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

  1. #21
    4 Star General emr's Avatar
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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.....
    Last edited by emr; 01-08-2010 at 09:35.

  2. #22
    4 Star General No.2Diesel's Avatar
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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

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

  3. #23
    4 Star General saddamsnightmare's Avatar
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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???!!!!

    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......

    Cheers,
    Kyle F. McGrogan

  4. #24
    2 Star General Rattlehead's Avatar
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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/s...MC/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.

  5. #25
    4 Star General emr's Avatar
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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,

  6. #26
    Moderator cranetruck's Avatar
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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.
    Bjorn

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

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  7. #27
    Moderator cranetruck's Avatar
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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?
    Bjorn

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

    1969 Ford XM757 8x8, 5-ton Pershing 1A truck tractor...the "improved MV".

    "Some things can't be made better, just differently......a lot of things actually"

  8. #28
    4 Star General ida34's Avatar
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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.
    Chuck Graham
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    Trailers, trailers, and more trailers.

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    2 Star General Rattlehead's Avatar
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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.
    Attached Images Attached Images

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  12. #30
    Moderator cranetruck's Avatar
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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"?
    Attached Images Attached Images
    Bjorn

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

    1969 Ford XM757 8x8, 5-ton Pershing 1A truck tractor...the "improved MV".

    "Some things can't be made better, just differently......a lot of things actually"

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