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

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emr

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I have noticed that i love the ABS in mine....I will also say that the older trucks are driven with old time driving experience and have no problem, But sure do under stand that a driver first MUST Do like u say here, experiment and create experince in there trucks in a few different condions to understand what they have and driver training in the hobby starts with the individual doing it them selves I must state, this is an awesome post. i have learned alot too. thanks...

I will note that shortly before and after the instalation of ABS brakes the Military did put a stricked speed limit of 40 MPH on these trucks, I do sure do believe they the ABS were most of it, but need to consider the lower speed limits, Today speed limits are very very taken seriously in Military trucks, A tid bit of info, If the vehicle gets pulled over for speeding or any violation, and there are a few guys in the truck the driver... AND... the highest ranking individual gets the ticket today... and it can get worse for all of them if the situation warrents.....
 
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No.2Diesel

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Hi,

Thank you Chris for posting your observations and the video links to the tests. It demonstrates how important it is to drive safely and not rely on Technology as a crutch. My brother and I have put almost 2,000miles on the M923 since getting it back in Sept. of 08' and have operated in many environments from the most urban streets and traffic conditions, to rural and offroad in all kinds of weather and never had a problem. If you have to emergency stop just pump the brakes quickly and you'll be fine.

Having the Torque Converter remain "locked up" in 5th no matter what is a huge factor that leads to stalling, loss of steering. I guess if you wanted to be extra careful during bad conditions the driver should keep the shifter in the "1-4" position to prevent Tq. Conv. lockup.

I do agree with members here that stated the video tests were executed without using the scientific method :p

Attached are some photos of our own testing at the "proving grounds." :driver:
 

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saddamsnightmare

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January 8th, 2010.

All this discussion is making me appreciate the simplicity of the unconverted M35A2 series truck and S404.114 Unimog that I own. The only worries I have is that neither truck's hydraulics are split, so that is the weak point in either system, having said that I have never seen an ABS system that I trusted enough to let it do the thinking for me. As I have always driven manual transmission light and medium trucks which did not have ABS applied, I always felt I had contriol of the vehicle and common sense and experience makes up for the lack of ABS.
The loss of steering power with the engine stall out would be far more bothersome, given the size of the truck, then any other factor, and it is caused by the torque converter lock up mechanism. It seems that they are trying to get manual transmission efficiency in an automatic transmission set up..... Why not just stick with the manual transmission....Oh, sorry, the average brain dead American can no longer drive a standard???!!!!:-D

This is gonna get like the Russian Army driving schools where some poor slob of an officer has to teach the peasants how to drive Uncle Mikhail's toys......

I can see the validity of the discussion and we all stand to gain from it, but I, for one, am going to continue to stay away from anything that has an automatic in it if it is at all possible...... The modern engineers can't design a solid reliable standard, witness all the trouble Chrysler has had with the drive trains on its Jeeps the last 9 years.....

Stay Safe and use your heads for more then hat racks, they work better when exercised......:twisted:

Cheers,
Kyle F. McGrogan:-D
 

Rattlehead

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Bjorn, my understanding of the 939 torque converter is that it locks in towards the top of 2nd gear and stays locked in for the rest of the upshifts. I can feel this as sort of an extra 1/2 gear "shift" right before going into 3rd. In the civvy junk, a brake pedal electronic input kicks out the TCC, not the case with the 939's. I cannot remember where I got this info from, so cannot confirm it.

I did find this, which says the lockup operation occurs as a function of governor pressure (vehicle speed).

http://www.lejeune.usmc.mil/mccsss/schools/los/files/lessons/AIMC/AIM5206.pdf

from page 8:
(9) Lockup operation. Power is transmitted mechanically through the
lockup clutch. Application of the lockup clutch occurs automatically as a function of governor pressure.​
 

emr

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I liked the tests, on the wet ones they looked pretty similer to me, from my view the right side was about very close to the same place on the wet, meaning the wetter side was on the drivers side, from the glare it looked very very close to the same,
 

cranetruck

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So, the problem has to do with the release of the lock-up condition. Not responsive enough? Is this peculiar to this transmission (MT654) only or would it apply to my 8x8 TX200-6 (MT31, MT41) also, I wonder.
 

cranetruck

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On a civy car (vacuum available), this is addressed as follows:
"

