Another "Don't Use Low-Reverse on 939 Series" Thread? Yes, But Fact Finding...

[74M35A2]

Another "Don't Use Low-Reverse on 939 Series" Thread? Yes, But Fact Finding...

The "Don't use low-reverse" on M939 series seems to really have a mixed bag of emotions and outcomes. What I'm trying to decipher is, are there any cases of the T-case breaking from low-reverse usage with the front axle drive off?

I see carnage pics of broken ones, I read the stories, various threads, and I also see people say they do it all the time with no ill effects. I have even read one story where somebody says simply the slam of shifting the trans into reverse cracks the T case housing. I want to use low range to climb hills, pull stumps, and go over cars at the junk yard, etc...., but I don't want to shift into high range to simply back the truck up under its own weight only with little to no additional load.

Bottom line on top, are there any known cases of the T-case breaking by simply backing the truck up in low-reverse with no additional load on the driveline, or even just putting it into reverse? If so, then I'll pass on doing so myself, and maybe even pick up one of the GL auction T-cases that are new and piled up at the Ohio outlet (if these fit).

 

[swbradley1]

I don't recall hearing of anyone saying that they were on a flat surface with no load putting it in low-reverse and breaking the case.

That being said too many broken cases seem to indicate that there was indeed a load of some sort on the trucks that have had the problem. Backing up a hill or pulling something out in L-R come to mind.

Why risk it unless you enjoying changing cases.

 

[wreckerman893]

I have backed my M927A2 up in reverse but it was basically idling along while I was moving back and forth to re-position the truck while I was pulling a stump.

The transfer cases can be a real bugger to shift sometimes. On one truck I had you actually had to bleed the air down to almost zero to get it to shift.

It can be done without damage but you really have to watch to make sure you don't put any torque to it while you are doing it.

My guess is that it was a design flaw that wasn't discovered until the trucks had been fielded and were out of warranty.

The fix was then determined to be not to back them up in reverse so the contractor would not have to be paid a ton of money to fix the problem.

We were told in the Army to never back them up in reverse, empty or loaded. Did it happen? Yep. Were trucks damaged? Yep.

Will trucks continue to be damaged by reversing in Low Range? Yep.

You own it so you bear the responsibility for what happens to it unless you let someone else drive it.

Since we all know it's an issue why take the chance?

 

[VPed]

Well, as I understand the thread about the exploding t-case where the brakes were locked up, 6WD was not engaged. I would not think the front brakes would have been locked up since they are not equipped with parking brakes, so even if it had been in 6x6, almost no resistance from the fronts. I think that is evidence that the case will grenade if subjected to sufficient torque regardless of 4x6 or 6x6 mode.

 

[VPed]

I also can say I regularly attempt to climb sand hills in low and if I do not make it, I disengage the front drive and reverse out. Of course, this is downhill and only if I did not dig in while climbing. I disabled the t-case air switch by flipping the linkage section with the washer.

 

[Floridianson]

Same here hard ground not the problem soft ground or stuck in soft ground just as bad as brakes locked up.

 

[Suprman]

Driving over cars at the junkyard will cause tire and underside damage. As the cars bend and tear under weight, sharp edges form. I have never had an issue backing up to something in low range. I wouldnt do it under load though.
Will

 

[74M35A2]

Thank you for your experience of reversing in low range. So far, I am putting together the picture it has been OK as long as there is no load beyond flat/level ground on an otherwise empty truck. As others have said, my truck is also resistive to shift amongst T-case ranges, so I try to avoid doing so. I need to check for correct air line routing, etc...

Still waiting to hear if anybody has broken their T-case from a simple reversing situation in low range.

 

[acme66]

We run a tour company in MT and the last mile or so including the turn around is all LR stuff. I have the front axle auto-engage removed leaving me with just the dash switch. I turn the truck around by driving past a side road and backing around into it, all as I said in low range. There is a slight downward slope and then as I get further into the road a slight uphill. I only back in far enough to get the front around. I always disengage the front before starting my turn but when you do something 5-6 times a day day after day sometimes it gets forgotten. The road surface is dirt not gravel. I have been backing in accidentally with the truck still in 6x6 and felt the steering lurch reminding me. I then kick it out of 6x6. Two different trucks over two years, hundreds of turn around cycles and no broken t-case yet. It is a zero power situation but this might be some empirical data for you. I will answer any questions you like.

Ken

 

[stonemeadow]

My M925A2 backs up on a 50% hill in low range, did it a bunch of times, no load though. Proofs to me it is not super fragile. I would not do it with a heavy load.
Cracking the T-case may also have to do with shifting in reverse while still rolling forward, they may have missed that test on the proving grounds, surprisingly.
I always come to a complete stop and than shift in reveres with engine at idle RPM, never had a problem.
On rocky trails I am in Low-range most of the time, slows the vehicle down to a comfortable crawl easy on the vehicle and passengers, it travels over 2 feet boulder fields slow but surely.
Overall I think they did a top-notch job designing the 900 series!!

 

[74M35A2]

Thank you to those that have written here, it is exactly what I am looking for. These are enough success stories to say it seems possible to do without splitting it as long as the vehicle is reasonably level. I am specifically looking for even one person to say they cracked theirs from a "normal" reverse maneuver, but have not found such yet.

 

[sandcobra164]

The M939's have an automatic trans and reverse is in excess of a 10 to 1 ratio with 685 ft lbs or torque. I think I can see why the transfer case may have issues when engaged in low range on a truck that is trying to move against locked brakes or a heavy load. The A2's made around 750 ft lbs or torque and that only magnified the problem. I use low range for Parades and have disabled the automatic front axle engagement. No issues rolling forwards or backwards on flat ground towing an M105. On an A0 or A1 truck, you can expect 685 or better foot pounds or torque sent to the transmission with a torque converter sending at worse 1.5 of that into a 10 to 1 ratio to the transfer case. Somethings gotta give and it seems it's the transfer case when loaded as such.

