Don't Use Low Range Reverse with 939's? Here's Why !!

[Ue413]

As an engineer, IMHO, this is clearly a design flaw. 1. Vehicles drive in reverse and we've been trained to do that 2. Warning tags don't keep people from doing what they have been trained to do.

 

[marchplumber]

Much easier to inspect the gears condition using the "Mark" method.................harder to refill and reuse though.................LMBO!

 

[frank8003]

It seems kits are available for the M939; M939A1; and M939A2 at
http://www.machinerytrader.com/list/partssearch.aspx?partnum=57k3623&pst=exact

Says they have maybe six kits total.
http://www.flightsourcepa.info/default.htm


search by 57K3622 and/or 57K3623

I saw this happen here in Ft. Lauderdale to a privately owned truck. He fixed it and sold it.

 

[frank8003]

Tracking down these kits would probably be a good idea for thsoe using these trucks in fleet service.

http://www.tecmotiv.com/57K3623/ says they have them and see post 23

 

[Hoefler]

The casting thickness looks very thin for such a large case.
Pete

 

[Tornadogt]

Bottom line is gearing is a TORQUE multiplier. Low Reverse is the largest torque multiplier available and will stress all components the greatest of all other gear ratios. When this is done and the drive-line is STRESSED it will find the weakest point and fail. Simply being in reverse in low does not stress the drive-line.. STRESSING the drive-line in Reverse Low will break the weakest part of the drive-line which is the transfer case housing. I REVERSE IN LOW.. It is an OPERATOR who understands the stress on his equipment, and to idle or low rpm and no extra load over just moving the truck in this gear configuration. I would NEVER use LOW Reverse to pull anything or to attempt to rock a truck out of a stuck position..

 

[74M35A2]

As an engineer, IMHO, this is clearly a design flaw. 1. Vehicles drive in reverse and we've been trained to do that 2. Warning tags don't keep people from doing what they have been trained to do.

I think somebody used the incorrect combinations of parts. The transfer case likely met design spec for the transfer case itself. Whoever decided to use this T case, combined with a trans of reverse ratio 10:1, and 6 wheel drive, is the one who struck out. As a divorced T case, the T case maker can't dictate what their input is. They can advise, but that's it.

 

[The HUlk]

When new model vehicles are in the design phases there are mocked up tests that occur along tbe way. Final alpha, beta, and preproduction tooling tests are standard procedure as well. Real world testing of samples at each stage using real world driving conditions and controlled conditions occur. These include destructive tests to identify failure modes and confirm working ranges. I find it hard to believe that someone didn't realize that there was a weakness in low-reverse driving before they hit the production line. As a previous poster mentioned, I learned a long time ago that warning signs telling you not to operate something in a manner in which most any compareable item can handle, is window dressing for a problem that is baked in and will persist until a desgin change occurs to correct it.

 

[Plane Fast]

See they still test stuff first!

 

[The HUlk]

I was doing some work at a proving ground for a tier one rear differential, among other things, supplier and witnessed some testing. They were steady jamming a truck back and forth between forward and reverse giving it h*** until it finally broke. When the tire smoke cleared they drug it into a garage. I have always wanted that job ever since.

 

[Csm Davis]

Okay I must say that this guy broke the transfer case because he tried to move a truck with the brakes locked up not just because he put it in low reverse.

 

[rhurey]

Well, in addition to all the testing normally done for development...

The 939's were an existing drivetrain that had a new transmission and cab dropped in. Was the Allison an off the self transmission, or was it a custom design?

 

[Csm Davis]

If you would have changed the switch to not engaged the front axle it would have split the transmission ask me how I know. Don't blame the truck for a bad driver. I do recommend that everyone undo that switch. We have trucks moving 100,000+ lbs and haven't busted a single transfer case. I have no idea who thought it was a good idea to make these trucks go in to 6x6 when you put it in low, you already have a switch on the dash. The military has come to the idea it is better to make the vehicles do what a driver was trained to know when to do something. I am sorry but I would take a few of 818 and trained drivers than a dozen 931 and the new under trained soldiers to do a job. Now I love all of the military men and women but I believe we are making a mistake in not keeping some of our older style equipment and training to go with it. Proof of this statement is take a 816 and hook it back to back with a 936A2 and have a pull off or a 936 if you think the big motor has a advantage, it won't matter, the 816 will win every time. Don't get me wrong there were improvements the all hydraulic bed and winches, bigger cab, tilt front, and more but the automatic wasn't a improvement for the most part, yeah you can throw a kid off the street in and say go but that doesn't make a trained driver.

