T138 jumping out of gear - fixed

[BigRig379]

My Tcase has been popping out of gear on a hard acceleration/ deceleration or hitting a pretty good bump. It’s slightly not resistant to this when it warms up. I was told the synchro was going bad and needing changed. I got the tcase split but I don’t really know what I’m looking for since I can’t compare what a new one looks like. I Took pics of the synchro and hi/low range it would mesh with. Only think I really noticed was high range had rust or something on it. Everything else was clean except for it. Shifter fork also had some slight wear.

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[eric67camino]

I've not been in one of those, but something looks to be wearing.

There does appear to be wear on the right side of the shift fork. I don't know how much is normal.

Possibly some notching un the long splines on the shaft in the second picture.

 

[BigRig379]

The fork is wearing because I’ve had to hold the selector in high. I’m just trying to figure out why it won’t stay.


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[simp5782]

Usually the shift fork is bent and doesn't allow a full seat.

Did you have the interlock solenoid disconnected or bypassed?

Did it pop out of gear with the linkage disconnected at the transfer case?

 

[BigRig379]

Everything’s original on it. But it would move as I added more torque going down the road. If I let off quick it would pop out. But it would go to neutral it was between neutral and in gear. I’d have to shift to neutral and back to high if rolling down the road.


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[simp5782]

It's also a T138. Not a 1138. So it doesn't have the interlock solenoid.

 

[BigRig379]

It's also a T138. Not a 1138. So it doesn't have the interlock solenoid.

Thanks it’s been corrected


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[Jbulach]

Check all your bearings and make sure the preload on the shafts is correct. I’m betting the shaft was slopping back and forth.

 

[BigRig379]

Check all your bearings and make sure the preload on the shafts is correct. I’m betting the shaft was slopping back and forth.

That’s actually what it felt like but I just couldn’t understand why or how. Is that I. The tm’s under assembly?


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[Jbulach]

I’m not sure you’ll have to check. Someone had a good post on the shimming a few years ago, but I could not find it in a quick search, if you have no luck let me know I will try to dig harder. On another thought if you’re feeling movement in your shifter, where your transfer case mount bushings good and tight? If the transfer case was moving back far enough the shifter could be hitting on the front of the slot in the floor and popping it out of gear.

 

[Jbulach]

Wait a minute, what truck are we talking about? I thought it was a T1138 in an M939 series, I think the singles on the truck in the picture must have thrown me off.

 

[BigRig379]

Wait a minute, what truck are we talking about? I thought it was a T1138 in an M939 series, I think the singles on the truck in the picture must have thrown me off.

M818 T138, the original post was for a T1138 simp corrected me


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[BigRig379]

From top to bottom
Right
Rear left
Front left

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[BigRig379]

Check all your bearings and make sure the preload on the shafts is correct. I’m betting the shaft was slopping back and forth.

So far the intermediate and output shaft have a preload but nothing on the input that’s moving


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[hethead]

Mine failed in the same way. Started popping out of high range on a cross country trip. I checked the cross member for the shifter and it was loose but tightening it didn't effect it. The lever wasn't hitting the bodywork. The "shaft" on the T-case was moving through it's full stroke, from what I could tell. No matter where I adjusted the linkage it took an increasingly higher level of force to hold it in as I went but it had no issues staying in low range. I ended up driving it in low range to the first heavy truck shop I could find (in Butte, MT) and, being on the road with only basic tools, I just had them replace it with a rebuilt one.

So no help with diagnosing it, but when I drained the oil before I left, I found it to be low and black. That was after a 1000 mile drive from Eastern Surplus. I can only assume thats how it was when I left their lot, since it wasn't leaking during my trip. With it being low on filthy oil, I assumed there would be wear throughout the gearbox, since I also didn't know how long it had been that way. I had just bought the truck from them.

I'd estimate around 2,000 highway miles from when it first started popping out, to when it was no longer drivable in high range.

'71 M820 w/395's. 30,000 miles.

Edit- Mine initially popped out during the transition from load to decel, but would stay in (with pressure on the lever) while loaded., so long grades were welcomed. Then, it deteriorated to the point that it wouldn't stay in no matter what I did.

 

[gimpyrobb]

Maybe gringeltaube will post his thoughts...

 

[BigRig379]

So far from all the info I’ve gotten is the synchro and high range gear need replacing. I found one bearing on the hi range gear that feels different than the others so I’m going to just replace the pair with that gear. Havnt looked at the other shafts, might as well since their out. Gonna pull the sprag out and other bearings to sandblast the case and prep for paint.


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[BigRig379]

Made some progress finally drove it and pulled a trailer the following day. I’ve determined for mine the high range gear and synchro were worn. I could have maybe just changed the synchro and been fine but I decided to fix anything that didn’t look right. The Input shaft os the only shaft not requiring shims. I replaced the 4 bearings on the gears for that shaft. Some were looking kind of rusty or just dirty of some sort. Also replaced the synchro, high range gear. And shifting fork. When assembling the high range gear bottoms out against the case after you connect the two halves together and tighten the input flange the gear will get pulled away from the case. I tried to test the case with only the input shaft connected. Everything seemed fine until I went to 5th gear. It sounded like it was going to come apart at idle, after accelerating it smoothed out. Best I figure is like an aircraft piston engine it has to be ran under a load of some sort. I hooked everything back up, took it for a drive and it’s never been this quiet before. I’m
Hoping it will run for quite some time before it ever has to be opened up again. I plan on running it awhile before draining fluids and checking for metal.

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[gringeltaube]

Hey, love that last picture!

Sorry I had missed this thread until now... Yes, replacing both- high rage gear and the syncro (with new bearings of course), is exactly what I would have recommended, after seeing the wear on the clutch dogs, on both parts. And that worn shift fork may still do its job, just have a little more travel.

Good job and glad to see someone having the "courage" of digging into- and actually fixing his T-case; not simply replace it entirely.
Keep on truckin' !

 

[marchplumber]

Impressive......

Small assistant mech is indispensable for those "tight" areas!!! Lol


CONGRATS and hope she serves you and your efforts well!!