Transmission Mystery!

[Scott T-Bird]

Photo of prop ...

It is a little far away but this seems to indicate that the clockwise rotation as you are looking from this position is forward, which is the way Mack says it should be (I believe).

 

[Maine Sail]

Gear ratios

According to the Yanmar service manual I have, dated 3/15/2000, the KM2-A came in three gear ratio's for forward/reverse. The reverse gear in all configurations was 3.06 as of the year 2000.

Ratio #1
Forward = 2.21
Reverse = 3.06
Forward Shaft RPM = 1540

Ratio #2
Forward = 2.62
Reverse = 3.06
Forward Shaft RPM = 1298

Ratio #3
Forward = 3.22
Reverse = 3.06
Forward Shaft RPM = 1055

 

[Maine Sail]

What I don't understand is the specific parts that cause the gear ratio in forward and reverse. Is there supposed to be a different ratio? I think I remember the first guy form Mack saying that forward is 2.62:1 and reverse is 2.13:1. If this component is reversed somehow it explains why what should be the correct rotation is actually reverse.

As I said before the transmission gears only work one way. There is no way to reverse or mess this up. A component installed backwards would not work at all and would not even go together because teeth would not match/mesh..

If you are now at 2:00 in neutral it sounds like your pedestal shifter throw is reversed. Do you have an Edson or a Yacht Specialties pedestal..??

 

[Scott T-Bird]

That's great info (I'm sure I misunderstood something about the reverse gear) ... The name plate for mine has 2.62:1 stamped on it for gear ratio so that would make it Ratio #2. Yes, I completely understand how the shift lever would work opposite when it was 180 d off. I am sure that it was previously installed the wrong way before (with the shift mechanism on the pedestal consistent) and it is now installed the correct way for the proper rotation in forward. Am I right that the issue then is internal where the gear ratio for forward is now reverse?

 

[Scott T-Bird]

I have an Edson pedestal. I know that the pedestal shifter is now reversed ... I'm really hung up on the perception that the engine is behaving like it is in reverse when the boat is moving forward and vice versa. I did hear the explanation that the transmission gears only work one way and can only be installed one way. What I can't grasp is that if there are different ratios for forward and reverse, why is my transmission in the reverse ratio mode when the transmission is driving the boat forward? The photo tach should tell the story, I am sure ... I need them to take a look at that.

 

[Witzend]

Your trying to Einstein this thing, just look at the bracket, the marks in the paint on the bracket will tell the truth as to how the shift cable was installed previously. What about the missing piece?

 

[Maine Sail]

What I can't grasp is that if there are different ratios for forward and reverse, why is my transmission in the reverse ratio mode when the transmission is driving the boat forward?

If your prop is RH rotation, and the boat is moving forward, and the shaft is spinning RH or clockwise from the stern then it is not in the reverse gear ratio and IS in the forward gear ratio.

As I mentioned earlier the gears may not be fully engaging because the throw needs to be adjusted in the linkage. This could explain the whine and slow speed due to slippage.

There is no way around it if it is spinning and moving your boat forwar, with the right prop, and right direction of rotation, the gear box is in FORWARD despite the position of your gear shift lever..

 

[Maine Sail]

Your trying to Einstein this thing, just look at the bracket, the marks in the paint on the bracket will tell the truth as to how the shift cable was installed previously. What about the missing piece?

Apparently the gear shift arm was installed incorrectly at 9:00 prior to the rebuild and the shifter assembly at the pedestal reversed to compensate for the incorrect throw direction.

According to Scott, now, after the rebuild, the shifter arm is in the correct 2:00 position yet pedestal the shifter is still reversed..

When the boat moves forward the transmission is in forward despite any shift lever positions..

 

[Scott T-Bird]

Hi Witzend,

Thanks for your response. I believe that the shift cable is installed in the same orientation as previously and that there are no marks on the bracket that indicate that anything was adjusted. However, it is evident that the shift lever is now in the correct position while it was 180 d opposite before. It does indicate that the shift lever on the pedestal needs to be changed to make it consistent with the throw on the shift lever, but it doesn't explain to me why the shift lever was installed in apparently an incorrect position before, yet the performance of the engine as indicated by the gear ratios was as it should be. Now it seems that with the shift lever position, the rotation and the forward motion of the boat all is consistent for forward, yet the gear is performing in the reverse ratio while in forward and in the forward ratio when in reverse.

 

[Scott T-Bird]

I haven't detected any grinding of the gears when in the "new" forward or reverse. The yard manager and the mechanic would detect this also if it were an issue so I can't quite accept the theory that it isn't fully engaged. We are noticing a distinct difference in performance and it is the wrong way. I should be able to get higher rpms when driving the boat forward, yet the rpms are less. We get better performance when the boat is moving backwards.

 

[Witzend]

I now understand the history of the linkage as you have described. I would suggest that in order to insure that the current linkage is adjuster correctly and is completely engaging in FWD, that you disconnect the linkage, manually move the lever into FWD, note it's position. While under way see if the boat performs any differently. Also are you sure your not fouled, prop or bottom? What was your FWD speed before verses your tack, what is it now?

 

[Scott T-Bird]

I know that you are saying that with the same rotation as before, clockwise looking at the stern, the boat is in forward no matter what the shift lever position. Well, the performance is still not adding up. I guess I won't be convinced that they didn't do something to change the gear ratios around until we get a photo tach on it to verify.

 

[Rich Stidger]

A long-shot question

Scott,

Since your transmission is available in three different forward ratio's, did you confirm that the transmission that you have now is the same one that you had originally?

Could it be possible that Mack was rebuilding a couple of KM2A units at the same time and they interchanged yours with another that had a different forward ratio?

