The Mercruiser shift system (the most common one) can come across as complicated but it is relatively simple once you get the hang of it. To understand the shift system, you have to start from the basics. So, what is a shift interrupt switch?
A shift interrupt switch is a part of the drive that allows boat owners to switch to neutral quickly. You will find the shift interrupt switch on the shift plate mechanism. Its appearance is that of a mini-switch. Experts also refer to it as the kill switch or shift switch.
A drive is synonymous with a motor or engine, so your shift switch sits on the marine boat engine. All boats made in the past twenty years come with a kill switch. The kill switch serves various purposes, but its primary role is to stop the motor from running.
Shift Interrupt Switch: What Is It?
Boats, no matter how they’re powered, come with a kill switch. The kill switch interrupts the full load current to the motor and sensor system. (source)
From a lanyard switch perspective, the kill switch lanyard is a red plastic cord that looks like a telephone cord from the 80s. You attach this red cord to you (the captain) and a special button on the boat. 
You connect the lanyard kill switch to your belt or pant hoop, and the other end connects to the plunger button on the boat. So how does it work? When you step away from the helm, the cord pulls away from the connection and the motor stops running. 
Different boat engines come with varying types of kill switch connectors. A kill switch lanyard is different from switch connectors present in sterndrives and inboards. You can find the switch connector on sterndrives and inboards near the gearshift and throttle levers. 
On the other hand, you have the kill switch from the sterndrive perspective. When the captain pulls back on the control to shift from forward to neutral, tension carries on to the drive shift cable.
The clutch ramps locking into the face of the gear is what create the tension.
Pulling the clutch to a neutral position creates tension that will activate the shift plate mechanism and stall the engine for a second.
The stalling of the engine will ‘scramble’ the clutch away from the gear, and the drive or engine will be in neutral. (source)
Where Do You Find The Shift Interrupt Switch?
You can find the kill switch on the shift plate mechanism at the engine. The little micro-switch is what you know as the shift interrupter switch. As shared earlier, the lanyard kill switch lies between you and the plunger button.
You really can’t miss it (again, it’s a bright red cord that looks like a telephone from the 80s). If you’re looking for the kill switch on your engine, it’s a small switch located on the engine or drive. The entire shift system involves a variety of parts; you have to understand how and when the parts function to grasp the location of the entire system.
How The Shift Interrupt Switch Works
Here’s a step-by-step process of how the shift interrupt system changes your boat to neutral.
- You place your boat into forward gear.
- The remote control pulls the dash shift cable.
- The dash shift cable connects to the shift plate on the motor.
- This shift plate then moves the drive shift cable.
- The drive shift cable moves a slide mechanism in the bell housing, causing the upper shift lever to rotate.
- Turning the upper shift lever rotates the upper, middle, and lower shift shafts.
- The shift shafts eventually turn a bell crank in the lower unit.
- The bell crank slides a spool assembly attached to the clutch allowing your boat to switch to neutral.
Remember that the kill switch allows your clutch to go into a neutral state.
Types Of Shift Interrupt Switches
There are different types of shift interrupt switches. You’d think that they function the same way. However, some slight variations set them apart.
Mercruiser are the most common shift switches in the market. Then you have the OMC system with the ESA module. The process mentioned above follows the Mercruiser shift system. The OMC system and the ESA module work in a similar manner but
Neutral Safety Switch Vs. Shift Interrupt Switch
A neutral safety switch aims to prevent your boat from moving unexpectedly when the engine transmission is in gear. You basically don’t want to start your boat when it’s in gear, as significant damage can occur. (source)
On the other hand, you have a shift interrupt switch that allows your clutch to go into neutral. Both the neutral safety switch and shift interrupt switch are safety features, important for operating your boat.
Vital Information About The Shift Interrupt Switch
The kill switch comes with its fair share of ‘breakdowns.’ What happens is that, like any other part of the boat, the shift interrupt switch can have performance issues. Here are the two points you need to know about:
1. Stalling while shifting
This problem is the most common and what happens is the kill switch is ‘over-killing’ the clutch. If water gets into the shift system, it causes the shift switch to stall the engine, essentially over-killing the system.
2. Shifting difficulty
The purpose of the kill switch is to switch the gears to neutral. If it cannot shift efficiently, the drive will stay in gear when the engine is running. In this case, the gears will most likely slam into each other.
The Kill Switch Conundrum
The shift interrupt switch is a point of contention for boat owners because of its complexity and specific repair requirements. However, you can quickly solve these problems with the right tools and a little bit of information.
Hopefully, the kill switch won’t be an issue for you moving forward. Happy sailing.
- Randy Vance, Power Boating for Dummies (New Jersey: Wiley Publishing Inc, 2009) 384.
- The University of North California, A Solar-Powered Boat: Northern California Solar Regatta, North California Solar Regatta, May 3, 2014, Accessed March 18, 2022, https://scholarlycommons.pacific.edu/purcc/2014/events/117/.
- Stern Drives, Replacing Your Shift Cable, Stern Drives, Accessed March 18, 2022, https://www.sterndrives.com/supplies/mdtips02.html.
- Marine Surveyors Inc, Vessel Engine Neutral Safety Switch, Christian & Co., May 30, 2018, Accessed March 18, 2022, https://themarinesurveyors.com/vessel-engine-neutral-safety-switch/.