M939/A1/A2 ABS vs. Non-ABS Trucks

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ida34

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I would not think larger tires would effect anything. On the civy trucks, there are sensors on the wheels to sense a difference between any of the wheels. If the fronts are going 0 and the rears are going 35 then the fronts are locked up. I do know that larger tires does effect the computer as it wants to know how fast the truck is going. I would bet the 900 series kit has a sensor system also as this is the only way I know of to tell if one or more axles' brakes are locked up. I could be wrong. It has happened before.
 

Nonotagain

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I looked around a little bit to see what articles were in public domain about M939 series trucks and braking issues.

Per the article posted on Global Security website the retrofit to ABS brakes aslo included the replacement of the NDCC tires as they bore most of the responsibility for loss on control on web pavement, or so I deduce.

Arvin Meritor has an aftermarket ABS system available, I just know at what cost.

http://www.meritorhvs.com/MeritorHVS_Documents/MM0112.PDF




http://www.globalsecurity.org/military/systems/ground/m939.htm



M-939 5-TON TRUCK

The M-939 series comes in six body styles: cargo, dump, wrecker, van and long wheel base cargo. As of late 2000, the most common versions of the M939 Series vehicles in service were the M923 Cargo Truck, M925 Cargo Truck W/Winch, M927 XLWB Truck, the M931, 5-ton Tractor, and the M936 Medium Wrecker.

The M-939A2 tactical truck is a five-ton capacity, six-wheel drive cargo truck used for transportation of all types of supplies. The basic M939A0 model featured improvements such as automatic transmission, improved power steering, a complete airbrake system, an improved cooling system, an improved electrical system, a three-crew member cab, a tilt hood, a hydraulically powered front winch and a simplified test equipment/internal combustion engine diagnostic connector. The M939A1 model added super single radial tires while a new CTIS, a new diesel engine and chemical-agent resistant paint were the main additional features of the M939A2 model.

The M939 Series vehicles are powered by a NHC-250 Cummins diesel engine. The truck's central tire inflation system enables the crew to increase or decrease the air pressure in the tires to improve mobility on or off roads. It can tow 21,000 pounds.

The M939 truck was introduced in 1983. It's a general-purpose military vehicle, primarily designed for tactical, off-road use, with a top speed of 65 mph and an automatic transmission. It augmented the older M809 series tactical truck. The major differences between the M939 Series vehicles and the M809 Series vehicles are as follows:

§Automatic transmission;
§Improved power steering system;
§Complete air brake system;
§Improved cooling system;
§Improved electrical system;
§Three crew member cab;
§Tilt hood; and
§Hydraulically powered front winch.

The M-939A2 is a fitting replacement for the famed Army "deuce-and-a-half" truck. The Army received the M-939A2 in 1989.

There were 10,807 in the Army's inventory in the early 1990s. As of mid-1998, there were about 32,000 M-939 trucks in the Army fleet.

The US Army settled on adding computer-controlled brakes to its M-939 five-ton tactical trucks to make them safer for soldier-operators. It began purchasing ABS hardware starting in fiscal year 1999, with retrofitting to begin in 2000, and the project to be completed in 2003.

The Medium Tactical Vehicle Replacement program was set to replace the Marine Corps' aging fleet of M-939/M-809 five-ton trucks with trucks with greater mobility, lift, and reliability. The MTVR is set to carry an increased payload of 7.1 tons cross-country; 15 tons on hard-surface roads while simultaneously being able to tow an 11-ton load.

The M-931 and M-932 are the tractor truck variants of the M939 Series vehicles. The M-931 and M-932, like the M818 tractor truck, is equipped with a fifth wheel used to haul semitrailers with loads up to 37,000 pounds cross country and 55,000 pounds on the highway. The only difference between the M-931 and the M-932 is that the M-932 has a front mounted winch which has a pulling capacity of 20,000 pounds. The medium wrecker/recovery vehicle variant of the M-939 Series vehicles is the M936. The winch and crane capacities of the M936 are identical to the M816 wrecker/recovery vehicle.

In FY95 the M939 series 5-Ton truck was responsible for 26% of the total Army Military Vehicle (AMV) accidents and 53% of the total AMV fatalities. In 1999, GAO report GAO/NSIAD-99-82 analysis indicated that from January 1987 through June 1998 accident data showed that while M939s made up an average of only 9 percent of the AMV fleet, the M939 accounted for 34% of the fleet's accidents resulting in fatalities. Comparison of U.S. Department of Transportation accident statistics to M939 accident statistics showed that over a 10-year period, the fatality rate of occupants of the M939 averaged about 30 times higher than the fatality rate for occupants of comparably sized commercial trucks.