  • The vacuum switch in the "TCC Automatic Lockup" wiring ensures that the TCC unlocks in situations of low or no vacuum. Since it's hooked to a ported vacuum source, this means the switch is open at idle and when the engine is under heavy load - exactly when you want the TCC to be unlocked. The factory switches I've seen have a small vacuum restrictor/delay valve inline just before the actual switch, and this helps delay lockup until you have a few seconds of good, stable vacuum going to the switch. So long as the vacuum rating of the switch and the delay time on the valve are reasonably matched to your engine combination, this should result in the effect of only locking up the TCC when the engine is under stable operation. TCI's kit uses an adjustable vacuum switch to enable you to fine-tune this.
  • The brake switch makes sure that the TCC is unlocked when you step on the brake - this is important in "panic stop" situations where you jam on the brakes and might lock the tires. It also ensures that the TCC is off when you at sitting at a stoplight."
How is this done on the M939 series?
 

ida34

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I drove the deuce, an M925 with Michelin super singles towing a M198 howitzer and a M977 HEMTT. I drove the M925 on the march into Iraq during the first gulf war. We did not have ABS and due to problems with the brakes on the M198 we never used the trailer brakes. The M198 weighs in at about 15,500 lbs. The truck was loaded with a couple of thousand pounds of people and equipment. I never had a problem with locking the brakes up. I had a few high pucker stops but never locked them up. Some were panic stops. The march was in darkness only using PVS-7 NVGs with blackout drives and chemlights on the tubes of the howitzers. The goggles cut our depth perception and when the truck in front stopped it was hard to see. This was on a road cut in the desert a few hours earlier by the engineers.

I think a lot of the problem lies with the air brakes on the 900 series. The older air over hydraulic brakes of the deuce and the 800 series were hard to lock up and when you went from driving these vehicles to driving a 900 series there was a steep leaning curve as to brake input from the driver. I don't remember the same problem with the M977 and IIRC it also has full air brakes. I was always looking for the brakes to lock up on the M925 but I attributed the touchy feeling of the brakes to the 15,500 lbs howitzer pushing it with no brakes. I am sure the air brakes ability to lock the brakes caused a lot of my personal caution also.

What year did the ABS MWO come out?

BTW I did drive the M925 while empty and did not lock it up at all either.
 

Rattlehead

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I have a suspicion that TCC disengagement from heavy accel or braking is not done at all on the M939. I can feel the TCC engage, but have never felt it re-engage after say, braking at moderate speed and then getting back into the throttle. I just don't see how the trans would know you are braking, but total system details are elusive in these manuals.

Here is a screen shot from a MT654 repair manual, for transmission dyno testing. This confirms the TCC lockup in 2nd gear. Also notice that the TCC doesn't disengage because of a decel until vehicle speed decreases.
 

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cranetruck

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Below is a page from the TM.
Question; is there an equivalent to the throttle position sensor on the M939 series? When you let off the "gas" pedal, the transmission knows when to release the lock-up, my 8x8 has this throttle position linkage, but I don't see it for the MT654. What is the "Modulator adjustment point"?
 

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steelsoldiers

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Chuck, the effective date was November, 1999 and it was due to be completed by November, 2006 so the MWO did not exist until after your experience. The brake lock-up leading to accidents was experienced on-road the majority of the time and on wet pavement and lightly loaded condition in an even greater proportion.

Bjorn, the throttle linkage has a TV cable attached to it much like a civi vehicle. It lets the transmission know what position the throttle is in. I will peruse the manual and get pics/locations of the rest of the sensors that could be involved in controlling TC lock-up.
 

BKubu

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I drove the deuce, an M925 with Michelin super singles towing a M198 howitzer and a M977 HEMTT. I drove the M925 on the march into Iraq during the first gulf war. We did not have ABS and due to problems with the brakes on the M198 we never used the trailer brakes. The M198 weighs in at about 15,500 lbs. The truck was loaded with a couple of thousand pounds of people and equipment. I never had a problem with locking the brakes up. I had a few high pucker stops but never locked them up. Some were panic stops. The march was in darkness only using PVS-7 NVGs with blackout drives and chemlights on the tubes of the howitzers. The goggles cut our depth perception and when the truck in front stopped it was hard to see. This was on a road cut in the desert a few hours earlier by the engineers.

I think a lot of the problem lies with the air brakes on the 900 series. The older air over hydraulic brakes of the deuce and the 800 series were hard to lock up and when you went from driving these vehicles to driving a 900 series there was a steep leaning curve as to brake input from the driver. I don't remember the same problem with the M977 and IIRC it also has full air brakes. I was always looking for the brakes to lock up on the M925 but I attributed the touchy feeling of the brakes to the 15,500 lbs howitzer pushing it with no brakes. I am sure the air brakes ability to lock the brakes caused a lot of my personal caution also.