There is a nameplate on the starboard side of the tranny. Do you know what your original number was to verify? If they repainted the tranny after rebuild it might not show the same scratches, wear spots, etc. that it had before rebuild.

 

[Maine Sail]

I think I remember the first guy form Mack saying that forward is 2.62:1 and reverse is 2.13:1.

Scott,

If this info from Mack Boring is correct about your reverse ratio, my service manulal is for the year 2000, then the tranny will spin faster in reverse, at a given RPM, then when in forward. So what you are seeing, faster in reverse, would be normal behavior.

As Tim stated you don't need a photo tach. Simply mark the crank pulley on the engine and the prop shaft with a Sharpie marker. Then turn the crank with a socket wrench 2.6 times and see of the shaft spun exactly once. For every 2.62 times the engine crank turns the prop shaft will spin once. For every 2.13 times the engine crank spins the prop shaft will turn once in reverse..

We are noticing a distinct difference in performance and it is the wrong way. I should be able to get higher rpms when driving the boat forward, yet the rpms are less. We get better performance when the boat is moving backwards.

This is not always the case. If you have a 3.06:1 reverse it would spin the shaft slower in reverse than it would in forward but if you have a 2.1:1 it would spin faster. There are plenty of gear boxes that spin faster in reverse than in forward and vice versa..

 

[Scott T-Bird]

Thanks All!

Yes, I can confirm that it is the same one since I have a picture of the name plate before it went in. There is no possibility that it is fouled. The tranny was re-painted, and I have no explanation for why the shift lever was in a position that was different before. I am going to ask Mack if they noticed this before they did the re-build ... it should have been obvious. There must have been a reason. We did verify that the lever was fully engaged so I can't believe that engaging the clutch was the issue for performance variance. I am going to post a spreadsheet that hopefully explains it.

Tim, about decompressing ... put it in gear and how do you decompress?? Is it by pumping the decompression lever on top? Then manually rotating, what am I looking for in regard to determining the gear ratios in forward and reverse?

file:///D:/Personal%20documents/Scott/Tranny.htm

 

[Witzend]

One other possibility is they rebuilt the tranny with a different ratio than what you shipped them. Or maybe upon the first re-build they changed it and now it has been changed back. The de-compression lever just needs to be held making it easier for the rotation of the crank.

 

[Scott T-Bird]

Thanks again ...

That is very useful to know and you answered my question! I'm sure I can handle that so I am going over now to check it out. MS, I will also ask again about the ratios because I doubt I relayed the numbers correctly. I know that the plate is stamped for 2.62:1 but I suspect that the reverse ratio is 3.06 as you listed rather than what I thought I heard ... but let's see!

Well, I better get moving if I'm going to get anything done today! I'll report back on what I find. I guess my spreadsheet doesn't show up so I'll work on that later ... its an xls file that I have to convert somehow. I thought it would work as an html but obviously its still linked to my hard drive, and I hope you don't have access to that!

 

[Witzend]

You can always take a screen shot, paste it in paint, and save as a .jpg.

 

[Scott T-Bird]

Mystery solved ...

Ok, I was convinced that somebody in the past reversed the gear ratios somehow. Of course that's not the case! It was simply a matter of somebody installing the shift lever upside down and compensating by reversing the linkage on the pedestal (or vice versa). Now, the shift lever is right side up and the linkage needs to be corrected in the pedestal. I think that the issue may be that the pedestal shifting mechanism cannot be assembled correctly. The yard had a brief look yesterday and told me that it looks like there is only enough room to put it in one way (the way it is now). If that is the case, then I suppose that they (the PO) solved the problem in the past by changing the shift lever on the tranny upside down. Tomorrow, when I pull it apart, I'll soon find out. If I have to, I'll get a different shifter because I'm not going to attempt to get my body and hands in a position to mess with the shift lever, it is too far back and hard to get at now.

Witzend, you were right about the bracket. The yard was puzzled about the way it directed the cable and rod down in a more vertical position. They did bend the bracket back to the original position to orient it correctly now. The mechanic told me about that today when I was talking this over with him.

Mack Boring may have been able to clue me in on this situation if they had noted the position of the shift lever when I brought it in and either re-assembled it the same way that it was or told me it was going to be different (the way it is supposed to be assembled). Regardless, they were helpful over the phone. I spent most of my time talking with a gentleman in the Chicago area who knew quite a bit about the engine and transmission. I did verify the gear ratio as 2.62:1 in forward and 3.06:1 in reverse.

The whole ordeal was actually kind of illuminating to me. I was able to verify the gear ratios by spinning the crank shaft and observing the prop shaft just as Tim described. I also learned that the engine spins clockwise (when observing from the cabin looking toward the stern, while the prop shaft turns counter-clock while in forward. The prop shaft turns the same direction as the engine when the tranny is in reverse. Not being a mechanic, this is counter-intuitive to me and although I feel a little embarrassed that this was not evident to me because it seems obvious now, I have to admit that I just never paid attention to that detail before.

The Mack Boring gentleman was speculating that my engine doesn't develop the rpms that it should in forward because I may have an oversized prop or too much pitch. He suggested that I check out the prop to determine if it is sized correctly. He thinks that the additional load of the mis-matched prop could result in producing an engine whine that I was noticing. I am going to look into this in more detail ... I've been wanting a folding prop so this may be my excuse to pull that trigger!

In any case, I really appreciate the interest and the help, and once again I am astounded by the level of help and the knowledge that is available on this forum! Sometimes it really amazes me how you folks are able to read things that can sound confusing or off-base and quickly get to the heart of the matter!

 

[Witzend]

Scott, your quit welcome, and right about all these wonderfully astute sailors who have this amazing forum to share their lifetime of knowledge with others. Our community moderator is the shinning among us and it's THREE CHEERS to MAINE SAIL!!!