Re: Delayed shift out of gear
One of two things is (probably) happening here.
1) The lower unit shift cable is bad, or
2) the ignition interrupt switch is bad
It could be both problems simultaneously.
Assuming you haven't been diddling around with the shift cable adjustments, and the problem just started this season, or got worse this season, these two aspects are the likely cause. The lower unit shift cable core is designed in such a way so that it reaches its limit of travel each time you shift into neutral and pulls on the cable sheath. This 'pulling' on the cable sheath causes the cable sheath mount to move. When the cable sheath mount moves, it moves against a spring and moves a cam which closes the ignition interrupt switch. When the ignition interrupt switch closes, it applies a ground to the engine ignition, stalling the engine. When the engine stalls, the gears (clutch) in the lower unit decouple and separate because the lower unit shift cable sheath is pulling on it. Once the gears decouple, the 'pull' on the lower unit cable sheath is relaxed, the spring-loaded shift interrupt cam returns to its center position, which opens the ignition interrupt switch, which removes the electrical ground to the engine ignition which allows the engine to contine to run. CleverClever. When working correctly, this happens in the blink of an eye.
You will only see this happen when the boat is in the water. The lower unit gears are back-cut so that they stay engaged when you have a load on the prop. Without a load on the prop, the shift cable has enough authority to disengage the lower unit gears. But, with water loading the prop (and the gears), the shift cable no longer has enough authority to disengage the gears. The only way to remove the load on the gears is to stall the engine momentarily.
So, to test this, put the boat in the water, it can be on the trailer at the ramp, enough so that you can submerge the prop. Have someone put the boat into forward and then to neutral while you watch the shift mechanism on the engine work. You will probably see that the ignition interrupt cam isn't moving enough to close the ignition interrupt switch. Or, you will see that its moving the cam and closing the switch, but the switch isn't stalling the engine. This will tell you where to focus your repair. Hope for the switch, its easy to replace in comparision to the lower unit shift cable. Another test you can do is to disconnect the switch wires. There are two. One goes to the engine ignition, the other to ground. With the prop submerged, try to shift the boat into neutral, then touch the two wires together momentarily. (Not the wires leading to the switch, the other ends, then 'engine' ends.) You will notice the moment you touch the wires together (simulating what the ignition interrupt switch does), the lower unit will disengage the prop and go into neutral. If you hold the wires together, the engine should stall (because you are applying a steady electrical ground to the engine's ignition).
You can also 'help' the shift interrupt cam to move by watching which way it moves when your helper shifts from forward to neutral and push on it a little in the same direction by hand to see if it goes into neutral when you do.
If the lower unit shift cable is worn and has excessive end play, the shift interrpt switch cam doesn't move enough to close the shift interrupt switch, which means you don't get neutral when you want, which leads to nice dock rash, as I'm sure you've discovered. Makes the wife mad trying to load the thing onto the trailer which leads to marital problems. So, its important to get this solved. If you find the lower unit shift cable is the culprit, its labor intensive to fix, best to take it to the shop. (translation: $500 ++) It can be done DIY but it'll take a while. The part is less than $100. Check the lower shift cable by disconnecting the UPPER shift cable (and shift assist spring, if equipped) from the shift arm (at the engine shift plate) then move the shift arm by hand to see if the lower shift cable is nice and free-feeling. Mercruiser Service Manual 14 prescribes end play on this cable (with the drive installed, in gear, and someone standing on the prop) no greater than 9/16". When checking the lower unit shift cable in the boat with the drive attached you are actually testing much more than the cable end play, so just because you find end play greater than the prescribed limit, doesn't mean your shift cable is bad, it just means you've isolated your problem to the shift cable, the shift arm and roller in the bell housing, the shift shaft and its bushings in the bell housing, the shift shaft, shift spool and shift crank in the lower unit (among other icky-to-fix parts) or a combination of these parts which individually may be useable, but together may form a system that isn't. Follow the procedures outlined in Mercruiser Service Manual 14-1c for Troubleshooting and you can further isolate the source of the excessive end play. On my boat, the end play is 1/2" and it works fine, so I suspect you can be right at the 9/16" limit and still be operationally fine.
EDIT Jun-25-2012
*** SOLVED ***
After many new parts and a nagging problem where MY 2000 Mercruiser 4.3 Alpha Gen II would NOT shift OUT of gear, I was able solve the issue conclusively. Conducting the troubleshooting tests for the shift system found in Mercruiser Service Manual 14, nothing was out of tolerance or noticeably bad. I found Mercruiser Service Bulletin 2000-01
View attachment Service Bulletin 2000_01 Delayed Shift.pdf
which identifies a shift interrupt cam spring with the wrong tension causing delayed shifting to neutral. I procured the proper mercruiser spring part number prescribed by the service bulletin and compared it to the existing one installed on the boat. They are identical. Both use 0.096" wire and have identical shape, the "V" of the springs are the same distance apart. Hmmm... The new spring was in a Mercruiser bag dated May of 2000. Could it also be a faulty part? I wasn't marked at all, as prescribed by the service bulletin. Its disappointing the service bulletin and/or the troubleshooting section in the Mercruiser manual doesn't prescribe some way to test the shift interrupt cam spring tension. Here's why its critical.
The issue that I have is the lower shift cable doesn't have enough authority to overcome the shift interrupt cam spring. I suspect the spring in the shift spool in the lower unit is compressing allowing the gearshift to find its neutral detent BEFORE the shift interrupt cam spring deflects enough to close the shift interrupt switch (thus preventing the engine to stutter momentarily relaxing the clutch dogs allowing them to decouple to their neutral position). Since I now have two shift interrupt cam springs, I elected to remove some of, but not all of, the spring's preload on the shift interrupt cam, hopefully allowing the shift interrupt cam to deflect before the shift spool spring does. After some trial and error getting the shift interrupt cam spring the right preload (I used pliers to tighten the "V" a little at at time) a on-the-trailer test with a helper standing on the prop revealed the shift interrupt cam moving to almost full deflection in both directions. A good sign. Water testing reveals the shift interrupt cam moving full deflection which allows neutral well before the neutral detent on the gearshift. EXACTLY how its supposed to work. The shift interrupt cam spring had way too much preload tension so instead of deflecting when it needed to, the shift spool spring compressed when the gearshift was in the neutrual detent allowing the clutch to remain engaged. Reducing the shift interrupt cam spring tension allows the shift interrupt cam to deflect BEFORE the shift spool spring compresses.
*** SOLVED ***
For folks that are having a hard time getting into neutral that have followed the shift cable adjustment procedure, and don't have any known bad parts identified by the troubleshooting section in the Mercruiser manual this nugget is for you. Reducing the shift interrupt cam spring preload is free and easy, the best sort of troubleshooting.
Also, for those with a shift assist spring (not to be confused with shift interrupt cam spring or shift spool spring), include Mercruiser Service Bulletin 1999-06
View attachment Service Bulletin 1999_06 Shift Adjustment with Commander 3000.pdf
in your shift cable adjustment procedure. Once you read the bulletin, you'll realize that the shift cable adjustments prescribed in Mercruiser Service Manual 14 are overly complicated.
