M923a2 Low range reverse

[simp5782]

So, you can't lock the front end in low range reverse in the A2 series trucks? Mine doesn't have any kind of warning plate or lockout as far as I know. Is the HEMMT transfer case a direct replacement or require modification? Does it solve the breakage issue?

HEMTT. Per the TM you should not use reverse in low range transfer with 6x6 engaged or without it engaged.

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[red]

Does anyone happen to have a picture of a cracked T1138 transfer case that they would be willing to share?

For example https://www.steelsoldiers.com/showt...-Use-Low-Range-Reverse-with-939-s-Here-s-Why-!!


The T1138 housing is the known weak link. Was discovered early on in the M939's career while active duty (hence all the warning plates, MWO's, stated in the driver training, etc). Driving in low range reverse leads to the housing breaking apart. Will get lucky doing it at idle to some throttle for awhile but it will fail.

To restate what others have said:

The allison MT654 has a 10:1 reverse gear. T1138 has a weaker housing than it's predecessor the T138.
10:1 from the trans combined with the 1.6:1 low range of the T1138 gives 16:1, which (as proven numerous times) the housing can't take.
Doing the MWO to disable the auto engaged 6x6 in reverse reduces SOME strain on the tcase. As others have learned first hand, it doesn't reduce it enough.


Is it a pain in the ass to shift out of low range every time to back up? Probably (mines an m809 series with T138 so no concern there). But if the tcase is not a disposable item in your opinion then the smart choice is to shift to high range in reverse. If the T1138 is disposable to you, then by all means leave it in low range with reverse and eventually experience the broken tcase.

 

[simp5782]

Is it just me, or does that look like a very solvable problem? Do they always crack in that same location?

Generally all the way around the case and they can fall to pieces. They will generally operate even blown apart or cracked for a short time period as long as you crawl with it.
Its not a solvable issue to my knowledge. Not ever a stronger girdle or cage would work.

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[Russ Knight]

So there is no other transfer case that will swap in and fix the issue?

 

[98G]

So there is no other transfer case that will swap in and fix the issue?

There's several. They all come with their own trade offs.

Just like there's no simple straightforward fix to it.

If either of these existed, you'd see all of us doing it....

Edit - the simple solution is to not use low range reverse. Just shift the t- case to high range when going into reverse. 10:1 is plenty low.

If you routinely need to pull loads that endanger the integrity of a 5ton, may I suggest a HEMTT or HET? Maybe you just need a bigger hammer.... (Or even an M818 - it's rated a good deal heavier than an M931)

 

[snowtrac nome]

there are some of the new mrap t cases, but look how much your going to spend. Oshkosh have some plenty stout t- cases to but you have to fit them in place which is no small task. Once you have fixed that weak link what else are you going to break. the only use I could see for an additional low range is backing down a hill and the 900 5 tons are all ready geared pretty low for backing up.

 

[red]

So there is no other transfer case that will swap in and fix the issue?

Only regularly economical alternative is the T138 from the older m39 and m809 series 5 tons. Stronger, but you now lose the overdrive ratio so with 16x20 tires top speed just below redline is now 55mph, 2000rpm cruise at 50mph. On 11x20"s it drops to 40mph cruising speed.

It's interesting that in the TM the m818 has a 55k trailer limit compared to the 35k of the m931.

 

[simp5782]

there are some of the new mrap t cases, but look how much your going to spend. Oshkosh have some plenty stout t- cases to but you have to fit them in place which is no small task. Once you have fixed that weak link what else are you going to break. the only use I could see for an additional low range is backing down a hill and the 900 5 tons are all ready geared pretty low for backing up.

They all have .95 or lower ratios in their single speed setup. They opted out for the double overdrive in the Allisons

The MTVR 7tons have a 30000series unit. The same as a HET with 1.271:1 single speed but their 5th gear is direct drive, 6th is .85, 7th is .68 i think so they can roll good on 1600s

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