T-1138 failures. Examples of what to do and not to do. Failure mode. How to prevent in general use and how to bulletproof if possible.

[Superthermal]

Hello, I am Jeff, I am an aspiring future owner of a kickin-it M923/925/927A2. The only thing about this rig that bugs me is the T-1138 case is touted as being a easy pencil to break. I am expecting to turn up the smoke a bit and maybe injectors but nothing too crazy on the 8.3. Just enough that I know it is better than stock on HP/Torque but even if I dont mod the 8.3 the T-1138 case is the proposed weak link.
What is it about the case that is so wussy? Is it the case or the gears? If I was to have a full insane overkill billet aluminum case made for $,$$$ could I check off the box that says this issue is solved? Or is it something else that gives out and the case is just the obvious wrapper that cracks when the gears have all been thrown?
I read from comments that the case is weak only in low range when using reverse where you gain and loose traction and as the traction hits the case cracks. Is it the front wheels grabbing that busts it or when the rears grab? Or something else? If you have broken one of these cases please describe specifically how it was done and what in/on the case broke so I can be clear on the mode of failure. Thanks to anyone who responds with clear descriptions to not just help me but others understand.
If the breakage occurs only in low reverse, can I drive this thing mean and ruff in 6x6 high range (not having one... can you do that.. 6x6 in high range?) forward and reverse and be fine as long as I dont touch low range? If this is the "safe option", to just not use low range? Will the T-1138 handle life if the 8.3 is juiced up a bit for general wheeling say in snow or muddy mountain roads with hills and rocks with a few tons of firewood as a load in high range running chains at times?

 

[Elijah95]

Although I never got into rock crawling, I had countless miles offroad banging off the governor in mud and rough trails in my maxed out M923A2 in 6x6 low. I’m not advising it, but after modding the lever below the cab to NOT engage 6x6 automatically in low, I even backed up in reverse low with very gentle throttle usage and no heavy load/heavy trailer quite frequently. Is it safe? Nah. Will it survive? Probably. Do I recommend it? No.

I believe the risk comes in when in low reverse gear reduction is so great that excess torque attempts to drive the main shaft up the countershaft (terminology?) and the case cannot withstand the spreading force therefor giving way.

The 1138 will give you excellent service, I definitely abused mine. I’d guess majority of failures are from downright abuse


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[simp5782]

You can break em in high. You can break em in low. You can bend shift forks. You can break the front output shaft on it.

If you are worried about it, pull it and sell it. Spend the 4hrs putting a Oshkosh 55000 case in it. Get a 1710 output yoke for the Allison 600 series. Find a surplus driveline that is 4in tube with 1710 yokes on it to go in place of your jackshaft. Other drivelines may require shortening depending on how you position it. You will be losing the companion yokes off both shafts so they will be 3in shorter ea anyway

 

[charlesmann]

You can break em in high. You can break em in low. You can bend shift forks. You can break the front output shaft on it.

If you are worried about it, pull it and sell it. Spend the 4hrs putting a Oshkosh 55000 case in it. Get a 1710 output yoke for the Allison 600 series. Find a surplus driveline that is 4in tube with 1710 yokes on it to go in place of your jackshaft. Other drivelines may require shortening depending on how you position it. You will be losing the companion yokes off both shafts so they will be 3in shorter ea anyway

Will those t-cases work with the 936 pto rear hydraulics?

 

[simp5782]

Will those t-cases work with the 936 pto rear hydraulics?

Yes the 55000 is designed to run a 7/8-13 spline pump to the rear intermediate gear. M1070s use this as a secondary power steering pump. You can see where it would bolt on below the pump. And above the yoke.

The PLS truck 55000 case has the ability to run dual pumps one front one rear.

It would be constant drive pump so you would want to use a splitter valve body so it's bypassing to the tank while driving.

Pls case pictured too. With rear pump and front power divider

 

[charlesmann]

@simp5782
Does the current setup on a 936 not have a bypass built in already? If the pto is engaged and operation are not happening, say while up/down rigging, the fluid has to go somewhere, correct? I need to crawl under my tck and look at the plumbing and routing from pump inlet, and back to the tank.


Do the hemmt wreckers have that splitter that could be installed inline? I would think an air actuated splitter would be better than cable or electrical.

 

[simp5782]

@simp5782
Does the current setup on a 936 not have a bypass built in already? If the pto is engaged and operation are not happening, say while up/down rigging, the fluid has to go somewhere, correct? I need to crawl under my tck and look at the plumbing and routing from pump inlet, and back to the tank.


Do the hemmt wreckers have that splitter that could be installed inline? I would think an air actuated splitter would be better than cable or electrical.

Hemtt wreckers use a transmission mounted pto.

I know it has a bypass already I'm just used it putting a bypass in a system I build. Not thinking in terms of a system already operating