Also noted in the GAO report were the results of a TACOM tire study conducted from 1995 - 1997. That study concluded that the M939s were being used on road more than originally planned. The original intent was to drive the M939s on highway and secondary roads 60% of the time and 40% off road. Current data indicates the M939 trucks are being driven on roads 80-90% of the time and only 10-20% off road. For the past four years, the M939 Series trucks have been operating under Safety of Use Message 98-07 (SOUM 98-07) limiting the highway speed to 40 miles per hour in an attempt to limit accidents, injuries and fatalities occurring under this highway operational scenario.

The accident scenario for all M939 trucks occurs during panic stop situations and is worsened on wet pavement. In panic stop situations the trucks wheels lock up causing engine stall. This causes loss of power steering resulting in uncontrolled skidding creating accident and roll-over situations. Extensive testing of ABS for this truck has shown that ABS will eliminate 100% of the engine stalls and wheel lock-up regardless of the skill level of the drivers.

The accident scenario for M939 basic vehicles with NDCC tires occurs during panic stop situations on wet pavement. The front wheels lock up, the NDCC bias tires react like ice skates and stopping distance is increased by 245-320 feet over trucks with radial tires. The M939 truck is expected to support the Army's transformation effort through FY30. Recently the program looked at removing radial tire applications to avoid cost. Recent test data concludes that ABS with NDCC tires is a deadly combination. The tires react like ice skates on wet roads and braking distance is increased by 245 to 320 feet over a truck with radial tires and the ABS kit installed. Once the ABS kit and radial tires are installed, SOUM 98-07 can be lifted, allowing the vehicles to once more be capable of safe operation up to their required operational capability and full mission requirements.

The nondirectional cross country (NDCC) tire design on the M939 basic truck was engineered for cross-country applications prior to World War II. Changes in vehicle speeds, road construction, mission requirements, as well as advances in tire technology have made this NDCC bias tire obsolete and unsafe. This modification will change the tires from the current bias ply NDCC tire to a radial tire designed for on/off road. Recent improvement in radial tire design will provide better traction and mobility, which will enhance system safety.

There are 32,000 M939 and 350 M945 trucks worldwide that must have ABS applied. Of those 32,000 trucks, 11,700 basic trucks are having their bias tires upgraded to radial tires to further improve vehicle safety.

The Army identified certain hazards associated with the M939 series truck in 1995 and informed units in the field through Ground Precautionary Message (GPM) 96-04. Army leadership applied more controls to reduce hazards by identifying a maximum speed limit of 40 mph in Tank-Automotive Command (TACOM) Safety of Use Message (SOUM) 98-07. This change was incorporated into Technical Manual (TM) 9-2320-272-10, with Change 1.

The Army determined the fix for reducing these hazards across the force was a modification work order (MWO) to place antilock braking systems on the entire M939 fleet. For the basic M939 fleet, the Army also decided to replace existing non-directional, cross-country tires with radials (TACOM SOUM 98-07). Yet, this endeavor took time to complete. Until its completion, leaders MUST be informed and proactive about addressing the hazards associated with this common system used by numerous units across the Army EVERY time they use it.

Despite these efforts, as of 2002 units continued to operate M939 series vehicles in the very conditions the messages warn against. Until the MWO is complete, M939 trucks are not to be driven above 40 mph, which means 40 mph is the extreme limit. Driving too fast for conditions creates an environment for compounding the effects of the other hazards listed below. Unit leaders must evaluate and re-evaluate the conditions the truck will be used in and apply the appropriate controls.
Tailgating can create an extremely hazardous condition when drivers overreact to vehicles braking to their front. Over-braking can lock up the wheels, causing the engine to stall. This can lead to loss of control of the vehicle.

Damp or wet conditions contribute to the vehicle losing traction when the brakes are applied suddenly and with too much pressure. Drivers must slow down when damp or wet conditions exist, and leaders MUST re-evaluate the need to operate the truck in these conditions and, at a minimum, implement additional control measures and inform their drivers of the increased risks.

