How does the Mercruiser Shift Switch work?

[CLi87]

Hey all, I know this has been discussed a lot but I wasn't able to find my specific answer. My symptom is the common engine dies when shifting into gear problem, either forward or reverse. Most posts reference either the shift switch itself or the lower cable as the problem, I believe it's my shift switch as when I manually push on the lever up or down quickly, the engine dies no matter what. 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 and only disengaged while in neutral?

 

[Rick Stephens]

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...

forums.iboats.com

 

[CLi87]

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...

forums.iboats.com

Thanks Rick! Very helpful, so from my understanding when actually shifting into gear and depressing the microswitch it shouldn't have any effect on the boat, if the switch were working correctly that is. It should only momentarily cut ignition when shifting out of gear into neutral. But in my case the engine dies the second I push the microswitch down manually, so definitely bad switch. So...I don't suppose anyone might know the part number for the switch I need? lol That's the only part missing from Don's great post on the shift switch system =D

 

[CLi87]

I also happen to have a spare 1997 parts boat, it's got a newer 4.3 EFI and it seems a different type of shift switch, can I retrofit the new type switch to my older 4.3 Carbed? or better to just buy a replacement OEM switch?

 

[CLi87]

I found the part number for the shift switch but it seems the part is discontinued? I can find used ones on ebay but would rather put a new one in.

I believe it's #18 in the diagram https://www.perfprotech.com/mercury...bracket-old-design/assembly/951/230?model=688

Part

#39670A28​

 

[Rick Stephens]

When working correctly, pressing the switch grounds the ignition.... I don't get your first statement that the switch is bad because the engine dies when you press it. Somehow I am missing the point.

 

[Rick Stephens]

If the engine dies when shifting into gear then the most common reason is a sticky lower cable.

 

[CLi87]

When working correctly, pressing the switch grounds the ignition.... I don't get your first statement that the switch is bad because the engine dies when you press it. Somehow I am missing the point.

This is the part that really confuses me, so when you shift into gear, it pushes the microswitch, so at this point it cuts the ignition? But if the ignition is cut when the switch is pressed then at what point does the ignition come back on? Or do you mean when the switch is pressed going into gear it just cuts ignition briefly and then when the switch is depressed going out of gear it once again cuts ignition briefly?

 

[CLi87]

If the engine dies when shifting into gear then the most common reason is a sticky lower cable.

I don't think it's my lower cable, I have the cable disconnected from the shifter mechanism. I start the engine and it's fine, but when I move the shift lever either up or down manually at the mount, not using the throttle shifter, the engine immediately dies, the second that microswitch is pressed down the engine dies.

 

[Rick Stephens]

Shifting into gear, and shifting either into or out of gear while not in the water, should not move the shift interrupt cam at all. The only time that should move is in gear, in the water, where the propeller turning has the gears under load.

Because it is moving and the lower cable is not even hooked up, something is wrong in the linkage. The switch appears to me to be working perfectly.

To somewhat repeat the link I sent you, in a properly working system, the switch only gets pressed when the gear lever is pulled to neutral while the prop is under load. This should kill ignition for just a few cylinders before the unloading of the gears allows unlock and neutral to occur. In a working system you won't even notice the motor stumble it is so short.

 

[flashback]

When going into gear the lever you speak of should not move, the roller should stay at the bottom of the V. The lever should only move when pulling out of gear under load..

 

[CLi87]

Shifting into gear, and shifting either into or out of gear while not in the water, should not move the shift interrupt cam at all. The only time that should move is in gear, in the water, where the propeller turning has the gears under load.

Because it is moving and the lower cable is not even hooked up, something is wrong in the linkage. The switch appears to me to be working perfectly.

To somewhat repeat the link I sent you, in a properly working system, the switch only gets pressed when the gear lever is pulled to neutral while the prop is under load. This should kill ignition for just a few cylinders before the unloading of the gears allows unlock and neutral to occur. In a working system you won't even notice the motor stumble it is so short.

O gotcha, sorry, I had completely misunderstood the fundamental way the system worked because the linkage on the boat I'm working on was not adjusted right to begin with, with the lower cable connected it's moving the cam up and down when I shift forward and reverse, I didn't realize the cam isn't supposed to move at all unless under load. I'll go through the procedure of adjusting the shift cable and if that doesn't work looks like i'll be replacing the cable. Thanks for all the help! I'll post back with the results after making the adjustments.

 

[nola mike]

O gotcha, sorry, I had completely misunderstood the fundamental way the system worked because the linkage on the boat I'm working on was not adjusted right to begin with, with the lower cable connected it's moving the cam up and down when I shift forward and reverse, I didn't realize the cam isn't supposed to move at all unless under load. I'll go through the procedure of adjusting the shift cable and if that doesn't work looks like i'll be replacing the cable. Thanks for all the help! I'll post back with the results after making the adjustments.

Save yourself the trouble. Disconnect the lower shift cable and manually move it. There should be almost no resistance to shifting the drive. If there's even a "well, that doesn't take too much effort" amount of effort, it's too much effort and your cable (or less likely something else in the drive linkage) needs replaced.

 

[MichaelBC]

The switch is only used when switched from gear into neutral. When switching from neutral into forward or reverse the switch is not operated.

 

[Scott06]

Here's a good video of adjustment

 

[Bondo]

The switch is only used when switched from gear into neutral. When switching from neutral into forward or reverse the switch is not operated.

Ayuh,..... 'n Only when the boat is In the water,..... not ever when on the wagon,.....

 

[MichaelBC]

If you want to test it on the trailer you can ask someone to push the propeller against the blockage when the gear is in. If you switch to neutral then you can see how the switch is pressed. It is very important for the fingers of the helping person that the engine is not running when you do that.

 

[achris]

If you want to test it on the trailer you can ask someone to push the propeller against the blockage when the gear is in. If you switch to neutral then you can see how the switch is pressed. It is very important for the fingers of the helping person that the engine is not running when you do that.

Not a good idea! To put enough load on the prop to cause the switch to be activated isn't going to be easy, or safe.

Do you understand how the whole system works? Only by understanding it can you properly diagnose any faults.

Chris..

 

[QBhoy]

In the while I have been about boats, there has been so much confusion about how this works. No doubt the guys above have given you the notion of how it works and you now understand the operation a little better...but I thought I would add my thoughts around your situation...

If she is cutting out going in and out of gear when in the water and all normal operating conditions...and she didn't do this previously during your ownership...the most common cause for this, that I am aware of...is that the running condition of the engine is compromised in other ways...most popular of which that I have seen, is bad fuel causing a lower than normal idle and suitable running condition...or one or more of your cylinders not firing as it should...
Advice given above about the shift cable assembly is also a good avenue to look at too. But most if not all I have seen doing this, is down to poor running or low idle, caused by poor fuel or if there has been a disturbance to the fuel delivery system..like carb or carb settings. If she is a fuel injected engine...the IAC could also cause similar issues. so could a vacuum leak or intake manifold being compromised.

 

[CLi87]

Thanks all, just wanted to report back got the issue fixed, the lower shift cable was so corroded on the outdrive side that it literally just fell apart when I was removing it, got the new lower shift cable installed, did the adjustment procedure and now she shifts just fine without cutting off.