manual transmission pops out of 3rd gear

[jeffc280sl]

I have recently had a lower rear bearing failure in my fluid leaking but otherwise working G72 manual transmission.  I installed new seals, gaskets and bearings and went for a test ride. I did not replace synchros because they looked okay to me.  Here is a picture of what I think is a acceptable synchro. The underside (no picture) of the synchro cone has well defined grooves.  On page 118 the Haynes manual talks about a clearance of .5mm between the synchro and dog teeth. My tolerance for a typical synchro was about 1mm. Does anyone have any comments to share concerning this tolerance?
Otherwise the synchros fit tightly against the gear cone.

Download Attachment: good synchro.jpg
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The transmission shifts into all gears fine.  But when not under load and coasting the transmission pops out of third gear.

Any ideas as to the cause of this problem?  

Thank you

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

Thought I would post a pic of the synchro grooves on the inside of the cone.  I don't see any wear here or on the inside ring surface.

Download Attachment: synchro groove.JPG
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Jeff C.
1970 280SL 4-speed

[Naj ✝︎]

Hello, Jeff,

Did you renew the change-speed lever linkage bushes?

I'm wondering if third gear if fully engaging when selected.
Could be the linkage 'bottoms out' before full engagement?
Just a thought...

naj

68 280SL

[jeffc280sl]

Naj,

Good idea!  I changed the bushings 2 years ago and inspected them prior to installing the transmission. They are in very good shape and I think I'm getting full engagement from the shifter.



Jeff C.
1970 280SL 4-speed

[jeffc280sl]

Thinking about what Naj said and after doing some reading and I've decided to recheck the linkage first since that's a good possibility and the easiest.  Previously my shifter was located to the very rear of the adjustment plate in the tunnel so if any adjustment were needed there would be no place to go.  I've decided to shorten the the support bars and shifter rod by 1/4 inch which should bring me in the middle of the shifter plate adjustment. I plan to install the shifter rods but leave the 4 shifter base bolts loose.  Next I plan to install but not tighten the intermediate drive shaft and transmission mount.  Once I get the flex ring bolted in place properly underneath I will lower the car to operating position.  At this point the engine and transmission will have "settled" into place.  I plan to make sure the drive shaft rotates freely and then go back and tighten the transmission mount and intermediate shaft.  This will be followed by tightening the shifter base bolts.  

Here is a pic of the transmission full of fluid with the top off.

Download Attachment: transinternal.JPG
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Jeff C.
1970 280SL 4-speed

[Naj ✝︎]

quote:Originally posted by jeffc280sl

 Previously my shifter was located to the very rear of the adjustment plate in the tunnel so if any adjustment were needed there would be no place to go.  I've decided to shorten the the support bars and shifter rod by 1/4 inch which should bring me in the middle of the shifter plate adjustment. I plan to install the shifter rods but leave the 4 shifter base bolts loose.  Next I plan to install but not tighten the intermediate drive shaft and transmission mount.  Once I get the flex ring bolted in place properly underneath I will lower the car to operating position.  At this point the engine and transmission will have "settled" into place.  I plan to make sure the drive shaft rotates freely and then go back and tighten the transmission mount and intermediate shaft.  This will be followed by tightening the shifter base bolts.  
Jeff C.
1970 280SL 4-speed

You can change the position of the vertical shifter by changing the position of the splined front of the horizontal link to the gearbox.
Take the top tunnel inspection cover off to see/adjust the splined joint.

naj
naj

68 280SL

[jeffc280sl]

Thanks again Naj.  Went through the linkage yesterday and made some minor adjustments to center the shifter base.  Went for a test ride and I still have the problem.  I can hold the gear in place with the shifter but as soon as I release the shifter and coast it pops out of gear.  While holding the shifter in gear and transitioning from acceleration to coasting I can feel the synchro body try to move away from 3rd gear.  Plan to take the transmission out this week and inspect the third gear synchro ring and main shaft end play.

While taking a test ride I left the tunnel cover off so that i could inspect the main shift rod and listen to the transmission.  Everything except popping out third gear when coasting was great.  The transmission is quiet and smooth.  Absolutely no vibration at high or low speeds.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

For those that may be interested I thought5 I would present my initial take on why the transmission popped out of third gear.  The attached pictures show the synchro body in neutral, partially engaged and engaged.  For some reason my synchro body will not fully engage and as a result it pops in to neutral while coasting.  I suspect that I will find something inside the synchro body that I did to cause the problem.  I should have caught it on the bench but at the time I was not as smart as I am today.  



