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Mercruiser 260, No Spark- Thunderbolt IV

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  • Mercruiser 260, No Spark- Thunderbolt IV

    Hello,I'm getting no spark in a Mid 80's Merc. I've replaced the distributor module, rotor, cap, & coil. The only thing I believe that is left, is the Thunderbot IV ignition module. To test (the only way I could think of) I pushed 14 volts through the module from a jump box and I'm getting 7 out the other side. Is that normal or is there something else I should be looking at to fix? Any help would be sincerely appreciated, I'm dry docked and just missed a great weekend.Thank you!


  • #2
    Re: Mercruiser 260, No Spark- Thunderbolt IV

    That is a test I have never done. If you are getting +12V to one side of the coil, and a pulsing negative (as you turn over the engine) to the other side of the coil you should be getting spark. As far as I know, the ignition module deals with the negative side of the coil. You are going to need at least a DVM to troubleshoot this problem. I would start by looking for +12V on the positive side of the coil (that would be a key switch,wiring problem if not there).

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    • #3
      Re: Mercruiser 260, No Spark- Thunderbolt IV

      Check out the flow chart.... web page

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      • #4
        Re: Mercruiser 260, No Spark- Thunderbolt IV

        Thank you! Have you ever heard of a shift cutoff switch? I'm being told that may be the actual problem. I have run through the flowchart and ended up at the "replace ignition module". However the chart does not mention the shift cutoff switch.

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        • #5
          Re: Mercruiser 260, No Spark- Thunderbolt IV

          The cutoff switch you have heard about, momentarily kills the motor to allow you to easily shift from forward to neutral and same for reverse. When they are working right you don't even notice it happening.

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          • #6
            Re: Mercruiser 260, No Spark- Thunderbolt IV

            Hopefully my last question.Would it be possible to get NO spark if this switch has gone bad? Can I permanently bypass the switch?Thanks again

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            • #7
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              Re: Mercruiser 260, No Spark- Thunderbolt IV

              Yes, that's what it does: interrupt the spark while shifting. No, don't bypass it, doing that will only make your gears grind and lead to eventual failure in your drive.
              Disclaimer!: In my inexpert opinion......

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