The problem lies in the shift lever in the gearbox. A poor initial design that was upgraded twice. The shift fork arms have spread enough for pins to come out of the groove they run in when the tiller is shifted to the last position. (Bending back into position would only be a temporary fix as would not be any stronger.) To replace shift lever, the gearbox cover has to be opened. Seperating the cover can be somewhat challenging. When prying cover apart try to not damage the cover gasket. Gearbox grease is very thick, so gasket's real purpose is to keep water and contaminants out. Leave all gearing and chains in place. The shift lever part number is 618-0245B. Or use assembly kit number 753-0869 (contains shift lever 618-0245B, position indicator, and roll pin.) Part will run in the $56-$68 range and can be found through MTD as well as Cub Cadet. Also can get through Sears PartsDirect.
I think you are correct in thinking the chain is broke or there is a gear that drives the chain in the gearbox that could be the problem, either way it is a major job. If you do take it apart,try and leave as much of the gears in place as possible to avoid confusion.
I had this same problem and took the first advice about the shift fork arms being too spread apart. I took apart the gearbox apart and found that the shift fork arms did indeed spread apart overtime and with wear. I'm having a buddy weld some reinforcement to it as long as he can maintain clearance in certain areas as to not disrupt the operation. If it doesn't work then I did find the shift fork assembly part for $50, I just googled the part number, clicked on various sites, and found a better price!
I have seen the same problem with mine. Now I am trying to figure out what keeps the detent ball from falling into the key way. The detent spring is good. Once the detent ball gets in the key way the clutch collar just floats around. I am also trying to figure out what is supposed to retract the key from the spur gear that drives the tiller shaft. When I move the shifter from T to F these keys should pull out of the spur gear but do not.
I find it interesting that I have been unable to find anyone that has actually dug into these things this far and fix them. Other than the ball and key problem they are pretty simple to work on.
There must be a ton of these things out there with broken gear boxes.
How does the detent ball and spring stay in the collar? Does the ball go in first with the spring on top or vise versa?
The spring goes in first followed by the ball. The ball should not get in the key way because that is filled by the slider pins that go up into the spur gear that drives the tines. Actually the slider pins drive all of the gears by engaging different gears as the gear collar is moved up and down. The detent spring and ball is the mechanism that is supposed to hold the the clutch collar in place. Mine jumps out of gear when tilling any depth except very shallow. I have reinforced the shifting forks and replaced the pins on the end of it. I believe that the pins wear and the arms spread because i have to keep pressure on the shifting rod or it will jump out of gear when tilling. Even with the reinforcing, pressure on the shifting rod will wear out the pins on the shifting arm. This will be my 3rd reassembly and I am leaning toward the detent spring being weak and allowing the shifting collar to move too easily. I may order an new spring and ball and see if that helps. I am getting pretty good at taking it apart.
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