Download Attachment: engaged.JPG
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Download Attachment: partial engage.JPG
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Download Attachment: neutral.JPG
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Jeff C.
1970 280SL 4-speed

[Benz Dr.]

I had this problem once. In that case it turned out to be a worn input shaft where it fits into the pilot bearing. This would cause the whole shaft to deflect and make the trans pop out of gear.
Another problem is too much end play on the input shaft - it should be pretty much zero. A loose nut on the main shaft can also factor into this.
 The syncro it'self could be worn. Place it on the cone and try to turn it which it should do no problem. Aply a small amount of downwards pressure as you turn it and it should lock up. This is how the syncro works but not why it will pop out of gear.
 There are small springs and a ball under each spring inside of the syncro unit. Maybe one ball is missing - it should be faily tight and take about 25 pounds of force to move it. If you decide to take the this thing apart wrap it in a rag before you start or you will have a bunch of little hoo - das flying out all over the place. Ask me how I know this...... took me hours of crawling around on my hands and knees seaching the shop floor for a little spring.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[jeffc280sl]

Thanks for the input Dan.  I'm sorry to say I must admit to searching for springs and ball bearings more than once as those little *** slipped out of place.

My problem is that I don't think the outer synchro body is fully engaging the dog teeth on third gear. It just covers the teeth a shown in the earlier picture.  This is the case when I orient the tooling grooves on the synchro body ring towards third gear.  With the ring oriented this way I can move the body towards and past 4th gears dog teeth.

If I reverse the outer synchro body ring and put the tooling grooves towards 4th gear the ring will slide fully on to third gear but only partially on to 4th  (it just covers the 4th gear dog teeth.  So depending which way I orient the tooling grooves I think the ring only partially engages which is causing it to pop out of gear.

My question is this.  To be fully engaged does the synchro body have to cover and go slightly beyond the dog teeth on the gear that has been actuated?  

To complicate matters I have another transmission from the same era.  If I use its synchro body/ring it fully engages both 3rd and 4th gears no matter the direction of the tooling grooves.

Basically I need to know which direction to orien the synchro body tooling grooves.  In addition is it normal for the synchro body to engage 3rd and 4th gear dog teeth differently as per my prior pictures.
 
Thanks for your help. Needless to say I can use it.


Jeff C.
1970 280SL 4-speed

[Benz Dr.]

Better ask Mr transmission.

I know that the syncro rings are different but I can't remember which one. I think first gear has a different width boss or something..
Anyway, I believe it should cover both sets of teeth. It think it's marked so that it has a forward facing side, possibly with small grooves on the ring. The brass syncro is supposed to slip a small amount so that both sets of teeth line up and then the ring will move and lock everything up. It should be noted that fourth gear has no actual gear engagement and the ring simply moves locking the main shaft to the input shaft. The power goes straight through the trans and any more gear reduction is done by the rear axel gears.

Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[jeffc280sl]

Thanks again Dan for your help.

Here are three pictures of the synchro body and 3rd and 4th gears.  
The first pic identifies the gears and orientation of the synchro body tooling grooves, in this case it faces 4th gear.  

The next picture shows 4th gear engaged.  Notice the synchro body is just covering the dog teeth on 4th gear.  There is some sort of mechanical limitation that will not permit the synchro body ring to progress past this popint on forth gear.

The third pic shows 3rd gear engaged.  The synchro body is completely engaged and fully covering the dog teath of third gear.

In the next post I will turn the synchro body ring 180 degrees and post the results.  Can you offer any comments concerning the difference in synchro body overlap of third gear dog teeth versus just covering 4th gears dog teeth.

Download Attachment: 3rd engaged.JPG
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Download Attachment: 4th engaged.JPG
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Download Attachment: neutral groove 4th.JPG
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Jeff C.
1970 280SL 4-speed

[jeffc280sl]

By reversing the outer synchro body ring direction the depth that the ring engages the dog teeth is reversed as well.

If I use the synchro body ring from the SL transmission 3rd and 4th gears will completely engage, no mechanical stop, no matter which direction the tooling grooves face.



Download Attachment: 3rd engaged a.JPG
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Download Attachment: 4th enagaged a.JPG
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Download Attachment: neutral groove 3rd.JPG
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Jeff C.
1970 280SL 4-speed

[jeffc280sl]

The sychro ring body has 2 rows of splines that slide back and forth on the synchro body.  After looking very closely I conclude that the splines in one row are different in width from the other.  Each row is uniform in width its just one row is slightly wider lets say.  The wider spline is on the side that has the tooling grooves.  Because of the width of the grooved side splines this side will not completely engage the dog teeth.  The orientation of the grooved side, whether it faces third or fourth gear, determines which gear will be fully engaged and which will be "partially" engaged.  

