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Posted By: e9coupe Need help with stuck gear selectors on trans - Wed Apr 17 2019 04:42 AM

'51 GMC 3100, stock engine and trans, three speed column shift. I drive the truck up and down the street today and the shifter started getting tight and wouldn't shift out of third. I limped it back to the shop in third and started taking things apart, starting with the selector box on the column. There a lot of slack between the two u-shaped cogs and the selector had squeezed past the cogs to the outside. So, that explained what happened up now the selectors on the gearbox are stuck. I drove it back in third gear so I don't know how they could both be stuck, but I'm not familiar with the gearbox. The rods are disconnected and both selectors are frozen in place. What should I do next?
Posted By: dgrinnan Re: Need help with stuck gear selectors on trans - Wed Apr 17 2019 11:52 AM
If you can get the vehicle in a position where there is no tension on the gears from the drive shaft you can manually move the shift selectors to the correct position. Do this with everything hooked up and on the column under the hood. Mine did this few times after I first got it. I would pop the hood, move the shifter rods to the neutral position and everything would be alright, for a while. As I said, it did it a few times when I first got it but has not done it again in the last 2 years? I am not sure what I fixed or changed that corrected the issue.

There is some additional detail in this thread.

Stuck gears
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Wed Apr 17 2019 01:46 PM
Thanks for the reply. I did jack up one wheel and move the driveshaft back and forth with no success. I guess I'll try lifting the whole back of the truck just to make sure and make it a two person job.
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Wed Apr 17 2019 03:52 PM
Assuming I get the gears to release, what's the best way to tighten the selector mechanism? Everything seems fairly good with no obvious wear on most pieces, except that the distance between the cogs allows the selector bar to squeeze between them and end up outside the "box" the cogs make. I'll add a photo later today. Can you shim behind one of the cogs/jaws to make them closer?
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Fri Apr 19 2019 04:51 AM
Okay, I got the gears free. I got both rear wheels off the ground and had my son move the wheel to create slack in the drive train and still couldn't get the selector to free up. Ended up taking the side cover off where the selector arms are and had to jostle the gears around quite a bit to get them loosened up. Now I need to fix the selector box tolerance.

What's the best way to tighten up the distance between the cogs so the selector can't squeeze between them and end up in the outside?



Attached picture gear selector box markup.jpg
Posted By: EdPruss Re: Need help with stuck gear selectors on trans - Sat Apr 20 2019 02:02 PM
It looks like your idea of adding a shim at the top or bottom of that shaft would work, could be just normal wear.

Ed
Posted By: sstock Re: Need help with stuck gear selectors on trans - Sat Apr 20 2019 02:36 PM
If you need another column shift mechanism, low mile, good working ,pm me.
Steve
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Sat Apr 20 2019 05:59 PM
I installed a shim and it looks much better. Still don't know what those two plates are for. I can't find them in any other photos or diagrams



Attached picture IMG_20190419_195340630~2.jpg
I can’t tell from your photo where the shift arms are in relation to each other. With the transmission in neutral the arms should line up perfectly with each other. If not take the rods off and turn the fittings to line up the arms. Makes shifting smoother.🛠
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Sat Apr 20 2019 11:38 PM

Just drove the truck again and it works great now. I've read how you're suppose to shift the column shifter, "H" pattern and all. How I described it to my son is that you shift into gear, then shift out of gear, then shift into gear, then shift out of gear. Basically starting over each shift. That slows you down and it works good.

Kinda glad it got stuck in gear, made me fix it instead of learning how to adapt to it.
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Sun Apr 28 2019 11:02 PM
More issues with third gear. I drove the truck down the street and it runs pretty good and shifts fine. I only drove it about four blocks, cruised in third and pulled up to my work and it wouldn't shift out of third again. I rattled the shifter a little bit without forcing anything and nothing helped so I limped it in the back of our place and put it on the lift. The selector and assy is fine and adjustments look good but the third gear lever on the gearbox is stuck. I pulled the cover off and took a few photos and eventually got it to release by hand. The big gear with the thin collar is the one that gets stuck and it doesn't feel right when it gets to the end of its movement, like it goes too far. I have no idea, I don't know these gearboxes at all. I watched a few videos of disassembly but nothing stood out for me.

The selector box and shift rods are all adjusted and work well, it's just the gears in the gearbox causing the problem.

Any thoughts?

UPDATE:. I managed to get it stuck again while messing with in while it was apart and then couldn't get it free. I ended up putting it together as-is so I could limp it back to my garage and while driving it back I got it to release and shift fine again. I read that others have had third stick if they stop without shifting out of gear. Would still like to know what's going on..

Attached picture IMG_20190428_150251233.jpg
Attached picture IMG_20190428_150309663.jpg
Attached picture IMG_20190428_152641380.jpg
Posted By: rfs56trk Re: Need help with stuck gear selectors on trans - Mon Apr 29 2019 05:30 PM
There is some wear on the 2-3 shift fork, it could cause some of the problems you are experiencing. The "big gear with the thin collar" is actually the synchronizer assembly. It is also referred to as a "drum". It has brass rings on both ends which help engage 2nd & 3rd gear.
It is engaged by the 2-3 shift fork moving the collar. 3rd gear is to the front. Take a close look at the ends to see if the brass rings move when it is moved manually. If the rings are loose or worn excessively it won't engage or disengage properly.
Fred
Posted By: e9coupe Re: Need help with stuck gear selectors on trans - Mon Apr 29 2019 05:58 PM
Originally Posted by rfs56trk
There is some wear on the 2-3 shift fork, it could cause some of the problems you are experiencing. The "big gear with the thin collar" is actually the synchronizer assembly. It is also referred to as a "drum". It has brass rings on both ends which help engage 2nd & 3rd gear.
It is engaged by the 2-3 shift fork moving the collar. 3rd gear is to the front. Take a close look at the ends to see if the brass rings move when it is moved manually. If the rings are loose or worn excessively it won't engage or disengage properly.
Fred


Thanks for the lesson. One of the things I thought was strange is that the rings under the assembly moved separate from the drum. As I moved the drum back and forth the edge of the brass ring was visible and had some slop in the movement. I wasn't sure if that was normal or not. Sounds like it isn't. Do you know of an exploded diagram of the gearbox, or drawings/illustrations showing how it works? When I had it apart a mechanic friend checked it out and now it's bugging him until he figures it out. 8-)
Posted By: rfs56trk Re: Need help with stuck gear selectors on trans - Tue Apr 30 2019 03:09 PM
Any movement in the brass ring is bad news. They are pressed in and staked in place. The steel ring has some normal movement as it engages over the energizing spring. I specialize in the later SM319 Muncie overdrive transmissions but don't normally do much with the early torque tube units. I'm sure the manual for these transmissions is online. Someone here can likely lead you to it. The internal components are similar to those used on the 55-59 cars and trucks. Those manuals are available from aftermarket sources.
One of my parts sources here locally is Waugh Transmissions in Santa Fe Springs, CA. 562-693-0855. They are experts in these units and rebuild the synchronizer units as well.
Fred
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