M939 warning notice.

[Jones]

We just got another heads-up bulletin so I thought I'd pass it along.
"Reverse Movements In M900 Series Trucks --
Do NOT back up M900 series trucks with the transfer case in low range. This WILL damage the transfer case up to and including breaking the case body casting; deadlining the vehicle".
You may be lucky and pull it off a time or two but the odds are stacked way against you and these guys aren't easy to tow out of the woods to home. And I guarantee, swapping out a transfer case isn't on the same effort level as checking tire pressures.
The military has a lock-out kit that prevents the transmission from being shifted into reverse with the t-case in low. Will try to get details on them.

 

[M1075]

Thanks for the heads up Leonard. This has been a concern of mine and has kept me out of the M939 league (for now anyway).

http://www.steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&t=14286

 

[Jones]

I'm told it's a design flaw in the t-case. My t-case is out and on the ground now and I'll try turning it backwards by hand; if it tries to hang up then something inside doesn't like being rotated the 'wrong way'. If it rolls through smoothly then it probably has to do with the torque generated by being in reverse, which is definately the lowest gear ratio in the box. If Rockwell didn't properly support a shaft's bearings then rotating it in reverse would load and force the gear in that unsupported direction. Since they're coupled to automatic transmissions, shifting from forward to reverse would be pretty close to instantaneous and that shock would further add to the strain.
The guy I got my replacement t-case from says most of the ones he sees in surplus sales have cracked cases.
The mod kit is some kind of linkage block that picks up off of the T-case high/low shift lever position and keeps the driver from being able to shift into reverse when in low range. Sure sounds like it would make for more shifter wand-waving when going from forward to reverse in off-road conditions.

 

[papercu]

kit

Here is a link to the kit. Wayne

http://www.steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&p=141271&highlight=#141271

 

[OPCOM]

RE: kit

Rings true, I had an Army soldier tell me about that at an Armed Forces Day open house. He said it chews up the gears. The truck had a similar warning sign on the dash, thus my question to him about it.

 

[D-Man]

Not that I would necessarily recommend doing it, but I have in the past gently used low range reverse with no ill effects. According to our motorpool NCO who has spent many years fixing these trucks here and overseas, it is the multiplication of torque in low range reverse coupled to an already powerful reverse which is simply too much for the T-case to handle under normal, loaded conditions. Just backing it up unloaded and with nominal throttle will not destroy it.

 

[BKubu]

I concur with D-MAN. I have used reverse in low range...once...the first time I drove an M939 series truck and before I noticed the warning on the data plate. I let the truck move backwards at an idle.

 

[Jones]

I think the moment of truth comes when you shift into reverse and torque is applied instantly through an automatic under partial throttle conditions. The relatively thin t-case walls, without benefit of the earlier t-case's outside re-enforcing ribs just won't take the shock. Empty and under idle or light throttle conditions would probably be OK but then again, I'm the Murphy's Law poster-child...
Once moving backwards the possibility of damage is lessened; unless you stab the throttle or power-up to get up a grade.

 

[BKubu]

I will add that I have not operated the truck in reverse while in low range since that first time. I don't need to press my luck...and I have rarely had a need to put the truck in low range anyway.

 

[D-Man]

Well said, Bruce!

 

[Jones]

Ditto, Bruce.