The M939 series truck was developed for heavy loads and off-road conditions. The accidents seen so often are M939 series trucks operating on asphalt roads. The trucks are generally hauling cargo on or around post, or they are hauling soldiers to and from training and details (see Army Regulation (AR) 385-55, Prevention of Motor Vehicle Accidents, for guidance on hauling soldiers). This does not mean you need to add weight to the truck to operate it safely, but it does mean that leaders need to recognize the increased risk of operating in these conditions and enforce speed limits and safe distances between vehicles, as well as inform drivers of the increased risks. Information and knowledge about the system is half the battle of operating any equipment safely. Soldiers will not know if leaders do not.
 

cranetruck

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Right, the question still remains; why does the engine stall? A delay in the transmission lock-up release? Is the driver braking with his/her left foot?
 

Rattlehead

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I cannot find any input in the schematics, electrical or pneumatic, that would "tell" the transmission that the brakes are being applied. The transmission shift points and TCC apply are controlled by the modulator, but this is only the throttle position input to the trans. Unless there is some electrical or pneumatic input to the trans during a brake apply, the TCC would stay locked until your vehicle speed reduced to 2nd gear range.

Bjorn, your trans may be different. Do you see any way that a brake apply would be communicated to the trans?
 

jwaller

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Right, the question still remains; why does the engine stall? A delay in the transmission lock-up release? Is the driver braking with his/her left foot?
if I were to guess I'd say that the trans converter isn't unlocking fast enough and there may or may not be a fix for that.

I suspect the only real fix might be a reprogramming of the unlock speed to a higher number such that it would give you a few more milliseconds of time before coming to a full stop.
 

cranetruck

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Bjorn, your trans may be different. Do you see any way that a brake apply would be communicated to the trans?
No there is no communication between the brake system and the transmission on the xm757 (linkages shown in image below). All six gears lock up, but only when the rpm is high enough (=governor pressure?), so perhaps a combination of throttle position and pressure in my xm757 case?

The xm757 transmission shift valve action is shown in image #2 below. I don't have the MT654 transmission details, but it may be very similar, being Allison and all.
 

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Rattlehead

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Without some sort of brake input to the trans, the only other way I could see the TCC disengaging is if it were designed to on a decel, via hydraulic circuits and the throttle input. I cannot tell with those schematics if your trans does or not, nor do I have any such charts for the MT654. But I don't feel a tcc engage/disengage event if I decel momentarily, and then get back into the throttle to maintain speed. That doesn't mean it is not occurring, though.
 

Rattlehead

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The transmission service manuals have to be purchased, but their user manual is available for download.

http://www.allisontransmission.com/servlet/DownloadFile?Dir=publications/pubs&FileToGet=OM1334EN.pdf

There is a section on lockup clutch function. It is not clear whether the lockup disengages during a decel after a delay, or simply not at all until vehicle speed gets to the programmed point. This is done for engine braking. But either way, if there is no brake input to the trans, locking up the brakes will stall the engine.
 

steelsoldiers

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Well, my Haldex Info Centre for the M939 ABS systems showed up yesterday. Now, I just need the ABS :p I am anxious to try out the Info Centre to see how it works. My M923A2 would kick on the ABS light every once in a while to indicate a fault. It would have been nice to have this thing to log the faults, miles, speed, etc...

Bruce, if I get a chance, I could hook it up to your M925A2 John's and scan the system to see if it is in good shape.
 

BKubu

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That is old news, Bjorn. Yeah, that is a design flaw for sure. I adhere to that warning. However, when I was a newbie to the trucks, the first time I drove one, I had the truck in low range and put it in reverse with no ill effects. The problem comes in when you apply a reasonable amount of throttle. The t-case will have a tendency to crack!
 

cranetruck

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That is old news, Bjorn. Yeah, that is a design flaw for sure. I adhere to that warning. However, when I was a newbie to the trucks, the first time I drove one, I had the truck in low range and put it in reverse with no ill effects. The problem comes in when you apply a reasonable amount of throttle. The t-case will have a tendency to crack!
At least that design compromise came with a warning...(unlike the panic brake-engine-stall problem)...
 

steelsoldiers

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I got my bedsides and tailgate on today. I will repeat the test next weekend and see what happens. I will make sure I do it in a straight stretch so I don't end up in John's field!
 

OPCOM

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in the abs wet video, the rattling sound is the drivetrain loading and unloading. it's not good for it. But better than crashing.

And the non-ABS wet pavement thing is exactly what the driving instructor at the National Guard said would happen. He did not alow us to try it..

Are those videos public domain or does someone own them? I assume they are military test videos and belong to "we the people" but it is right to ask.

Finally, the ABS kits on the M900 series we were trained on were said to be a very simple trailer-type ABS kit based on commercial items.
 