 

[rivercreek]

Okay I must say that this guy broke the transfer case because he tried to move a truck with the brakes locked up not just because he put it in low reverse.

Csm Davis, I believe that you are absolutely correct. The transfer case grenaded not solely because the transfer case was in low range and the transmission was in reverse. The t/case grenaded because BOTH sides of the center axle had frozen brakes. If only one side was frozen, the opposite wheel would have spun twice as fast as road speed, but it wouldn't have broken the case. The power/torque still had somewhere to go (spinning one wheel twice as fast as road speed). While ANY use of low range reverse is strongly and publicly discouraged (and frankly fairly useless - high range reverse is plenty low), just dropping the t/case into low range and the trans into reverse will not by itself cause catastrophic failure. I also believe that the front axle being engaged or disengaged would not have changed the outcome in this circumstance. Even in low range reverse/front axle engaged, one would have to duplicate the "Mark Method" or really dog on it to break anything in normal circumstances.

I also agree with the idea that instead of attempting to make everything stupid-proof, time, time and money would be best spent to make the operator "less stupid". Perhaps the US Military should incorporate this picture into their driver training course. Although I agree that this weak point IS a design flaw, a reasonably intelligent operator can be instructed on how to avoid "The Mark Effect". One last thing in my defense - "Mark" is NOT a fictitious person! It really wasn't me, although I've done plenty of other stupid @#$% in my day And poor Mark, he isn't gonna live this down for a loooooong time. We're going to make him a "Biggest Screw Up of the Season" trophy out of a piece of the t/case, and hope that it is indeed the biggest screw up this season... Gonna mount it on a nice piece of wood with a little brass plaque and all. LOL

 

[Chief_919]

Okay I must say that this guy broke the transfer case because he tried to move a truck with the brakes locked up not just because he put it in low reverse.

While true, I have seem them grenade just like that when use in low reverse without anything locked up- just backing up with a load in the bed.

 

[Jeepsinker]

Never any occurrences of this happening in the older M 8xx series trucks at all?

 

[Chief_919]

Never any occurrences of this happening in the older M 8xx series trucks at all?

Not that i know of, but they used a different transmission and transfer case.

The T-138 case in the 800 series is a fair bit larger than the T-1138 in the 900 series vehicles, and the 900 series has a much, much lower geared reverse,

 

[Csm Davis]

While true, I have seem them grenade just like that when use in low reverse without anything locked up- just backing up with a load in the bed.

And those were in 6x6 not just rear wheel drive the case will not split like that unless there is a binding difference of torque between the front axle and the rear axles in the older transfer case it would take out the Sprague but the new air shift engagement is much stronger and you now have a new weakest link the case. You can disable the switch and put all the weight on that you want and back up in low range because there will not be any torque on the case just the big solid through shaft, this is a design flaw not in the case being weak but in installation of the case to this truck if the case manufacturer would have thought that it needed to be in low and 6x6 the external switch wouldn't exist it would have been designed into the case from go like the smaller 4x4 cases that have only the one shifter. I do believe that the case is not as strong as could be but the solution should have been handled differently and I also have a question does anyone know of an 939A2 that has split a case? The torque the 8.3 puts out is much less than the 855 with them both at 250hp. Oh by the way I have beat on and broken more of these trucks than all but a few, my record was 4 wreckers in 4 days before the girls said stop and would not let me tear up anything for a few days.

 

[The HUlk]

Oh by the way I have beat on and broken more of these trucks than all but a few, my record was 4 wreckers in 4 days before the girls said stop and would not let me tear up anything for a few days.

 

[73m819]

I have to agree with Jeff, the big problem if the AUTO front wheel drive in low, swing the contract arm out of the way, there by Deactivating the AUTO engage, them backing up in low will be no big deal.