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Mercruiser 4.3 Shift Interrupt Switch

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  • Mercruiser 4.3 Shift Interrupt Switch

    I recently read an excellent thread started by rowndman who was getting intermittent artifically high RPM readings and pinging on his Mercruiser 4.3 engine. After much testing, with the help of Don S and others, he finally determined that the shift interrupt switch was the problem.

    My 1997 Mercruiser 4.3LXH Gen+ 4bbl is starting to have similar symptons to the ones experienced by rowndman. My questions are pretty basic: Where is the shift interrupt switch? Is it easy to access? What does it do? To disconnect it do I simply uplug it? Assuming that it affects shifting, can I shift from neutral to forward so as to test if the symptoms are still present with the switch disconnected?

  • #2
    Re: Mercruiser 4.3 Shift Interrupt Switch

    1) Located on the shift plate, follow the cables....
    2) Should be
    3) The shift interrupt momentarily kills the engine to allow the gears to disengage...
    4) Yes
    5) Yes, but do not try to get back out of gear with engine running.....
    Why don't you post your symptoms, just because you have some of the same symptoms, doesn't mean his solution is your solution.....JK
    Disclaimer!: In my inexpert opinion......


    • #3
      Re: Mercruiser 4.3 Shift Interrupt Switch

      A couple of questions, do you have an Alpha drive or Bravo drive.
      There is a plate on the engine where both shift cables attach.That is where the shift interrup switch lives IF you have an Alpha drive. Look at the plate, and compare to picture below. Do you have a switch like item 17 or 19.
      Also, do you have Thunderbolt IV or Thunderbolt V ignition.
      The plunger type switch (like item 19) is the ones that have the problem, but I've never seen them cause problems in engines that are not EFI. On the EFI engines, the switch controls the ECM, not directly to the coil for for ign. control.
      Have you already done the basic, tuneup, filter checks for water, etc.
      As stated in the other post, you MUST hook the switch back up before pulling it out of gear, or you may break something, but going from Neu to Fwd, the shift interrupt system is not used, so it doesn't matter if it's hooked up or not.

      Don S.


      Please, no PM's (Private Messages) regarding boat/engine problems.
      That is what the forums are for.
      Only forum/moderator issues will be answered in PM's.


      • #4
        Re: Mercruiser 4.3 Shift Interrupt Switch

        Its an Alpha drive. I am not sure if it is a Thunderbolt IV or V as the boat is at the lake and I am at the office (although I am guessing that its a Thunderbolt IV). The last tune up was about 40 engine hours ago at the beginning of the season. I also changed the fuel/water separator filter sometime in July.

        My symptons include RPM readings that were about 1000 higher than actual that occur immediately upon starting. When this first started happening, the RPMs would return to normal after accelerating so it didn't seem like a big deal. More recently, the artifically high RPM readings have been persisting for longer (while eventually returning to normal eventually) and I have started to notice some pinging and knocking at higher speeds without any backfiring or other problems. At one point, I tried some premium fuel, which seemed to help initially.

        After reading rowndman's thread, I concluded that my symptons were similar enough that the shift interrupt switch may be my problem. However, my engine is not EFI as was rowndman's.


        • #5
          Re: Mercruiser 4.3 Shift Interrupt Switch

          You might try checking the Timing,.......
          Pull a couple of Sparkplugs,+ See what they can tell you,........
          It's Possible it's going Lean on you.......

          For the Tach Issue,........ I'd try another Tach against it......

          I Really Doubt the Shift-Interupter has Anything to do with Your Issues............
          Any Grease is Better,..... Than No Grease at All.......


          • #6
            Re: Mercruiser 4.3 Shift Interrupt Switch

            Don S & bond-o are giving you the straight skinny. If your engine is not EFI, chances are the shift interruptor is not the culprit. On an EFI system, the interruptor is in the communication circuit between the ECM (the computer?) and the ignition control module inside the distributor. The signals in this circuit are very low voltage, so a little resistance here will scramble the conversation between these 2 components. On your system (as I recall), the switch is just an On - Off device in a relatively high voltage (12V) circuit. Doesn't matter a lot if it works well or not, it just has to work.

            Its pretty easy to jump it out, though, if you want to try. Unplug the switch at the 2-prong connector about 6" from the switch and install a jumper between the 2 female connections in the engine harness side plug. I made one out of a piece of No. 12 romex wire about 4" long. Strip each end about 3/8", flatten the exposed ends of the copper wire so that they fit snugly into the plug, and bend it into a U-shape so that it matches the terminals in the plug. Push her in & give it a try. Do this somewhere that won't require you do any shifting out of forward in a hurry, though.


            • #7
              Re: Mercruiser 4.3 Shift Interrupt Switch

              If he jumpers those two connections on his engine, the engine Will NOT run.
              I've never really checked the EFI switch, but on the Carbed switches, they are a NO (Normally Open) switch. When they close they ground out the circuit and stop the ignition. Jumping the wires is esecntially closing the switch.
              Don S.


              Please, no PM's (Private Messages) regarding boat/engine problems.
              That is what the forums are for.
              Only forum/moderator issues will be answered in PM's.


              • #8
                Re: Mercruiser 4.3 Shift Interrupt Switch

                Should be a Tbolt V system. Check for plug wires touching the igntion circuit or knock sensor wires.