What year did the ABS MWO come out?

BTW I did drive the M925 while empty and did not lock it up at all either.
\
Thanks, Chuck, for this great post. I was reading this thread and planning to post some of these same thoughts. The brakes on this truck were designed to stop heavy loads and there is no question that they are better than what is found on the older trucks. The trucks were designed to be driven with a load and the brakes were designed to stop that load. Without a load, one needs to be careful to avoid locking up the brakes (without ABS, that is). You don't need to hit the brakes as hard as you can to get the vehicle to stop quickly and without locking the brakes (again, non-ABS truck). The problem, I believe, lies with the fact that many of the guys driving these trucks don't have experience driving vehicles with air brakes and don't really know how much pressure you have to apply to get the truck to stop. I was one of these inexperienced drivers, but two guys with hundreds of thousands of miles in over the road trucks explained how to drive a truck with air brakes. They do not really work like the brakes in your deuce, Unimog, or earlier 5 ton. Sure, the pedal is basically the same and the theory is the same, but the pressure required to stop a truck, even fully loaded, is different. I now have hundreds and hundreds of hours in both ABS equipped trucks and non-ABS trucks and I have never had a problem. Just as Chuck stated, I have had panic stops where people pulled out and stopped in front of me when I was driving at 50 mph. The truck stopped without any issues, the brakes never locked and I was able to maintain control. Basically, the ABS idiot proofs the truck. If you are aware of your surroundings, drive defensively, reduce your speed in bad weather, and avoid standing on the brake pedal, you should be fine. I currently own non-ABS and ABS M939 series trucks and I drive them the same way. To me, the ABS simply allows the driver to think less. Sure, it is a great safety item that I wish were on all of my trucks...who wouldn't?!?!
 

cranetruck

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Well Chris experienced the "stall" problem and that's what, at least I, am looking into...

The treadle valve may be an important part of this problem, here is a patent (for the sake of discussion) showing a valve which limits the amount of force applied to the brakes.
 

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emmado22

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Good luck finding the ABS MWO kit. They are kinda like the M998 parking brake MWO kit. I have only seen ONE for sale (on ebay), and I check ebay and MV shows regularly. It went for over $700. Im glad my truck had one put on by the USMC.
 

Nonotagain

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While I have never driven a M939 series truck, I was however unfortunately lucky enough to have driven some of the first ABS equipped OTR trucks. This would have dated back to the late 1970’s or early 1980’s. Back then, they were referred to as 121 brakes, for the Federal vehicle code FMVSS-121. They sucked.

The ABS system in your truck was probably designed by either Bendix or Kelsey Hayes. Both systems had problems with computers (remember the first electronic ignition systems), sensors and the modulating valves.

Back then you never knew when you hit the brakes whether they were going to lock-up or if you were going to roll thru (the computer would not allow the brakes to lock-up) whatever the obstacle was in front of you.

We had some in-town trucks that had Allison transmissions, but I never heard of complaints involving engine stalls during hard braking action.

I would contact an Allison dealer and see if you can talk with one of their older mechanics to see if the stalls were due to the design of the older transmission and if that’s the case whatever service bulletins might have been issued to correct the problem.

In Chris’s case, with just getting this truck running, it’s possible that there are some transmission gremlins that need to be worked out as well as getting the brakes adjusted and sensors cleaned up.
 

steelsoldiers

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My experience is not unique. There are hundreds of reported cases of brake lock-up and engine stalls documented in these non-ABS trucks, with a good percentage ending up in vehicle damage, injuries or fatalities. I mentioned earlier and Bruce just mentioned again that there is a much higher likelihood of that happening with the truck unloaded, on paved roads, and on wet pavement. That is why I mentioned in my original post that the bedsides and tailgate were off. It was kinda like jamming on the brakes on a bob-tail tractor. I could lock up the rears pretty easily in my 818 running unloaded and especially on wet streets.

The ABS system on these trucks was designed and built by Haldex. Haldex Selected to Supply Air Brake Systems for 32,000 United States Government Vehicles | Cision Wire
 

BKubu

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I would also take a kit if anyone locates a bunch or if people do some type of group buy. I have heard they are very costly to the govt so I bet they will be expensive if someone finds them. As I said above, I'd rather have the kit than not.

One question: would adding larger tires affect the ABS? I have heard that putting larger tires on a civilian pickup truck with ABS, for example, will impact the ability of the ABS to work.
 
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