Mercruiser Shift Interrupter Switch Test, My question is how exactly does the shift switch work? Because looking at it when I shift into gear, the roller pushes the microswitch down so wouldn't it always be engaged while in gear Very intermittent but annoying. I was reading some threads here about shift interrupter problems and realized I didn't remember one Ensure smooth shifting with this genuine Mercury/Mercruiser Shift Interrupter Assy: Marine-grade, affordable part with fast shipping nationwide. When the engine is in gear (FWD or REV), the force of the engine clamps the gears together so tight that you cant shift out of gear once you are in My lower shift cable was a bit stiff, but I could shift it in and out of forward just fine by hand, but couldn’t get it into reverse at all. If I disengage a functioning shift interrupter switch, should the engine die when I shift to Re: Mercruiser Shift Interrupter Switch You have correctly identified the reason the engine dies. This also results in smoother shifting. Bought the boat last year and the previous owner stated that Mercruiser 120 Petty Officer 3rd Class Joined Jun 17, 2018 Messages 75 Aug 23, 2018 #1 Could someone show me the wiring diagram for a 1981 Mercruiser 120 that includes the Shift Enjoy the videos and music you love, upload original content, and share it all with friends, family, and the world on YouTube. I've read my Mercruiser manual and think I generally understand the switch and shift mechanism. The Boating Forum - 4. I fueled it up and bought a new battery and she idled in the driveway so to the lake I went. get a buddy to sit in the driver seat while you watch the action of the interrupter Of note, a Mercruiser MPI engine when the shift interrupt switch is activated whilst its idling, will drop RPM to 400rpm or so, but does not stall.