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Checking for Intermittent Spark

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  • Checking for Intermittent Spark

    Diagnosing a high speed driveabilty problem can be tougher, because you usually cannot replicate it in your driveway. The first thing i like to know is what system is causing the issue spark, fuel or mechanical. Once this is determined one can then start troubleshooting that particular area. I use this style spark tester that you can get from Amazon, which enables plug wires to be snapped onto each end and the neon bulb is of a good quality. (the snap-on / blue point testers are the same repackaged) I take the existing coil wire out of the distributor cap and run it up and out of the hood in front of the windshield. Hook the end of the coil wire to one side of the spark tester, and then get yourself a long modern car spark plug wire that will reach from the tester back to the cap. ( if needed cut the boot off so it goes into the cap) tape up the ends of the spark tester with electrical tape to prevent any arcing to the cars body. Now you can drive the vehicle at high speed and watch the rhythmic flash of the tester. A break up or stutter of the spark would indicate an issue
    Four can be hooked up at once if you want to see the spark going to each plug.
    You do not have permission to view this gallery.
    This gallery has 1 photos.
    3~ Tudor's & 1~ Coupe
    Henry Ford said,
    "It's all nuts and bolts"
    "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

    Mitch's Auto Service ctr

  • #2
    them there spark testers are worth their weight in gold!


    • #3
      Mitch, allI get when clipping on you're "spark tester" highlight is a "fuel cap "tester?


      • tbirdtbird
        tbirdtbird commented
        Editing a comment
        weird, works for me

      • Mitch
        Mitch commented
        Editing a comment
        You should see a thumbnail without clicking

    • #4
      Hot damn tamale what a great and simple way to monitor spark! Now I can take the trailer wheels and hitch off my Sun machine.
      Mechanical engineering 101: If you put an adjustment knob, screw, bolt, or tolerance specs on something, some people will immediately fiddle with it. If you mark it DO NOT TOUCH everyone will mess with it.


      • #5
        On a long distance trip our A developed an occasional miss, and when it happened, it appeared that the spark was weak on two cylinders. Shorting them out with a screwdriver showed which ones had the weak spark.
        I fitted a new distributor cap which did not cure the problem.
        Finally after 2500 miles the car died and had to be put on a truck for the last 50 miles to home.
        After much checking and testing it finally proved to be a loose connection at the ignition switch.
        I'm not sure that a spark tester would have been any more use than a screw driver?


        • Mitch
          Mitch commented
          Editing a comment
          I have hooked up 4 of these spark testers at once to visually see the intensity and pattern to each cylinder. Much better and easier to see whats going on in live mode

      • #6
        well, it is a place to start.

        As far as loose connections at the ign switch, gosh we have seen that sooooo many times, and it can be intermittent as hell, and make you crazy. At this point, we now check for that first, since it is so easy to do. Then we check the pigtail wire between the 2 plates in the dizzy. We always check BOTH. Between these 2 issues and a bad capacitor you'll cover about 90% of ignition trouble

        Be sure to put the star lock washer between the switch body and the ring terminal, not between the ring terminal and the screw head. This seems counter intuitive but works well. When you are pushing the wires back into the tunnel, they want to walk and thus loosen the screw. If the ring terminals cannot walk around, the screw will stay tight on its own. My nephew was surprised to see that method, but if you look carefully at a lot of original wiring stuff on a lot of vintage makes you will observe it was done that way

        It is beyond wild that loose ign sw connections would only drop out 2 cyls!


        • Mitch
          Mitch commented
          Editing a comment
          and add the wireless plates which are famous for intermittent glitches.

      • #7
        I have been looking at inline spark plug testers.

        How would you compare this tester vs the tester shown in post #1?

        Start video at 4:34.
        Last edited by Fordman; 01-17-2022, 12:41 PM.


        • #8
          Originally posted by Fordman View Post
          I have been looking at inline spark plug testers.

          How would you compare this tester vs the tester shown in post #1?

          Start video at 4:34.
          That would be hard to see when driving down the road. I would stick to the one in #1
          3~ Tudor's & 1~ Coupe
          Henry Ford said,
          "It's all nuts and bolts"
          "Start by doing what's necessary; then do what's possible; and suddenly you are doing the impossible."

          Mitch's Auto Service ctr


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