Merc Shift Cam

[joeveebee]

I have a question about a lower unit problem that I'm hoping someone can help with.
I have a 1973 mercury 850 85 HP outboard.
I have torn into the lower unit to try and find what is wrong and I think I have but am not sure.
The unit would not go into reverse, it would click and grind and just seemed to not be getting into reverse at all. Then last summer, it started making some noise in forward as well, it would still engage into forward but was noisy.

When I opened up the lower unit, I expected to find some gear damage but the gears all looked good. The one part that had wear or damage was the shift cam, I haven't looked at a new one to compare and see just how worn down this one is but it looks like there is a piece missing and/or it is worn smooth on one side of it. I've looked at a picture of a new one and mine looks like it is missing quite a bit of itself.

My question is, could this worn shift cam cause the symptoms that I described?

It would be easy enough to just replace it and see if that fixes it but I want to avoid spending unneeded money since I'll need to replace all the seals and will have to buy a new gear housing cover as I had to drill the old one out.

Thanks in advance for any help..
joe-

 

[Texasmark]

Re: Merc Shift Cam

Not an expert, but have been there, done that.

The Merc shifter mechanism is a "shuttle gear" commonly referred to as a "clutch dog" that slides back and forth on the propshaft. It is pinned to the prop shaft so it spins with it.

The insides (dog sides) of both F and R gears are notched to accept the teeth of the clutch dog when it is forced into them. Some of these teeth are sawtoothed, and some are square. F is usually sawtoothed allowing for prop overrun when decelerating rpm's under load, and R is usually square.

So shifting is merely positioning the clutch dog, via the plunger riding on the shifter cam.

The clutch dog is spring mounted against the cam plunger whose forward point rests in grooves in the cam. If you push on it (the semipointed plunger) in the aft direction, it will push back at you so it is more or less relaxed in F and has some forced compression against it in N, and more in R.

The cam is driven by the splined shift shaft which shifts by twisting (cw or ccw) and is splined into the shifter cam. The shift shaft provides the pivot for the cam as it protrudes thru it and is mounted into the lower unit housing for support. You can see the cam if you pull the prop shaft, dog, and R gear out and look thru the hole in the F gear.

The cam has 3 positions which are at different distances from the pivot point; the shift shaft. This difference in length is what positions the dog in F, N, or R.

The cam has 3 slight (round) detents with a very slight groove between detents which help to keep the semi-pointed tip of the cam plunger centered in/on the cam. So as you rotate the shifter, the plunger just moves in and out, following the cam.

That's it.

HTH

Mark