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Need help checking for spark

Discussion in 'XJ Technical Chat' started by jchevelle70, Jan 1, 2008.

  1. jchevelle70

    jchevelle70 New Member

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    I have heard that it is really easy to burn up a coil on these bikes if you are not carefull when checking for spark. I do NOT want to find out the hard way. So, what is the correct way to do this? When checking one plug at a time, do I simpley remove one plug, ground it, leave the other three hooked up and crank the engine over? If so, what's saying the bike isn't going to fire up on the other three cylinders and scare the crap out of me? :lol:

    Or do I remove the caps from all the spark plugs, pull one plug at a time, ground it, and check it that way? If so, shouldn't the other three plugs be grounded so that it does not harm the coil?

    Maybe I should try it this way? remove all 4 plugs, ground all of them, and crank engine and check for spark on all of the plugs at one time. My main concerns are that I do not want to ruin a coil when checking for spark and I also do not want the bike to fire up while checking for spark.
     
  2. BlueMaxim

    BlueMaxim Active Member

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    The sparkplug is the ground for the coils. So to make sure the ground isn't interrupted one plug should stay connected. One coil fires plugs 1 & 4 and the other fires 2 & 3. So never remove "1 & 4" or "2 & 3" together or you may damage the coil. Since some PO may have misswired the plugs, I always check for correct connections before checking for spark.
    To check for spark I remove the plug connect, the plug wire, ground it to the head and look for spark.
    The plug caps are resistor caps so this shows me if they are working also. No spark means either a bad/unconnected coil, plug or cap. Removing the cap will produce a good spark if the coil is good but will not tell you if the cap is bad.
    I do one plug at a time so there's no way to burn a coil. This also helps make sure you reconnect all plug wires so you don't forget and burn a coil by accident. I also use an insulated tool to hold the plug while testing since I really don't like getting shocked.
    Also note that a spark outside the cylinder doesn't mean the plug is firing while under compression, but a good spark usually does.
     
  3. ArizonaSteve

    ArizonaSteve Member

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    Or you could just get a spark tester from the auto parts store.
     
  4. Polock

    Polock Well-Known Member

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    any of those ways should work but i don't like having one cylinder with no compression, don't know why but it doesn't sound good, and blows a lot of air
    or take a extra good plug and move it around
    or if it runs just slowly pull off each wire, you'll hear the spark
    or you can use a timing light on each wire
    if you don't use any choke and hold the throttle open it won't start, better yet pull the fuel line and run it out of gas

    what's up with "easy to burn up a coil" ?
     
  5. BlueMaxim

    BlueMaxim Active Member

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    Not sure if the XJ coils should be included but it was common knowledge on older bikes like the BSA, Triumph and Honda's that the coils could be burned quite easily by removing both leads and checking for spark. Most of these bikes had analog spark or points and a condenser instead of a TCI. So this may not apply to XJ's but I don't take any chances. I instruct bikers to do what I know won't hurt their machine until I know for certain the other way or ways won't.
     
  6. MiCarl

    MiCarl Active Member

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    Every service manual I've ever read for any coil fired engine warns to ground the coil secondary (spark plug lead).

    The secondary in the coil generates very high voltages (greater than 10,000 volts), that electricity has to go somewhere. By design it is supposed to go to ground through the spark plugs.

    I can imagine that it's possible that the excess voltage could break down the insulator in the coil and create a new path which will be easier for the electricity to travel than through the spark plug. No spark at the plug after that.

    If the charge doesn't leak out by breaking down an insulator it will have to flow backward through the secondary. This could induce some effects upstream to the electronics (TCI). Potentially harmful.
     

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