M939 Transmission Operational Question

[rickycotte]

Before posting my request for information to this honorable community, I must say I did search on the TM before daring to ask for information. I found no information on this. Maybe it is there but I did not find it.

Ok, that aside, I would like to know the reasoning for the warning on the dash placard against using LOW RANGE in REVERSE. It lists all the permissible speeds for the different gear combinations but warns against using low range while in reverse gear.

Any explanation on this? Is reverse gear ratio already so low as to put too much strain on the powertrain by going to a lower ratio?

Someday, someone may need to get out of a really tight/steep spot in reverse, with your rig loaded, off road without any other recourse.

Your sharing of knowledge is greatly appreciated.

 

[73m819]

R, LOW will BREAK the transfer

 

[builderboy]

if you read the tm's or asked around everything says this " R low will be to much torque for the transfer case" If you don't believe that or just want to find out go for it. Oh, please post pics of transfer case.

 

[TIGERFANS2]

On a side note, I bet driving an M939 through the rain forest in PR would be a hoot.
But not backwards in Low range.

 

[rickycotte]

if you read the tm's or asked around everything says this " R low will be to much torque for the transfer case" If you don't believe that or just want to find out go for it. Oh, please post pics of transfer case.

Ok builderboy, you seem to be very smart or was able to read further than me. If you could just point to the correct TM I would greatly appreciate it.

I can understand the torque/strain limitation. What is hard to believe is how the government or the designers got away with this. A super expensive truck you can breakdown in a most inconvenient time by just shifting reverse.

BTW, I am not contesting anything. I was just curious about the reasoning behind the warning.

Thanks for your comments.

 

[73m819]

Ok builderboy, you seem to be very smart or was able to read further than me. If you could just point to the correct TM I would greatly appreciate it.

I can understand the torque/strain limitation. What is hard to believe is how the government or the designers got away with this. A super expensive truck you can breakdown in a most inconvenient time by just shifting reverse.

BTW, I am not contesting anything. I was just curious about the reasoning behind the warning.

Thanks for your comments.

These trucks were ALL built by COMMITIES, but NO one talked to each other, all ASSUMED that all the others KNEW what the OTHERS were doing.

This is why the 936 rear winch was DE rated from 45000 to 25000 lbs, because the hyds will only pull 25000 , eather the pump or the motor is to small, each designer thought the other was specing the right unit .

PLUS they were DESIGNED and BUILT by the LOW BIDDER.

 

[Dukeman]

Yeah, but nobody answered the question. Why? What happens? What does the transfer case care if it is going in forward or reverse, its just a gear box. Is it something to to with the sprag? Is it because with the TC in low range you are automatically locking in the front axle? Right? The TC low range over rides the TC air lock switch and automatically locks in the front axle. Anyway, it would be nice to understand what actually happens.

 

[Derrickl112]

The question has been answered in other threads before. The torque from the transmission can crack the transfer case housing.

 

[Dukeman]

Yeah, sorry, this old thread helped, just didn't spend enough time searching.

http://www.steelsoldiers.com/5-ton-up/13783-m939-warning-notice-2.html

 

[rickycotte]

Hello,

Yes, I got my answer too. And some opinions. If you read my very first post in this thread, I just wanted to receive input on my implied reason for the warning.

I am still baffled at how these things go on without consequence for the designers. A kit to prevent engaging reverse gear is insult over injury or just plain old "sweeping dirt under the rug".

 

[Tinydragon2001]

I have used reverse in low range to move old trailers with locked up brakes. I know that this is not recommended, but it was the only way to get the right amount of torque and wheel spin to make the trailers move with out spinning out. I tried to doing it in hi range, but the amount of wheel spin would cause the truck to just sit there and spin its wheels. I completely understood that I could have potentially split the transfer case, but sometimes you just have to take a chance. I was no where near full throttle, and was carefully not to push it to far. I can tell you that these trucks are tough as ****, as long as you keep it within reason, and have no intention of turning mine into large yard ornaments.

 

[wheelspinner]

You ask about the designers not being penalized, but that is not necessarily the case. I worked for an OEM that supplied long lead time government contracts (Not these trucks!) and sometimes through no malice you end up here, with a compromised design. Becasue the design / production run is so far advanced, sometimes its just not feasible to go back and start over and still maintain a parts supply that is common. In these case the contractor generally either extends the warranty, provides additional spares based on the anticipated failure rate, or pays liquidated damages for the design issues. I am quite sure that they did not get off with no consequences as evidenced by the dash tag, so everyone at some point discovered the discrepancy. Just MHO.