When the transmission was popping out of third gear the grooved side of the synchro body ring was facing third gear.  Should the grooved side be facing 4th gear?

Now that i have gone back and looked at pictures I can see that there are 3 tooling stripes on the 280SL sychro body.  When I assembled the tranmission I used the synchro body from the other transmission that had two stripes.  This is the one that would not engage 3rd gear completely causing it to pop out of gear.

Thank you for your help.

Jeff C.
1970 280SL 4-speed

[Shvegel]

Having rebuilt a few manual transmissions I would look at a few things.

Make sure the transmission is being fully shifted into gear.

Make sure the faces of the shifter fork that slides the 3rd 4th collar are not worn. The fork should fit fairly tight in the collar.

Compare how third gear and forth gear fit into the collar. Slide the gear into the collar and twist. A loose fit here is trouble. Look hard for evidence of wear in the slots and teeth. The small teeth on the gear and the slots on the slider are really the most important thing to look at in the trans. they carry the torque of the engine and when you shift from power on to coast this area can get worn. If it gets worn it can wiggle and cause the gear to jump
out.

The points on the teeth and the points on the slot help to get the slider across the gap and get it into gear. the area behing the points should be flat and free of any wear.

I wish I could give you a quantatative number as to radial play but It is something I have just developed a feeling for.

[jeffc280sl]

Shvegel,

Thank you for your input.  Your points are great and I've check them as follows.

The shifter forks are moving the transmission into gear.

There is a .010in gap in the fork and the 3rd/4th gear collar.

The collar moves onto third and fourth gear nicely with no play.  

Slots and teeth are in very good shape.

The area behind the dog teeth points is flat and free.


I found the problem!!

I went back and looked closely at pictures  of both transmissions as I took them apart.  I discovered that I mixed the synchro body from my second transmission with my 280SL transmission.  As I look closely I can see that the SL body had three tooling stripes on one side.  The dog teeth of third gear on the SL transmission also had one tooling stripe.

My other transmission synchro body had two tooling stripes and the dog teeth had none.

As I compared third gears I noticed that the SL gear had wider dog teeth than the other tranny.  I apparently mixed up synchro bodies when I reassembled the SL transmission and used the body from the other transmission.  The splines on it were not wide enough to fit the dog teeth of SL 3rd gear so it did not engage fully and popped out of gear.  

I will make a note of this difference along with some others  so that those who choose to swap another models transmission into there SL know what they are getting into.  



Jeff C.
1970 280SL 4-speed

[hands_aus]

ever thought about writing it up in the Technical manual?
You have the pics.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL, auto

[jeffc280sl]

I want to put this issue to rest and hope the attached pictures will help others should they follow the same route.  

The first picture of third gear and the correct synchro body are from the SL transmission.  Note the 3 tooling grooves on the body and one tooling groove on the dog teeth of third gear.  The SL dog teeth are wider than those from my second 1960's era MB transmission.  The splines on the SL synchro body are also wider apart than the other body.  These parts need to be matched correctly so that the gear can be engaged properly when shifting.

 

Download Attachment: 3rd gear tooling SL.JPG
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The second picture shows the equivalent parts from the other transmission.  Note the body has two tooling grooves and third gear dog teeth have none.



Download Attachment: 3rd gear tooling other.JPG
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You should be very carefull when thinking about mixing and matching parts from one transmission to the other.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

I think we can put this topic to rest.  Reinstalled the transmission and took it for a test drive.  It felt great and all gears shift fine.  Nice and quiet also.

Lessons learned are many.  The one that took up most of my time was the mistake of mixing parts from one transmission to the other.  There are subtile yet import and differences that in my case caused third gear to pop into neutral.

While I was servicing the transmission I took care of some other related issues as follows.  Here are the parts I used for the entire project.

new
clutch disk
throwout bearing
pilot bearing
drive shaft bearing
transmission mount
4 transmission bearings
layshaft key
3 new transmission seals
4 new gaskets

All in all a good project that would like most be much easier a second time

Jeff C.
1970 280SL 4-speed

[Peter van Es]

Jeff,

Please, please, please write it up for the Technical manual!

Peter


Also known as 'admin@sl113.org' and organiser of the Technical Manual (http://www.sl113.org/wiki/pmwiki.php). Check out http://bali.esweb.nl for photographs of classic car events and my 1970 280SL.