Mercruiser 3.0 won't "lock in"

tr@v1s

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Aug 21, 2025
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Hey all. Haven't been doing anything with boating for about 12 years. I used to be on iboats forum back then and messed around with little boats and old OMC outboards. I don't have the old email I used then to access that account, so started a new one. I have just purchased a 1989 28' Harris Flotebote with a 3.0 mercruiser I/O (Alpha 1 Gen 1). Was not able to water test, but got to run it with muffs on and shift forward and reverses multiple times. Was able to get it around $3,000 under book price, and at a price we thought was a great deal, so we did it. That was last Friday evening. This is my first I/O.

Took the boat out last Saturday morning with the 20 year old son to test and make sure all was good before momma and the 21 year old daughter, 17 month old son, and 5 week old son joined us (yes, I have two generations of kids! lol). Everything went smooth. Motor ran great and seemed to move us across the lake at an expected speed (I certainly need to learn better how to set trim and all that stuff, but it was going nicely). We had about an hour to kill waiting on the rest of the family to show up to the dock, so the boy wanted to fish. we dropped anchor and unsuccessfully fished a few minutes and realized we were drifting toward the cove where there were lots of trees/stumps just below the surface. pulled anchor and put it in reverse and bumped right into a stump. Stopped the engine and tried to paddle out of the situation. Got where things looked clear, but could not get the prop to lock in to gear - just clicking.

Called for a tow. They broke down and had to bring seadoos for the tow after we'd waited a couple of hours. They looked at it in the water and though maybe busted u-joints. Got it to the trailer and took to their shop where they decided they didn't want to check it out until Tuesday. No communication or call backs from them Tuesday meant I was going to pick it up and check it out myself Tuesday night. I got it home and found that it seemed to engage fine if I pressed the shift linkage between the upper and lower units a little extra (whichever direction I was trying to shift). So I speculated that the shift cable was not adjusted correctly. Maybe I had stretched it or something when trying to shift out of gear when we hit the stump? I played with the lower shift cable last night and got it to where forward gear locks in pretty solid, but reverse with sometimes pop out (this is all tested motor not running and just turning prop opposite direct of shifted direction). I could never get to where both engaged equally well.

So, here's my questions:

1. Am I just not adjusting correctly?
2. Did I screw up my dog gear and need a rebuild?
3. Is there something else I'm completely missing?
4. Once I get this addressed, should I replace the stainless prop that was put on by the previous owner and go back to an aluminum prop?
5. Is it worth having a second out drive ready to go during boating season just to be able to swap out in event of breakdowns if you can pick one up fairly cheap?

Motor s/n: B849523
Drive unit s/n: 0B934426
Transom Assembly s/n: 0B943249
 
Last edited:

alldodge

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VIDEO links
The vids will show how to adjust cable.

If you are unable to get it to work both ways consistently than the lower cable most likely needs to be replaced
 

tr@v1s

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Aug 21, 2025
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VIDEO links
The vids will show how to adjust cable.

If you are unable to get it to work both ways consistently than the lower cable most likely needs to be replaced
Thank you dodge!! My shift plate is a different style than what is shown in the video. I assume the process is still the same? If the console cable has more than the specified 3" (+/- 1/8"), I assume that means the cable has to be replaced? What about the locking nut that locks the tube to the brass lead to the cable sheething? Is there a distance for setting that tube or should it be screwed all the way in??

Sorry, I have zero experience with these things...
 

alldodge

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I think you might be getting the shift bracket distance from the cable length distance

You screw the tube in as far as it will go, than tighten the nut down
 

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tr@v1s

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Aug 21, 2025
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Good news: adjustment worked.
Bad news: Started motor to test shifting with motor running after verifying manually that the shift was locking in on the clutch dog and the motor was not turning the outdrive shaft. Assuming I have a trashed motor coupler...
🫩🫩🫩
 

tr@v1s

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Aug 21, 2025
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9
Good news: adjustment worked.
Bad news: Started motor to test shifting with motor running after verifying manually that the shift was locking in on the clutch dog and the motor was not turning the outdrive shaft. Assuming I have a trashed motor coupler...
🫩🫩🫩
Question now:

Pull motor, pull out drive, or pull both? (I'd like to be on the water next weekend if possible, but also want to do it right.)
 

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tr@v1s

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Aug 21, 2025
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A very good chance the vertical shaft is the problem.
What do you mean by 'the vertical shaft'? The drive shaft to the lower unit? Pretty sure everything in the drive is good. You can see in the video I posted me turning the prop with it in gear and the shaft going into the motor coupler spinning without the motor coupler spinning. Am I not understanding something?
 

nola mike

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Apr 22, 2009
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What do you mean by 'the vertical shaft'? The drive shaft to the lower unit? Pretty sure everything in the drive is good. You can see in the video I posted me turning the prop with it in gear and the shaft going into the motor coupler spinning without the motor coupler spinning. Am I not understanding something?
vertical shaft in the drive. would be a good guess given your prop strike, but vid shows blown coupler.
 

tr@v1s

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Aug 21, 2025
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Can anyone help me locate a part number number and vendor for the motor coupler? Recommendations on process/steps?
Any other things I should definitely do while im replacing the coupler?
 

kd4pbs

Petty Officer 2nd Class
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Mar 5, 2012
Messages
142
I see them on eBay and Amazon. I imagine every marine supplier worth their weight sells them. Call a supplier with your engine info and they'll sell you the exact one you need if you can't find a part number on the old one or don't trust parts lists. You'll need to remove the drive, then the engine.
If it were me, I'd make sure any corrosion issues are addressed while there. While the drive is out, you should also replace the seawater pump impeller. If the old one is in good shape, keep it for a spare.
The worst part of the job is devising an overhead lifting point for the engine. I have used stout tree branches that I was absolutely sure could support the weight beforehand by lifting the front of a Jeep with the same rigging, sections of Rohn 45G tower ratchet strapped together, wooden beams laid on top of the gunnels, and a long steel beam interlaced into the top side of the bottom roof trusses over the garage. On a 'toon, you might be able to just use a cherry picker on deck.
 

kenny nunez

Captain
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Jun 20, 2017
Messages
3,443
If the splines completely went away there has to be a alignment problem.
sierra #18-2321 looks like the coupler that you need
Mercruiser # 12632A6
sierra#18-4442 alinement tool. Every Mercruiser owner needs this.
 

tr@v1s

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Aug 21, 2025
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I was thinking I would go ahead and do bellows and gimbal bearing while in there. I assumed the same about the alignment, so I figure best to just knock all that out while I have the unit out. Got good numbers for those and the puller/install tool for the gimbal bearing?
 

Fun Times

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Motor s/n: B849523
Drive unit s/n: 0B934426
Transom Assembly s/n: 0B943249
Here's your parts catalog for your serial numbers...
0B849523 = https://www.mercruiserparts.com/3-0l-gm-181-i-l4-1987-1989-0b450801-thru-0c856558

0B934426 = https://www.mercruiserparts.com/r-mr-alpha-one-sterndrive-1983-1990-6225577-thru-0f386995
^One in the same as,
0B943249 = https://www.mercruiserparts.com/r-mr-alpha-one-transom-assembly-1983-1990-6216687-thru-0f386995

If you're going to be your own work to the gimbal transom assembly then one of your best bets would be to pick one of the tools sets with the most tools available for the lowest cost..., https://www.ebay.com/sch/i.html?_nk...ruiser+transom+alignment+tool+sets+&_osacat=0


 
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