Transmission stuck in 3rd Gear

[tnosal]

Greetings!

I’m having an issue with my T84 transmission. Things have been running fine until one day I was driving, in 3rd gear when I was approaching a turn and went to downshift. As I attempted to downshift the would not budge out of third. I was able to pull over and I checked my clutch linkages and all appeared to be fine. I tried to push it out of 3rd with no luck. There was no indication of a problem throughout my drive that day.

Over this last year the only issue that I noticed has been a periodic kick out of 3rd. This didn’t happen the day when it got stuck in 3rd. In fact, it has been so infrequent that I really didn’t consider it an issue.

Once I got the jeep home, I started checking things. Once again, everything with the clutch looked fine. I drained the transmission fluid and it was clean with no metal shavings present (I filtered the gear oil through a coffee filter). I inspected the transmission from the top and I can see no visible damage like broken/worn teeth. I have a thin rod with a magnet on the end and I probed the case for savings or other bits of metal and found nothing.

I replaced my transmission in 2011 because it was kicking out of 2nd all of the time and the gears were clearly worn in the old transmission. So I’m familiar with how a bad transmission operates. My current transmission has been running smooth.

With all of this in mind, here are my questions to the group:

What is a possible cause of the transmission locking into 3rd without something visibly breaking?

Is there something that I may be missing with the clutch?

Should I be looking at an issue in the transfer case?

Thank you for reading this and I truly appreciate any advice.

Sincerely,

Tim

[dinof]

I had the same nightmare with my jeep. My trans was built by a "professional" and had 5,000 miles on it and one day while driving , I went to come to a stop and it would not move out to neutral. I eventually got it to unstuck and dismissed it as a fluke. Then, a year later, it starting to do it again. Come to find out, the plates on the synchro hub were jamming up against the brass synchro ring and hense, causing the lock up in 3rd.

That was my story. The hub of the synchro was moving to far and exposing the dimples on the shift plates and then they would tip upwards and jam up against the brass synchro. I was forced to pull the transmission and rebuild it myself.

[slatgrille]

The interlock pins in the synchro hub are 'popping' up in their slots and jamming. Had this issue with a T90 in a 2A many years ago. Had to use the transfer from low to high while moving to complete a few trips. Being a novice and young at the time, I went in and pried the bad one(s) out completely. Had a bit of double clutching to do for a smoother shift, but it worked. Not suggesting you do that to a restored T84...I was young and impulsive...but I think that may be your problem as suggested.

Craig

[tnosal]

Thank you to both of you. I appreciate your posts. I have an idea of what to look for and what will need to be corrected. I was hoping to avoid a rebuild but I guess that is the best way to go. Such in life with a antique jeep. Thank you again!

Tim

[Wolfman]

Agreed. One of the synchronizer blocking bars popping out is the usual cause for your problem.
One of two causes.
The mainshaft has too much end play allowing the synchronizer hub and blocking bars to move too far to the rear, allowing one of the bars top pop up or the guide plate in the transmission shift cover is worn or loose, allowing the shift collar to move too far forward. The later is where I would start. A lot easier to get to.
There has also been a rash of bad blocking bars going around. The flat tab on the end breaks off, letting the bar pop up.

[dinof]

How to confirm this is not too hard. I pulled over when this happened and took the tower off and was able to see the plate that was causing this.

[Wolfman]

Do that in your 2017 grocery taxi !!
I love old jeeps !!

[Chuck Lutz]

OK....good suggestions here,
When setting up the slop/axial play on the main shaft you need to actually either bolt up the T/C or use a dummy plate so you can get some PULL on the shaft when tightening the castle nut.

If you don't, then when you finally assemble T84 & T/C, the TORQUE you put on the castle nut will pull the main shaft back further and that will pull the HUB back further and allow the three plates to pop up on the 2nd gear end of the tub when you are in 3rd gear and the sleeve/fork has pushed forward into 3rd gear.

