Shift Interrupter Switch Operation

[rkilpa]

I started a thread asking about my 5.0L with an Alpha 1 Gen 2 shutting off while docking and my questions were directed at the IAC valve and IAC filter/muffler that I thought the problem might be.

Several experts asked if the shift interrupter switch was tripping due to a worn lower shift cable. I had a Volvo Penta outdrive for 20 years and never had this issue, so I didn't even know what a shift interrupter switch was or where to locate it.

After doing some reading on this topic from several posts by some very knowledgeable people, I now know what to look for and where to look when I return to my boat. It sounds like this might be the problem because the motor idles and runs perfectly fine until I move the control from forward to neutral or reverse to neutral at docking speed. Sounds like the switch might be closing for too long when engaged not allowing the motor to continue running.

My question is - because this only happens sometimes, not every time, is it possible that instead of a worn lower shift cable (only 175 hours) that I might be causing the issue myself by the way the controller is being manipulated?

If the shift interrupter switch is supposed to just ground the coil for a split second so that the engine stalls just enough to take pressure off of the drive to allow the gears to disengage and then continue running, I would assume that the shifter would need to be moved swiftly from gear to neutral. If the controller were moved slowly from gear to neutral, would that not keep the switch grounded longer than it should be and cause the engine to shut off?

I am still not used to the "clunk" that my Alpha drive makes when putting it into gear compared to the Volvo, which was velvety smooth, so I find myself "easing" the controller in and out of gear. I'm thinking if I just moved the controller a little faster, it might not ground out and stall the engine.

Could it really be as simple as operator error? Would a lower shift cable even wear out at 129 hours, which is when I got the boat 3 years ago and experienced my first stall.

Thoughts?

 

[Lou C]

You might be on the right track, a dog clutch drive has to be shifted briskly or else it will grind going into gear. It is possible though if the shift cable is internally corroded it might be dragging and keeping the interrupter engaged too long.

 

[Scott Danforth]

shift cables dont know hours. they know force, number of cycles, corrosion, neglect and manufacturing defects.

so yes it is possible the shift cable is stretched a bit out of adjustment, or the inner sleeve is corroded.

yes, an alpha bangs into gear just like outboards because of the dog clutch.

the Volvo drives use a cone clutch, which is what Mercruiser adopted for the Bravo drives.

you want to be crisp and decisive with your shifts on an alpha or any drive. you do not want to be slow shifting an alpha into gear, those little grinding bangs you hear is the dog clutch pieces trying to mate together.

 

[Furbird]

It's kind of like a sportbike clutch. If you hold the clutch in too long then shift into gear it sounds like a box of marbles. Clutch, engage gear. You can always tell the people that have been taught wrong because they sit at red lights with the clutch pulled in. They are the ones most likely to stall at a red light, because that stopped clutch is engaging against that spinning motor and the load is too much. That's why it's called a throwout bearing, it's designed to let it spin, not just completely disengage. Sorry, back to topic.

I figured that out the first time I put my boat in forward. I was trying to be easy on it thinking it would slowly engage but it kinda grinded some. Now it's just try not to go too far past engagement and get the lurch. You can only learn by feel when it comes to this though, just like a manual trans. People can tell you how to do it, but until you do it, you don't get the lightbulb moment.

 

[Lou C]

Another thing, if the boat was stored with the drive up, the jacket around the cable can develop cracks which can let water in and cause corrosion. That causes excessive cable drag. In fact when I adjust the shift cable on my OMC Cobra part of the procedure is to measure cable drag with it disconnected on both ends. Measured with a fish scale not to exceed 2.5 lbs. I have kept the drive down as much as possible and this cable has lasted a very long time (at least 20 years)...

 

[rkilpa]

After replacing the IAC valve and filter, I attempted to duplicate the stall problem by slowly moving the shifter from gear to neutral. I found that it didn't matter if the shifter was moved slowly or not. It seams that there is a point where it "snaps" out of gear and goes into neutral so you can't really drag the shift interrupter switch to make it stall.

The IAC valve and filter replacement cured my stall problem as explained in my IAC valve thread.