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Toro self propelled lawn mower ignition problem

Discussion in 'Electronic Repair' started by klem kedidelhopper, Sep 1, 2012.

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  1. Someone gave us this non working self propelled mower that he says
    just stopped working. When my son first tried to start it he says that
    it would run for a few seconds and then quit. After this it would
    apparently take about 20 minutes before it would again start, and
    again run for about two seconds and then the process was repeated.
    This morning we worked on it a little and I could not get it started
    at all. We ultimately found that there was no spark. I don't know what
    happened when he worked on it. The ignition kill switch is working,
    There is no short on that terminal in "run" position, the rust has
    been cleaned off both the rotor and stator areas of the flywheel and
    transformer and the plug of course looks OK too. There are no points
    or capacitor on this mower and there is just a magnet on the
    flywheel.The coil has a secondary wire to the plug, a terminal that is
    grounded to the transformer laminations with a self taping screw, and
    a terminal connected to the kill switch. Unless I'm missing something
    the operation of this system seems pretty cut and dried without
    timing or anything else being an issue, that as the magnet passes by
    the coil a voltage should be induced into the secondary. Am I correct
    in this assumption? Is there a way the positively bench test this coil
    before we go out a buy a new one that will no doubt be non returnable
    if it's not needed? Thanks, Lenny
     
  2. Guest

    It's been a long time since I worked on lawn mower engines, but every
    one I ever saw had points and a capacitor. On vertical shaft engines
    they were typically under a cover under the flywheel. Now I will
    admit that this experience was well before the wide spread use of
    solid state devices, but there MUST be some method of interupting the
    primary circuit as the crankshaft nears top dead center. Also, the
    presence of a capacitor is pretty well mandatory.

    PlainBill
     
  3. These appear to be owners manuals. How to change the oil, etc. There
    really isn't anything mentioned that goes into anything that technical
    like bench testing a coil. I really think it is the coil. I just don't
    want to order one, about 20.00 and then possibly be stuck with it.
    Anyone have any ideas? Thanks, Lenny
     
  4. I just went through this. I didn't want to believe that the coil was
    bad (a new one was $40). But there isn't a way, that I know of, to
    bench test. If you don't have a spark, it's bad.

    To test:
    - remove the kill wire
    - remove the spark plug, attach the coil wire, & ground the plug
    - pull the cord & watch for a spark on the plug

    This is better than trying to get a spark between the coil wire & ground.

    Bob
     
  5. Wow, 40.00 for a coil! I am probably facing the same thing. I hope you
    at least got a kiss out of it...Lenny
     
  6. Guest

    Yes. Look here:
    http://home.howstuffworks.com/home-improvement/repair/how-to-repair-small-engines3.htm

    The pictorial diagram and the two pictures match what I remember from
    50 years ago. I also see references to solid state ignition systems
    used on these engines, that came well after the time I worked on them.

    If you can pull the coil without finding any wires other than to the
    kill switch and to the spark plug, the problem must be either the
    coil, the magnet in the flywheel, the spark plug, or the spark plug
    wire. The spark plug would seem to be the only replaceable part other
    than the coil.

    PlainBill
     
  7. Lawnmowers haven't had points for 20 (30?, 40?) years.
     
  8. Wow Eric thanks for the theory. I think that it is a good bet that the
    electronics in the coil has failed. I'm pretty sure that I can get a
    used coil from a lawnmower repair guy in this area. There are a number
    of people that work on these and sell used mowers. I'm thinking that
    they must keep used parts around. I wish I could find the model number
    of this thing. Lenny
     
  9. I have eliminated an internally arcing spark plug as a possibility
    because I tried sticking a screw driver into the boot and acing it to
    ground and there was no spark with that method. In addition, with the
    shroud (with the starter pull rope) removed, and turning the engine
    with a drill the high voltage wire is well removed from chassis ground
    so it arcing is not the problem either. However now I think that I
    know what may have happened. I ran across the sites below that had
    really good writeups on small engine ignition theory. The second one
    happened to mention that you should never fire an ignition coil with
    the secondary disconnected from the spark plug or as they put it a
    "suitable load". The article went on to say that doing so could cause
    internal acing in the coil and damage it. I never knew this but it
    makes sense and incidentally adds up as well. I happened to mention
    this to my son this morning and he admitted that during the course of
    his working on the engine when he had it running for a few seconds and
    then it would die he had done this very thing. And mysteriously now we
    have no spark where he did before. So a new coil is about twenty five
    dollars and I'm thinking that should hopefully take us back to square
    one where we should be back to the two second run problem, (I hope).
    If I knew that it was just the coil I guess I'd spring for the twenty
    five
    bucks plus freight. However given the history with this engine I'd
    rather not go throwing a lot of money at something that may need a
    carburetor or more as well. I gotta go see the used lawn mower guys
    this week and hopefully find a coil. Lenny
     
  10. Although that sounds plausible, I'm skeptical. First, I don't see how
    an internal spark would happen. I.e., what the path would be. But even
    if it could, I've fired disconnected coils lots of times without problems.

    An alternative explanation would be that the coil was marginal at the
    start - good enough to run for the few seconds. Then failed altogether.
    I don't know if there is a coil failure mode like that, but I wouldn't
    rule it out.

    Bob
     
  11. ... you should never fire an ignition coil with the secondary
    And how is external arcing "somewhere else" different from arcing
    across the spark plug's gap?
     
  12. You're right, but there are limitations. First, the voltage isn't
    unlimited, it has some maximum, whether there's a gap or not. I don't
    know the mechanisms limiting it - parasitic capacitance, leakage
    current, I dunno. Hopefully the coil designer knew and insulated
    against it, maybe.

    Then there's the path of an internal arc. There's certainly not enough
    voltage for an arc between adjacent turns. Also very unlikely between
    adjacent layers. The highest voltage is between the last turn and the
    core. But that's the most heavily insulated path.

    I'd be convinced by an actual case of an internal arc - anybody ever see
    one?

    Bob
     
  13. bw

    bw Guest

    Yes. Many. Thats how they fail. Temperature, vibration, aging induce small
    cracks in the insulation. Dirt, moisture infiltrate, etc. Better coils are
    designed with internal aux gaps greater than the normal spark plug gap. I've
    dissected failed coils, not hard to find traces of internal arcing, melting.
     
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