backup in low

tobyS

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What (if any) 5 ton transfer cases do NOT have a problem with backing in low range?
 

Elijah95

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That I am aware of, it is only a problem with M939 series trucks equipped with the Allison because of the reverse gear ratio and torque multiplication combined causes the gears to attempt to climb themselves under a load and push the case apart.

In theory, we could create something like the Gorilla Brace used in the Dodge Cummins realm to reinforce the aluminum case G56 6 speed from counter shaft attempting to walk the main shaft.

Wes has (or had) a T Case or two from MTVRs that would probably create a nearly indestructible upgrade for a 5 ton, but fabrication work and loss of top speed being an under-drive transfer case may not be worth while. It is also a single speed case


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simp5782

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The Oshkosh 18000 from the 916 is a direct drive single speed that could easily be molded in. It is direct drive. Top speed with 1600s would be 54mph at 2150rpms with the m939 trucks

The Oshkosh 30000 from the MTVR and A1 HETs is an underdrive unit. 1.36:1. Meaning a 5 ton on 1600s would top out at 40mph at 2150rpms.

T138 from the m809 series is direct drive high range so the OD on the 5spd helps
 

tobyS

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I find that having a dump (929A2) that I cannot use low reverse....sucks. My first dump, an M817, had problems caused by the military forgetting to set the shaft(s) bearings on a new T138, but once I had it right, it performed flawlessly. I should not have gotten rid of that truck. Now I'm on pins and needles with the 929 in low and reversing, but it is unavoidable sometimes.
 

Elijah95

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@tobyS have you removed the large washer that engages the front axle by default when in low?


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tobyS

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I'm not sure I know what you mean about a washer. I really have not gotten into the A2 that far....thank GOD.

From my understanding, which is limited to disassembly and reassembly of the T-138 out of my M817 (not a T1138), the center gear shaft wedges at a low angle (even lower in the T-1138), forcing the gears apart, breaking their shaft mounts in the case. I could see how that could happen, even in the T-138 under extreme reversing torque.
 
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Jbulach

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I'm not sure I know what you mean about a washer. I really have not gotten into the A2 that far....thank GOD.

From my understanding, which is limited to disassembly and reassembly of the T-138 out of my M817 (not a T1138), the center gear shaft wedges at a low angle (even lower in the T-1138), forcing the gears apart, breaking their shaft mounts in the case. I could see how that could happen, even in the T-138 under extreme reversing torque.
Adjustments.jpg
Remove or trim the washer so the front axle switch does not auto engage when you put the transfer-case in low range. Supposedly helps prevent transfer-case breakage. I trimmed mine quite some time ago, not sure what the thinking was making the front auto engage in low range was in the first place. In my experience, it’s amazing how infrequent you need assistance from the front even in low.
 

Elijah95

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You nailed it! It doesn’t guarantee your t case to live, but it does remove some stress.

I bypassed my sticky t case air actuated interlock and although I didn’t hesitate to backup in low gently, I wanted to be able to easily shift back and forth from low to high with zero issues. Be advised though, once you remove the interlock there is no safety, forget to shift into neutral before shifting case and possible breakage. Off-roading in high simply creates way too much heat from torque converter slippage


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Jbulach

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My thought for at least the front side of the case, if someone with access to a burning table and machining capabilities could make a girdle to bolt on over the caps for the shafts. Should be fairly simple to make and install as well as quite marketableIMG_9239.jpgAdjustments.jpg.
 

tobyS

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Got it. I didn't know the front engaged at any time it is put in low. Therefor there are two air valves in parallel and by trimming it, the TC front will no longer activate by the one that hits the washer. So capping the air line and removing the valve is also an option, it still works manually in the cab.

Good idea on the support Jbulach. Having a machined indent for each of the caps would make it rely on more than just the bolt strength. Thanks for the clear photo's too.
 

Jbulach

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I was thinking a half moon welded underneath each end by the installer to help with spread and shearing of the bolts, would make a tighter custom fit for variances between different cases and caps.

Also the photos are not mine. From another thread around here someplace. Thinking Wes posted them originally.
 

tobyS

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Take a look at post 8 and run a straight line between the input and output shaft. The center shaft is almost in line and when the output is stopped, it makes the center (gear) try to climb the input gear, forcing the case apart. While I don't know for a fact, I'll bet that the T-138 is not as much (close to) a straight line as the T-1138, if it does not have the breakage problems that the T-1138 has. I can't compare, just remember thinking that there was a very slight difference...that has big implications.

It makes me wonder if a case from a T-138 could be used but get rid of the sprag in favor of the air engagement and have a different approach angle. The reason I say "case" is that the gear ratio on the T-138 is different front to rear while the T-1138 gears are the same ratio, so the guts would need to change too. Or the T-138 could be used as a pattern for new (reinforced) cast steel case.

I've had a lot of parts cast, mostly ductile iron, but will check with the local foundry.
 

topo

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In some oilfield trucks grinding a ring around the axel to make a weak spot extra axles where carried and easy to change .
 

tobyS

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How about just not using low in reverse? With the 10:1 reverse trans ratio, I don't see low + reverse being needed.
Not possible. I drive too far onto my soft dirt when filling a hole (backing onto the pile) and need it in muck for both backing and then especially forward when loaded. Loads of the muck are a one shot deal, like working on top of a bowl of jello. If my fuel was turned up some, it might do better getting out. If I'm not in low range to begin with backing in, getting it into low can be difficult with a load.
 
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