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

[steelsoldiers]

So, I had some more seat time in my M925A2 w/o ABS recently. I drove it for around 10 miles on dry and wet pavement. I was way out in the boonies and there was nothing around except farmland so it seemed like a good time to check the brakes. I was cruising around 50-55 mph and executed some hard stops, not panic stops, but hard stops. The first time was in a straight stretch on dry pavement. I applied the brakes firmly and could feel the back end start to get light and then the rears locked up. All 4 rears were locked as far as I could tell and the truck skidded forward in a straight line. I did not experience any fish-tailing and the engine did not quit running. The second hard stop was on wet asphalt so it was a little easier to break the rears loose. The truck skidded to a stop in a straight line. The engine did not die and there was no loss of steering control.

These are obviously not controlled, scientific experiments, but I thought a little more anecdotal evidence for or against the ABS would be nice. I will post up more experiences as they happen. I am feeling more confident in my truck now after my most recent test drive. It definitely handles a little different than my 923A2 w/ ABS, but not necessarily in a bad way. I like it!

 

[wbdodgeiv]

If the engine stalls when the wheels lock up because the transmission is locked in, why doesn't it roll start like a manual would when the brakes are released?

 

[IHCBigJohn]

I have to say that the wet slide 180 he pulled looked a little too perfect. Perhaps the driver has practiced a little Ace Ventura "like a glove" parking style.

 

[OPCOM]

Thanks for moving this discussion into a new thread Chris.

So the engine stalls because you break so hard the transmission doesnt get out of lock up. That sounds like a big problem to me, especially with losing power steering, traction and possibly break boost. Did not expect these big rigs to be automatics.

Even with all the training in the world, theres no excuse for not using the available options for safety. But just like your training might fail with that minivan with kids pulls in front of you, your abs (or whichever safety device) might fail at any other point, and you're back to training. Training is essential, the rest is adviseable. The problem arises when automatic assistance is mistaken for automatic safety, but there are plenty of topics about human ignorance on this forum already.

To bring it back to the trucking topic, I would install ABS if I had the option. But I'd make sure a failing ABS system can never interfere with braking. And have a way of turning it off easily.

Possibly a latching relay could be used to cut the juice to the lockup solenoid and activate a lamp when ABS activates. A simple momentary normally-closed push-button on the dash would restore the function by interrupting the juice to the relay. What I have just described is extremely simple and would work on something for example a 1986 Chevrolet Caprice that has an actual wire coming from the lockup solenoid (valve) to the computer, but the Allison transmission may be too mental and fussy for it.

 

[OPCOM]

If the engine stalls when the wheels lock up because the transmission is locked in, why doesn't it roll start like a manual would when the brakes are released?

Because once the engine has stalled out below a certain RPM the transmission no longer has enough speed from the engine to keep itself engaged and if it is computer operated may additionally disengage itself. - -just speculation.

 

[Vintage iron]

I hate when people compare stuff and the comparison is not very scientific. The videos are taken from different angles in both tests! taking a video from the side is the best way to judge speed. Why would you stand in front of a 5 ton going 45MPH unless you knew what it would do. Doesn't sound to smart or scientific.

 

[Karl kostman]

Thanks guys for starting this thread I just won a 925A2 with ABS (yes there will be pics as soon as my EUC clears and I recover it) and never understood what the issues were that caused the military to convert so many over to ABS, I know I have a lot to learn but you certainly cleared up a small part of this process for me! Thanks very much
KK

 

[emmado22]

The biggest unofficial issue is that most 18 year old kids don't know how to drive with air brakes...

 

[Bighurt]

I havn't got the ABS to activate yet, when I first heard about ABS on air brake systems (CDL test book) it made me very nervous. I don't even like ABS on standard trucks. But I agree I think its more the 18 year old troops that in all liklyhood hadn't driven before their enlistment, and issues with not only driving but operating the tranny as well. Thus ABS, automatic trucks.

Well that's my theory, I've said since the day I joined the service I can drive anything with wheels, but then again I'm a different nut.

 

[quickfarms]

What year did they start adding ABS to airbrakes?

 

[Bighurt]

what year did they start adding abs to airbrakes?

1981

 

[spicergear]

I think there are some things that are being overlooked. Take for example the REB kit that was available for the GMC 6x6 auto trans that converted it to full manual valve body. They made that transmission awesome but very, very positive to the point that you could stall it if you didn’t downshift for a stop.

On my M931A2 I will set the throttle faster while it’s cold because for the reason that it will drag then engine down to a point of near stalling as the shifting lags behind. Keep in mind that these trucks mostly had 15/40 engine oil in the Allisons which makes them act very differently. My truck after it warms up has much, MUCH better manners than when it does cold and will also come to a stop and take care of downshifting with not a hint of acting like it’s going to stall. The Allison is a very good transmission and I would be willing to bet that the guys that have already changed over to ATF from 15/40 won’t be telling us about their trucks stalling when stopping from the engine bogging down.

It's like thinking of a manual transmission... ...is it easier to stall your truck in 1st gear or 5th gear. 5th, right- so if the cold thick dumb oil is allowing the shifting to lag behind it stands a chance of stalling the engine as it drops to idle rpm and the tranny is still a few gears up.

 

[emmado22]

I hope someone downloaded all the stuff from https://www.absmodtrack.com/m939/upl...r%20Manual.pdf because now it is gone.... Right when we can use it the most.....

 

[emmado22]

I hope someone downloaded all the stuff from https://www.absmodtrack.com/m939/upl...r Manual.pdf because now it is gone.... Right when we can use it the most.....

 

[emmado22]

https://premiernorth.com/m939/uploads/AMG ABS MWO Field Service, 14 Feb 03.pdf

 

[castirondude]

15/40 in an allison?? First time I have ever heard of that. I was very pleased how mine shifts, pretty comfy just like transit buses etc

 

[rmgill]

I would posit that a transfer case transmission design that will crack the housing when you shift into an easily set control configuration is a design flaw.

You should not, in general automotive terms be able to destroy something by just setting two controls. There should be an interlock or some other mechanisme to prevent that OR be designed to not have this failure mode to begin with. That's bad engineering in my opinion. That would be like having an oil drain button next to the starter switch.

 

[castirondude]

Yea. On these trucks there is a label that you cannot reverse in low range. Wtf...

 

[ichudov]

OK, suppose that I have a truck without ABS, and I need to perform a "panic stop", which is of course something to be avoided.

Would it be true to say that if, when I start stopping, I flip my transmission into Neutral, then I can panic stop and the engine will never lock up, so I do not lose steering?

Will Neutral prevent engine stalling?

Assuming that the answer is affirmative, would I need to bring the truck to complete stop before re-engaging Drive from Neutral?

 

[castirondude]

I think the transmission locking is a defect. A panic stop will not cause the transmission to lock. As I understand the problem, it's the other way around - if the transmission malfunctions and locks, then when you try to slow down to go around the corner, the engine will reach idle speed and not want to slow down any further (diesels have an idle governor and will throttle up somewhat to prevent stalling).
If you are in 4x6 mode the rear tires keep pushing against the brakes but the front tires will get the full braking force. The front tires will lock and then you cannot steer anymore.
I think neutral should work, you can just slam the gear selector forward with the palm of your hand and it will lock in neutral. You could also shut off the engine but then you lose power steering.