craftsman transmission problem

[ACF]

have a craftsman 13AQA2ZWA93 serial number 1D21PH40002. the problem is that the transmission will not drive the mower belt good and tension is good. mower has tuff torq k46ds transmission. the mower will move when the bypass lever is pulled out and will move freely when the bypass lever is pushed in. looking to see what could be the possible causes for this situation. have a drawing from tuff torq it shows the shaft has a spring installed with a snap-ring holding spring on shaft. understand will not for sure until opened up, just looking to see might be going in right direction so i can put together an estimate of parts needed and cost for owner.
thanks
AC

 

[StarTech]

When I order make sure you use the transaxle serial number as there 4 serial numbers break.

And I don't see any spring of the bypass lever. IPL says the bypass is just a rod held on with a clip at the bypass lever.