Backing Up a M939 in Low Range

[M1075]

Here is a question for Bruce, GIJoeCzar, or other 900 series gurus...

The operator's manual and dash placards on the M939 series state "Do not back up with transfer case lever in low range." I have heard about blown transfer cases when you do. I would like to know is this a design defect, operator error, or something else? Does anyone have some knowledge or first hand experience on the topic?

Is this related in anyway to lockers in rear axles? Do any of the Army M939s have rear lockers?

This inherrent weakness makes me a little gun shy of spending top dollar for a M939. How can you effectively operate a loaded truck off road without being able to use reverse in low range?

 

[ida34]

We towed 15000 lbs howitzers in the sand in Saudi and through the mud in Kentucky. We never had a problem that needed low range reverse to get out of. I think it is more of a design limitation not a defect. They were new when we got them in the late 80's so this is the way they were since new. It was not like the situation that prompted them to install anti lock brakes. We did lock them up a lot but that was because we never used the brakes on the howitzer ( the brakes on the howitzer would lock up frequently). We did get them stuck from time to time but when they were stuck low range reverse would not have helped out. Most of the time they would get stuck after a night rain and the trucks being in position all night. The ground would soften and the truck would sink a little and pulling the howitzer really would hold it up. Most of the time a Hemitt would get it out. One time we stuck the hemitt too and an engineer dozer had to respond to pull it all out. It pulled the hemitt still attached to the 5 ton and the gun. It unstuck them all at the same time. I love the 900 series and would not hesitate to get one. If only I had the money. They are really easy to drive too. The ability to actually have three people in the cab is cool too.

 

[GIJoeCzar]

Hi,

The M939 series 5 Ton trucks do in fact have a transfer case design flaw. As the manual and dash plate state, you can't back up in low range; if you do back up in low range you risk cracking the transfer case. In fact, there's a cracked transer case at Mechanicsburg, PA GL site right now.

The Detroit Lockers equipped trucks have nothing to do with the transfer case problem. When the Army first identified the problem (late 80s) they went back to AM General. AM General in turn went back to the transfer case manufacturer. The transfer case manufacturer in turn blamed it on Allison and Cummins, stating that their transfer case WAS within design specs, so it must be the fault of the motor or transmission. In the end it never got resolved and the military got left holding the bag -- to this day.

As for the Detroit Lockers, only the USMC had them installed in the rear two axles of their M809 and M939 series trucks. I just answered someone elses questions on the lockers, but to sum it up, don't get lockers UNLESS the vast majority of your trucking will be done off road. The lockers, when run on hard ball roads wear out tires and transfer cases well before their time. As well, if you haul a substantial amount of wieght in a lockers equipped truck, you'll start snapping axle shafts. Now, if the majority of your work is off road, then the lockers might not be a bad idea.

Hope this answers your questions.

Are you in the market for a M923 or M925?

Joe

http://www.czarnik.net/joe/My_Stuff/5_Ton_Trucks/

P.S. I'm new to steel soldiers and never would have seen that your question was directed to me, expect I was milling around in the forum. Is there a way that I can search forums to see if my user name is mentioned so that I can answer questions???

 

[M1075]

Joe-

Thanks for your input and answers. I saw your other response regarding the transfer cases & lockers, so I thought I would ask. I could have sent you a Private Message (PM) instead, but I knew you and Bruce were "active" on the site. Besides, others have good input to (as shown above). You can use the search function to look up anything, including questions to you on the site.

I am always in the market for good trucks. I have been looking into M809s and M939s for a while now, especially bridge trucks. I'm sure I will have a few more questions for you before I buy.

 

[jeli]

Don't do it!!!!! The M929 we had ripped the Tcase off backing up in reverse. I didn't see the damage because it went to depot right away. They were stuck and loaded with sand. I'm sure a boat load of torque was being applied.

