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

  1. #31
    Administrator steelsoldiers's Avatar
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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.
    Chris Stansbury, founder of Chris' CUCV Homepage in 1998 and Steel Soldiers in 1999.

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  2. #32
    Moderator cranetruck's Avatar
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    I'll keep looking as well on the assumption that the engine stalls because it's still locked up due to some delay in the release mechanism.
    Bjorn

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  3. #33
    4 Star General BKubu's Avatar
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    Quote Originally Posted by ida34 View Post
    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?!?!
    Bruce

  4. #34
    Moderator cranetruck's Avatar
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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.
    Attached Images Attached Images
    Last edited by cranetruck; 01-10-2010 at 10:51.
    Bjorn

    Avatar: XM757 in OK prepared for 1,000 mile trip home. Part of 6,000 mile journey in 2006.

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  5. #35
    4 Star General jwaller's Avatar
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    Quote Originally Posted by steelsoldiers View Post
    I don't want an aftermarket kit..
    I'd like to have a abs kit as well so if you find 2, count me in please.
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  6. #36
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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.

  7. #37
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    On a lighter note, though, at least now you know how to do a quick 180 in your 5-ton.
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  8. #38
    4 Star General Nonotagain's Avatar
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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 1970ís or early 1980ís. 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 thatís the case whatever service bulletins might have been issued to correct the problem.

    In Chrisís case, with just getting this truck running, itís possible that there are some transmission gremlins that need to be worked out as well as getting the brakes adjusted and sensors cleaned up.
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  9. #39
    Administrator steelsoldiers's Avatar
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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
    Chris Stansbury, founder of Chris' CUCV Homepage in 1998 and Steel Soldiers in 1999.

    Current:
    1969 Kaiser Jeep M756A2
    1962 Dodge M37B1

    Gone, but not forgotten:
    M1009, M35A2, M1009, M561, 404 Mog, M818, M923A2, M931A1, M1008, M925A2, M998

    HUGE M925A2 Build Thread: https://bit.ly/2uPlSnZ

    HUGE M923A2 Build Thread: https://bit.ly/2uO2g3K

    Father, lead me, 'cause I can't do this alone.

  10. #40
    4 Star General BKubu's Avatar
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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.
    Bruce

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