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HOW TO: How the Shift Interrupt system works

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  • HOW TO: How the Shift Interrupt system works

    Usually in the spring and summer we get a lot of posts regarding the shift system for the MC-1 (falsely referred to as Pre-alpha drives) to the Alpha 1 drives. There are a lot of wrong ideas on how and why the system works and why it's there to begin with. Maybe this will help explain how it actually works and why it's needed.


    The ONLY purpose of the shift interrupt system in the Mercruiser Alpha style outdrives is to get the drive out of gear (forward or reverse) when the engine is running and the boat is in the water with a load on the prop.

    The lower gears (Both Forward and reverse) have a dog type clutch. These dogs are reverse cut so they try to hold themselves into gear, and will stay in gear even under heavy load. Even at idle they are under a fair amount of load and without the shift interrupt system, it’s almost impossible to get out of gear. You can see the undercut and how the shift dog fits in the gears in the picture below.



    What happens with the shift interrupt system when moving the shifter from Fwd, or Rev back to the Neutral position goes something like this.

    When you start moving the shifter into neutral there is resistance from the engaged shift dogs that prevents them from coming out of gear. The outer sheath of the lower shift cable is attracted to the Y shaped spring loaded bracket that operates the shift interrupt switch. When this Y plate moves and activates the shift interrupt switch, the ignition coil is grounded and the engine starts to die due to lack of spark, and pressure on the shift dog is released. With no pressure on the shift dogs, the inner cable then pulls the drive easily into neutral. When this happens, the Y plate releases the shift interrupt switch and ignition is returned and the engine keeps running.
    All this only takes a split second, but if you listen, you can hear the engine drop a few rpm before running normal again when coming out of gear.
    With an old, corroded lower shift cable, the cable will not allow the Y plate to release and the shift interrupt switch remains engaged. Since the shift interrupt switch remains engaged for a long period of time which keeps the ignition system grounded out, the engine dies completely.



    The system is NOT designed to work when putting the drive into gear, or when the boat is out of the water and on muffs. Without the load on the prop, there is not enough resistance on the shift dogs in the lower unit to cause resistance to the shift cable and engage the shift interrupt system.

    As a trouble shooting note: If your shift interrupt switch is being activated going into either forward or reverse or coming out of gear when the boat is out of the water, then you have a problem with something binding from the lower shift cable to the drive itself.

    The above describes a Mercruiser setup, but the OMC system with the ESA module works basically the same. The difference is the ESA module only stops spark to half the cylinders and will cause the engine to miss, which releases the pressure on the shift dogs.
    Attached Files
    Last edited by Don S; November 12th, 2012, 12:39 PM.
    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.

  • #2
    Re: HOW TO: How the Shift Interrupt system works

    Great post Don!! This would explain why I could not get my boat out of gear last summer at the dock. I bought the boat last summer and only had time to take it out once. I had rebuilt the complete outdrive (upper and lower) and the boat ran and shifted great!!! Except that I had great difficulty getting it in neutral after running around on the lake. I am assuming by looking at the diagrams you posted that this linkage which actuates the shift interrupt switch is adjustable. I believe mine is out of adjustment enough that the switch is not being activated. I will consult the service manual and make neccessary repairs. Thanks again for the great information. I love this forum for all the great advice!!!

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    • #3
      Re: HOW TO: How the Shift Interrupt system works

      Can this be pinned???

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      • #4
        Re: HOW TO: How the Shift Interrupt system works

        AWESOME! most likely just what I needed. Great post.
        sigpicI reject your reality and substitute my own.... Adam Savage-Mythbusters

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        • #5
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          Re: HOW TO: How the Shift Interrupt system works

          Very good lesson, especially the end. I have been having cutout issue while on muffs and this explains it. I must have a sticking cable, since it shouldn't be activating the cutout at all.
          1990 Bayliner Capri 2050 BR - (My first runabout; Never seen her in water) ***Restore on hold***
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          Progress so far: Teardown - Transom - Stringers - Deck - Paint - Re-power - Interior Panels - Seating - Water Trial - Upholstery


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