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emr

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Actually it is NOT a design flaw, the Army, Alison and AM General all did separate studies and all came up with the same answer, driver error, the Army ordered a trans with an 11 to 1 gear ratio in reverse, a cat D 6 has a 9 to one, so it is easy to see how low these trucks are geared, there has never been a trans problem that did not" occur during shifting from D to R with the R's up, Meaning shifting with out letting the revs settle to idle will crack the trans in low gear reverse because of the 11 to 1 ratio in a side sshaft trans...that is a catostrofic failure do to the driver, and a lack of training in the begining, These trucks are robust and so are there trans's, it has been proven thru hundreds of thousands of miles thru the deserts of iraq, I will add from the first units in the first war and the first units of the current conflick, the Nj guard saw more miles on the road then most for sure, and the leaders of these groups are most any way, have crossed paths with me at least and alot are good friends, and from Vietnam Vets in theater to new guys the 939 series of trucks are haled the finest cargo trucks in use from the guys who drove em and from there mouths...even over the 800 series from the guys with experince in theater for what its worth . And no surprise the FM class's too, Now it is important to note that there were absolutly no mass trans failures in threater, and that is du to driver training and the fact these trucks and transes are tough, And i have to say having one for 6 years i will say they are tough, They said impossible to over load, and that is a direct link to trans use in my eyes, they performed and stayed on the road, Now there were alot of 939s here at the Armory i hang and volunteer at, And in my quest for info they ran low in reverse all the time, so i have also for 6 years, and the BIG thing is If I were to shift with the R's up I would split the T case like they say, it is a side shaft Trans and one may call it a design flaw, but really it is still in use and will be in many trans's ...It is a matter of what this truck is and how to drive it, the novice should never drive this truck in low reverse because they will ruin it, the Driver who respects His truck and what it is will never have a problem period...:-D... I will add the problems were reported under severe use also, so in my opinion from all I have read they were not even that easy to split in low. But since I pay for it I will baby it, but man she pulls and We have been pretty stuck and reverse in low works real well, but like said I let her settle before shifting thats the key...
 
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cranetruck

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Actually it is NOT a design flaw, the Army, Alison and AM General all did separate studies and all came up with the same answer, driver error, the Army ordered a trans with an 11 to 1 gear ratio in reverse, ..
To nitpick a little, according to the warning label posted above, the Army didn't get the 11:1 ratio for reverse since the transfer isn't supposed to be in "LOW" when in reverse.
I like to compare features with other vehicles, in particular the m656/xm757 series, which kind of got lost in history. The transfer case for this series is a 1:1 unit, but with a six speed transmission, the reverse is still very low, a 6:1 ratio (first gear is 5.3:1).

Looking forward to results from some further testing Chris... The Army placed a speed limit of 40 mph on these trucks, is there a reason for this particular speed that you can see based on rpm and transmission lock-up conditions? You can drive 40 in fourth or fifth gear, perhaps it doesn't lock up at 40 mph when in 5th?
 

steelsoldiers

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Looking forward to results from some further testing Chris... The Army placed a speed limit of 40 mph on these trucks, is there a reason for this particular speed that you can see based on rpm and transmission lock-up conditions? You can drive 40 in fourth or fifth gear, perhaps it doesn't lock up at 40 mph when in 5th?
Well, since we got the bedsides and tailgate mounted on Sunday, I should be able to do some testing soon. I will try it on hard packed dirt/gravel first and then try the pavement.
 

bevanet

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Did anyone else notice that in the Wet pavement non-ABS test that the driver actually turned the steering wheel to the right when the the truck started to skid around. Watch his hands and head as the truck starts to skid. You can see him lean to the right and crank the wheel. If he would have turned left, the truck might have stayed straight. I think the skid was enhanced to help sell ABS systems.
 
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NEIOWA

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Very interesting thread. My M923A1 experience was in 1986/87 as a truck (Support Plt) PL. Brand spanking new. I know we didn't have any of the reverse low range/crack the transfer case or max 40mph w/nondirectional tires or you're gonna wreck. Little NET when we turned in M813 that would have trained operators (11B and a couple 88M) what NOT to do. We did LOTS of driving in rain (Washington State) on pavement including I-5 without killing breaking.

Obviously anything since the GMC 6x6 is junk, but ABS with Stability Control is a HUGE improvement in operator and public safety in heavy trucks. I'm coming from a fire truck standpoint. My personal newest unit is a 2007 M2-112 64000lb. I WISH Frtliner had a stability control system available. IH is leading the industry in safety improvements.

I doubt Sam has discovered stability control yet. Might be "issues" with offroad in any case.
 
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