I think that with just the shift plate on top removed you can rotate the main shaft with the T/C in neutral and a friend depressing the clutch.

Try that and it may confirm what the guy have been saying.

[dinof]

I think it's the back bearing that moves a little back that allows for more slop when you tighten it all up. Something that was lacking in those "sloop" videos on youtube. I Set mine up a little tight and when it was all bolted up, the clearances were right.

[artificer]

I think it's the back bearing that moves a little back

This has been done to death in most 10 page transmission threads & it is not correct.
The bearing doesn't move back.
For correct positioning of the synchro & all mainshaft components, when the castellated nut is torqued, the mainshaft is pulled fully rearward, not the bearing.
This in the encapsulated bearing's inner bore until the mainshaft, circlip, spacer, slinger etc. become 1, correctly positioned, solid part.

[Chuck Lutz]

Hmmm...and why exactly did the French Army come up with the spacer to be placed between the bearing and the T/C again?

[artificer]

Any spacers, shims, slinger etc. are used as an adjustment to govern the position of the rear bearing on the mainshaft.
The torqued castellated nut locks this bearing in that position.
That locked bearing position [on the mainshaft] controls the depth of the mainshaft & associated parts including synchro's & 2nd in the transmission, when the bearing is encapsulated between the transmission & transfer cases.

[artificer]

Here is a pic of what the guys are talking about as most likely being @ fault:
https://encrypted-tbn0.gstatic.com/imag ... _wxP6PVI-Q

[Michael Browne]

Hi Tim,

Almost all jeep owners have had a problem with their T84 transmission

Greetings!

I’m having an issue with my T84 transmission. Things have been running fine until one day I was driving, in 3rd gear when I was approaching a turn and went to downshift. As I attempted to downshift the would not budge out of third. I was able to pull over and I checked my clutch linkages and all appeared to be fine. I tried to push it out of 3rd with no luck. There was no indication of a problem throughout my drive that day.

Jamming in 3rd gear can be the result of a couple of things but is usually narrowed down to one, misplaced syncro keys as shown by the picture John G provided a link ( which by the way is not a MB/GPW jeep transmission but will do for now)

The root cause for the keys coming out of position and jamming the syncro sleeve and preventing movement is in most cases OVER THROWING the syncro sleeve when selecting gear. This happens with a loose gear shift gate by itself or combined with excessive tower ball wear that lowers the shift cane into the box increasing the throw of the sleeve.

With transmissions in good order the syncro sleeve can only move the required amount along the hub to select the gear and when on the bench a definite "snick" can be heard as the keys locate in the groove in the sleeve ( the shift rail detent balls will also make the same noise but I like to check the hub keys without the detent ball in position first)

When there is a loose shift cane gate ( plate riveted to underside of tower top cover) the leverage you have on the end of the stick shift allows the bottom of the cane to OVERTHROW the syncro sleeve along the syncro splined hub and the keys can pop out of position and jam the transmission in that gear.

A worn tower ball socket for the cane will allow the shift cane to sit lower into the case and this will increase the amount of movement the cane has on the shift plate/rail and the sleeve moves more.

Mainshaft position has little bearing on the action of the syncro keys unless the spacer between the snap ring and rear bearing is missing as the correct movement amount of the syncro sleeve prevents the keys from coming out.

Over this last year the only issue that I noticed has been a periodic kick out of 3rd. This didn’t happen the day when it got stuck in 3rd. In fact, it has been so infrequent that I really didn’t consider it an issue.

Now this is a separate issue and may be as a result of a worn flywheel bushing or weak or broken syncro key springs. more on that later


Is there something that I may be missing with the clutch?

Nothing clutch related is happening here from your description

Should I be looking at an issue in the transfer case?

Not at this stage, lets do some more looking first

Thank you for reading this and I truly appreciate any advice.

Sincerely,

Tim

[artificer]

I was aware it was a T90 but so similar makes no illustrative difference.