4 speed transmission internals

[jeffc280sl]

Having no prior experience with the insides of a manual transmission I must tell you it is a work of art.  I've come to understand the importance of the shims used in the tranny.  The front gear and shaft as shown in the attached picture is mated with the rest of the main gear assembly and held in place by the clip you see in the picture and the bearing snap ring.

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It is not bolted to the rest of the main shaft.  If the front gear assembly were to slide to the rear of the tranny the synchro ring would be compressed against the gear cone and the gear would seem like it is engaged.  A shim can be placed under the snap ring of the bearing to pull the front shaft assembly forward so that the synchro ring has enough room to slide of and on the cone gear assembly.  The shim and bearing ring keep the gear shaft from moving to the rear and the bell housing and transmission shaft cover keep it from sliding forward.  Same goes for the rear portion of the main shaft.  The rear cover and shims keep the bearing from moving rearward which inturn hold the main gear shaft in place front to rear.  I guess you have to see it to really appreciate the design.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

Some may know that I picked up a spare 4 speed manual transmiossion and I'm now finished with the cleaning and refurbishment of it.

I intended to replace the old transmission because of leaks and redo the clutch sometime this winter.  No other problems in the old transmission.  Then I plan to refurb the old transmission and maybe sell it.  I need to move my plans forward.  Last night went out for a spin and heard a clunk and then whinning sound from the trans/clutch area.  Fortunately I was able to make it home and back into the garage.  Drained the oil out of the transmission this afternoon and the metal pieces you see in the picture came out as well.

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Having just been inside a transmission I can say the noise is coming from a failed bearing.  The pieces that came out with the fluid are part of the individual ball bearing separator.  Will find out in the next week or so if any other damage has taken place as a result on the bearing failure.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

Removed the clutch and transmission from my SL this morning.  Tore the gearbox down to confirm the bearing failure and thought I would post a picture. It is the lower rear bearing. Will have to go inside to see if there is any other damage.  I count 7 balls so atleast one of them was not rolling around inside doing damage.

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Jeff C.
1970 280SL 4-speed

[jeffc280sl]

I've now taken the SL manual transmission completely apart for inspection.  Noticed play on the layshaft.  Took the input and third gears off the shaft and found a failed key.  I think there is supposed to be a single key fixing these gears into position.  The key broke into pieces at the point where these gears butt up to each other.  Here is a pic.

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The next post will include a pics of the key from a different angle.  You can see the key was under stress and became loose over time causing the failure.

Jeff C.
1970 280SL 4-speed

[jeffc280sl]

This key is another area to look at if you decide to tear your transmission down and repair.

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Jeff C.
1970 280SL 4-speed

[George Des]

Jeff,

I know this post is really old, but I just tore apart a transmission and found the exact same situation with the broken key on the countershaft. Must be a waek point on these boxes. Do you recall where you got a new key and how much it was? Thanks.

George

[ja17]

Hello,

These keys are badly worn or broken a lot of the time. Both the small key on the top shaft and the large key on the bottom shaft are fairly inexpensive even from Mercedes. The large key was becoming hard to get the last time I ordered. It may be NA at this point? However in many cases, MB will have NA parts made again and available in the future.

I try to have both keys on hand when I have a re-build coming in. Synchros are seldom bad.

[George Des]

Joe,

Checked with Tom Hanson. The large key as you said is no longer available. I was able to salvage a usable one from another countershaft. Curious though as to why these would fracture the way they do. Must have been some significant stress put on the countershaft at some point like maybe shifting into reverse while moving forward or vice versa??
 Another interesting thing I have been finding after pulling three or four of these apart is that some have single metal shielded bearings in the same application where some are totally open. Seems to me the shields would block some of the AT fluid from lubricating and cooling the bearings.
George