the circuit is open. That leaves only
the ignition unit and high tension leads as possible suspects, unless
the wire going from the safety switches to the ignition unit is shorted
to ground someplace, which would cause the ignition unit to stop
funtioning. Not sure exactly what's in this "unit" because there is no
coil, points, anything, just this unit. I assume everything is sealed
inside so there's not much to debug. There is also a stator called out
on the parts diagram, but I haven't dug that far and don't want to.
I don't believe the high tension leads are replaceable. I'm afraid to
yank on them hard enough to find out if they come out of the ignition
unit. It looks like they were molded in there. I don't want to make the
problem worse. The parts diagram does not call them out separately, they
are shown as part of the unit. That could just be the big money shakedown.
Thanks to whoever suggested the spark tester. I'm going to look for one
m Ransley wrote:
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