 

[my76fj40]

I have been using reverse gingerly, disabled the auto front engage and under no load, empty. Generally with no added throttle. Watching here to see if I am flirting with disaster...

 

[RobertoGatos]

Use reverse in low all the time, pushing trailers on and offroad, both in 6 and 4wd. I sometimes have shifted the truck between high and low a bit too fast as well. No breaks for me yet. I just don't romp on it needlessly.

 

[mcmullag]

From a former thread and I know I have seen this somewhere else too.
1st - 4.17
2nd - 2.21
3rd - 1.66
4th - 1.27
5th - 1.00
Rev - 10.76

Pretty crazy how the reverse gearing is so much more reduction than first gear. Would think the engineers and during testing they would have changed that reverse ratio. Can't see why reverse would be more than twice reduction of first gear.

 

[rangereter]

I like the transmission reverse ratio (high range w/duels, not singles) especially when backing up trailers and disabled vehicles...don't usually need the slower reverse ground speeds of the low range transfer. That being said, in the mid 80's I had to take a M936 (37,000lbs +/-) over to Pope AFB to do a "test/evaluation" load of the wrecker into a C-130 to determine air transport feasibility. The wrecker height had to be reduced to minimum (soft top removed, windshield dropped, boom braces removed and lowered, etc.) and had to be backed into the aircraft in "low range". I said that as a rule this family of trucks should not be "reverse low-ranged" but for safety of the aircraft low range had to be used (no subsequent transfer damage occurred). Well, a couple of weeks later, one of my guys had one of the 936's out on the DZ chopping into the sand with the old bar grip tires...put the xfer into low because high range 1st didn't enough "umph"...dug in a little more, without thinking, put the girl into reverse, romped on it and the "crack/split" was set as indicated by the puddle of gear oil under the truck on the concrete back at the motorpool. So the "loaded" reverse low range lesson was learned pretty early on...I still really like a slow reverse, gives you more time to react.
Regards, Bob

 

[Csm Davis]

The problem as rangereter shows above is two fold one is the truck being in 6x6 which is automatically engaged in low, stupid thing to leave when they knew that there was a problem, and secondary part is lead foot.
I personally have put hundreds of trucks in low and reverse and never broken a transfer case, and many of these were 936 wreckers that had a load on them of 20,000 lbs so the total load that the transfer case saw was around 60,000 lbs. Now this being said I recommend everyone disable that switch valve and look at your transfer case closely to make sure it is not already cracked by young men in green that didn't care, like I did many times, I have backed a many up over the 2 foot wall of a stepdeck trailer in low reverse. Many folks will say it is because I was easy on the go peddle and that maybe part of the reason why I haven't broken a transfer case but I will say that it is largely due to a strong transfer case, because if you ask Amy or Kimmie they will say that I gave our wreckers heck and broke them regularly, 4 in one week in fact, my list of broken parts is huge but doesn't include a transfer case yet. So I say anybody that gets a broken transfer case in low reverse with no load and easy throttle had a cracked case to start with.

 

[Csm Davis]

The "Don't use low-reverse" on M939 series seems to really have a mixed bag of emotions and outcomes. What I'm trying to decipher is, are there any cases of the T-case breaking from low-reverse usage with the front axle drive off?

I see carnage pics of broken ones, I read the stories, various threads, and I also see people say they do it all the time with no ill effects. I have even read one story where somebody says simply the slam of shifting the trans into reverse cracks the T case housing. I want to use low range to climb hills, pull stumps, and go over cars at the junk yard, etc...., but I don't want to shift into high range to simply back the truck up under its own weight only with little to no additional load.

Bottom line on top, are there any known cases of the T-case breaking by simply backing the truck up in low-reverse with no additional load on the driveline, or even just putting it into reverse? If so, then I'll pass on doing so myself, and maybe even pick up one of the GL auction T-cases that are new and piled up at the Ohio outlet (if these fit).

Okay guys I have one case that I will share about a non 6x6 broken transfer case. Didn't get to see the broken case but sold the owner a take out. He said that one of his drivers went to pull out with a full load of logs and the rear output area of the case broke out a major portion of the case. He was in low range 1-5 front axle disengaged and it just ripped. So moral of the story is be gentle and your truck will last for a long beautiful life, beat on it and something will break. It boils down to this, you guy's that baby them will never break the case in any situation, guy's that beat on it like it's a unbreakable will break it,and the guy's that work them like the great pieces of equipment they are probably won't ever break the transfer case or anything else until it wears out.

 

[Csm Davis]

One more thing I think I have told this in another thread but here it is again we moved around 2500 trucks out of camp shelby I would say that we backed about 1000 onto stepdeck trucks and up onto the upper deck without use of any ramps or cribbing and it is straight up about 24" and most were done in low range 6x6 reverse because it was easier on the truck and driver and not one transfer case ever broke, the trucks made it look easy, almost like a snake over a step, if you had to do it in high it was doable but more throttle was needed and it would slip the back tires sometimes and the driver would get a severe pucker factor and rear end of the truck would try to slip one way or the other. Point is if they were so easy to break I think I would have broken at least one.

 

[74M35A2]

Very useful info, thank you. That is like 1000 people writing in to say they have done it and nothing broke. I am seeking the one single case where it did break under little to no load, or even when shifting into R, but I can't find one. At this point, I don't think I'll be too scared to try it if need be. Thanks all for contributing.