 

[GIJoeCzar]

Roger, If looking to buy soon I've got four 5 tons for sale now. All need restoration, but all start, run, drive, and stop. Most have many new driveline parts including three of four with new government rebuilt motors. Go to (http://www.czarnik.net/joe/My_Stuff...9 Series 5 Ton trucks That I Want TO Sell.doc) for more info. If a password box comes up, simply click on the "cancel" button and the MS Word document will open. If truly serious, make reasonable offer for all four. I've got a trucker who can get two on a 45 foot step deck and can get them to you for $1.50/mi per 5 Ton truck. Joe

 

[m139h2otruck]

Backing up

It is interesting that this topic came up. While test driving our truck (gasser M139), I stopped at a friend's yard with a steep drive. The gate was locked, and I placed the truck in reverse and rolled back about 50 feet with the clutch in. Then, engaged the clutch to back around some cars and there was a very loud bang and shudder in the truck like something let go. Has done this before in forward, after backing up in gear on hard pavement, cutting the wheels hard, and then engaging a forward gear and driving off, but not as loud or violent. Truck still shifts fine, including low range and sprag seems to be working correctly and engages like it should. Any ideas? Truck was empty and uses single 1400x20 tires on rear. Air pressure was above 75 psig and all gear boxes warm from driving. All gear oil has been changed to 80w-90 just recently. Found a small piece of gear/sprag in the bottom of the transfer case when changed, that looked too small to be a main gear tooth. No idea of how long it was in there, as the oil was very bad, black and like asphalt. May have even been original, as this truck had only about 12-14000 miles and no rebuild tags (1961 IH truck).
Any suggestions? Is it OK?

 

[DDoyle]

RE: Backing up

H2O truck......All the sounds and symptoms of sprag problem. Sounds like the sprag is not shifting properly from forward to reverse - this is confirmed by your comments in another thread.

I'd verify the air valve is working properly, and if it is look toward flushing the transfer case out in hopes that the mechanism is simply gummed up.

Regards,
David

 

[m139h2otruck]

Adjustments?

Are there any adjustments for the sprag? Looks like the shifter fork is connected to the piston/cylinder inside the case with no adjustment. The manual I have has no info at all on the transfer case and required adjustments. Could it be that the fork/slider parts of the sprag are dragging and the air cylinder does not have enough force to slam it into the opposite position before you release the clutch? Maybe have to change gear oil again right away, because like I noted before, the stuff that came out was some of the worst I have ever seen, and that's going some. I still think that I read somewhere that the sprag unit goes to neutral with no air pressure, and this is how you can run hubs on the front of a 5 ton. The member with the big block Mopar powered 5 ton has some sort of valve arrangement for his due to a different transmission. Also, how would you be able to pull or push one of these beasts around with a dead engine if the sprag stayed engaged. Just trying to think of all possibilities! Also, M1075, sorry to hijack your post.

 

[DDoyle]

RE: Adjustments?

Towing forward is no problem - the forward sprag is the one that is engaged. Pulling backward is the problem. If the oil in the transfer is terrible, then I'd definately flush/change it. I know of one fellow who had similar problems and filled the transfer case with diesel and rolled the truck back and forth a few feet, then drained it and refilled it with gear oil. I dunno how I feel about his method, but it did solve his problem.

 

[bluehorse]

Anyone have pictures of where the transfer cases break or crack from using them in reverse? Also I have read a lot of conflicting information on using low range in reverse, seems if your not hammering on it in reverse fully loaded its ok, but other things I have read have said just backing up flat ground not loaded breaks them which I find hard to believe.

My guess was people were hammering on it in reverse low range loaded down stuck or pushing stuck trailers or trying to pull someone else out that was stuck by backing up in low range. Maybe the military just decided it was easier to say do not use low range reverse ever instead of saying just be careful because where is the line on be careful?

Anyone have any experience with a transfer case cracking or breaking on them?

 

[doghead]

There have been several threads on this.

I'd try searching, Low Range lockout, Transfer case cracked, in the 5 ton forums.

 

[doghead]

https://www.steelsoldiers.com/showt...ken-transfer-case&highlight=low+range+lockout

I did an advanced search in the 5 ton forum, for transfercase low lockout.

Lots of threads listed.

 

[quickfarms]

The issue is the choice of components used in the truck.

Each component is within there respective specification.

The problem is they chose the wrong Allison. The gear ratio for reverse in the Mt 654 is several times lower, more reduction, than first gear.

 

[lindsey97]

The issue is the choice of components used in the truck.

Each component is within there respective specification.

The problem is they chose the wrong Allison. The gear ratio for reverse in the Mt 654 is several times lower, more reduction, than first gear.

Correct. Reverse ratio is like 10:1 ? Needs to be 6:1. I have heard a rumor that a MT653 has a numerically lower reverse gear ratio, and that it will interchange with a MT654cr reverse gearset. Anyone done it yet?