10 Speed popping out of 4th & 9th...Need to Fix

Bent shift forks are not common, they are beefy. The shift fork and sliding clutch can wear where the fork rides in the sliding clutch gear. The clutching teeth on the slider and 4th/9th could also be worn down. 4th/9th gear could also be moving on the mainshaft from locater washer and or snap ring failure. You would need to pull the shift cover to inspect the internals. This can sometimes be done in the truck, but on most trucks it's a pull.

 

Start with the shift tower springs. Had this a few times. Unbolt the shift tower from the Transmission, you will see three springs in three holes. One might be a different color so pay attention to which holes the color of springs go in. These springs are there to prevent the exact problem you describing and hold the detente in Place so it doesn't pop out of gear. The springs may even look good but have lost there tension. Eeeeeeeasy fix.

 

Also check the detente balls under the springs they can an do get flat spots wore in them a little pocket were driver works good for getting them out I if this is a eaton metric 10 speed there will be no springs under the shirt tower its located under the top plate on the shift rail I'm pretty sure its the old style 10 speed unless it has been swapped out

 

i had a problem like that in an 8LL and it turned out that a tranny mount was broken or loose i believe.

 

I hate to resurrect an old thread, but I think some of the details are really good in here and I am having the same problem on an Eaton-Fuller FRO in one of our Western Stars. My understanding is that if it were something like an isolator, detent, engine or trans mount, etc it would pop out of all the gears, specifically on rough roads. The important thing is it pops out of 4th and 9th immediately after engagement, and only out of 4th and 9th, no other gears. I was thinking the same thing that the first answerer wrote - specifically the first sliding clutch is probably worn. It also grinds on range shift, so I was thinking the range synchro (there is no air problem), both of which are pull and teardown or replace the tranny or run it till you blow it. Trans isn't really my area of expertise, though I can do an R&R pretty easily and struggle my way through a teardown with the right references (which I have), but if there are any other options, any easy answers, it'd be nice to try them first.

 

Just r&r the trans with a rebuild unit, replace your clutch as the trans is out and you can get back on the highway making money!