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

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steelsoldiers

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I would like to address braking with a non-ABS truck. My 923A2 truck had the ABS MWO and would not lock up the brakes in a panic stop. I was driving my 925A2 on the farm road a few weeks ago and wanted to see what happens when you lock the brakes up on a non-ABS truck so I would be more prepared if it happened on the street. I tried this without the front clip, bed sides, or tailgate, so the truck was considerably lighter than it will be when I am done.

I got the truck up to around 35-40 mph and then jammed on the brakes to simulate a panic stop. It pulled the speed down very quickly, but it killed the engine. That is one of the safety concerns addressed by the ABS MWO because when the engine dies, so does the power steering and it makes it very hard to correct a skid situation. I freaked out a little when I lost steering even though I was on a farm road in a controlled situation. My first instinct was to flip the start switch to restart the engine, but the transmission has a neutral switch that requires you to be in neutral before you can start the truck. That caused me to panic a little more because I was rolling toward a soft field. Finally, I flipped the gear selector to N, restarted the truck, and steered away from the field.

All of this transpired in a matter of seconds. You can imagine the panic that would ensue if you were driving 50-60 down the highway, traffic stops, you slam on the brakes, the rear end skids out, the engine dies, you lose steering, and go careening toward the opposing lanes of traffic. Anyone that has driven a non-ABS truck with straight air brakes knows that you don't slam the brakes on when you are running empty, but all of that common sense will fly out the window if you are in a panic situation.

I plan on practicing some with mine to make sure I can maintain control during a panic stop. I am not recommending that any of the rest of you 939-series owners try this for obvious legal reasons, but please exercise caution if you own a non-ABS truck. Maintain the 40 mph speed limit suggested by the -10 especially if you are running NDT's. I am going to be keeping my eyes peeled for an ABS MWO kit to install on my truck as an added layer of safety.

Check out these videos from the ABS Mod website for an idea of what could happen:
 

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halftrack

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AWESOME info Chris,

If there were any doubt about the "braking stall" problem with the trucks, you just cleared it up.

Now, just put a 18 yr PVT behind the wheel who doesn't have a care in the world and you just made the situation worst by 10 fold!
 

Crash_AF

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In your search for ABS parts, I need a new ABS module for my 936 if you happen to come across one. The Army robbed the one off of mine before it was released.

Later,
Joe
 

NRG

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I'm not buying that wet pavement non-abs test. It looks pretty flawed. On the first wet pavement test the truck enters on the edge of the wet pavement and if you notice there is a strip of dry or considerately less wet pavement that he keeps the passenger front tire on, when he nails the brakes, of coarse its going to spin around, one side is almost or is dry and the other is soaked, they are setting it up to spin.

On the second test the truck enters on test in the middle of the wet pavement course and then hits the brakes. Both sides are in heavy water at that point and it does fine. In both tests the front tires locked up non-ABS and ABS. The only difference is they put one front tire on dry pavement on the non-ABS test to make it spin.

Also, a comment about the dry pavement tests, I have never seen a competent driver just leave the tires locked up and let it slide where ever it wants to. If it locks up then you let off and jab it again very quickly. You want to brake as hard as you can without locking the tires up. I think these guys tried make non-ABS tests look as dramatic and biased as possible. But thats just my opinion.
 
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duncan

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Also, ofcourse the ABS is going to be better when you SLAM the brakes. It's a part.. wait, used to be a part, of good drivers training to stop a vehicle as fast as possible. Currently people are taught to slam the brakes and let ABS handle it. That's not how you brake though, you apply the brakes slowly and build them up untill they almost skid, then keep them there.

ABS fine in situations where:
- ABS works
- Your trailer has ABS as well
- The surface is solid

Ive seen people lose all braking because the ABS sensor failed, telling the computer the wheel was skidding, to which to computer tries to get it out of a skid by releasing the brakes.

Then the people slamming brakes "cause we have abs" with a trailer with regular brakes. Trailer locks up, jackknife.

On the surface... a proper braking move will outperform ABS on a lot of offroad surfaces like gravel and snow.

Sorry for the ABS offtopic rant Chris, quick, post some more excellent project progress photos!
 

Oldvw2

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IMO, braking performance isn't near the problem that the engine stall that Chris mentions in his post is. With this series of trucks an engine stall can lead to loss of control real quick. This is the big advantage to the ABS MWO.
FWIW,
Chad
 

duncan

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I can see how stalling the engine is a problem, but I dont see how your engine stalls on an emergency stop? Dont you use the clutch? What am I missing here?
 

cranetruck

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Does it stall in any of the 5 gears, or just the 5th? Seems like the torque converter should provide slip and keep the engine from stalling...

Is there a transmission expert out there, who can explain why this "stall test" (image below) would or would not apply to the situation discussed above?...and can explain if there is an adjustment/tuning problem involved here or not.
(The transmission is not the one used in the m939 series, but there is probably a similar test for it.)
 

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D-Man

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It is my understanding that it stalls because of the Allison's 5th gear lockup. I have many hundreds of hours in my M925A2 on icy and snowpacked roads here in Colorado and am not sold on the ABS. I have absolute confidence in the non-ABS trucks in any weather conditions (provided the truck has the proportioning valve MWO and radials).

There is no substitute for proper training in these vehicles.
 

steelsoldiers

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I am not suggesting that the trucks have to be ABS trucks to be safe. I was merely relating some anecdotal evidence in support of ABS to prevent stall in the event of a panic stop. Training is great, but as I said all the training in the world may fly out the window if a minivan pulls out in front of you chock full of kids.

The ABS would also provide a margin of safety to a 939-series newbie who drives their truck off of the GL lot thinking it's just like their deuce.

I see I should have started a new thread just for this discussion.
 
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duncan

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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.
http://www.steelsoldiers.com/members/d-man.html
 

steelsoldiers

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Good points Duncan. There are many areas of society these days that are facilitated with computer controlled automation and when those systems fail, people stand around shrugging their shoulders or generally freaking out. Look at GPS in automobiles. I can't tell you how many people show up late at my office blaming their $#@! GPS systems for getting them lost. Duh!

Anyways, back to ABS. You can disable the 939 ABS system by yanking the fuse if you prefer to offroad without it or if the system has failed. There is also a diagnostic tool made by Haldex that plugs into the system. It will do a series of self tests, clear stored faults, and live monitoring. It can be mounted in the truck and will take 12v power from a cigarette lighter outlet. I picked one up on eBay in anticipation of having ABS on my current truck some day. Here's the manual: https://www.absmodtrack.com/m939/uploads/Haldex M939 Infocenter Manual.pdf

Here's a good summary of the accident stats from the GAO and a list of the MWO's that were performed to make the trucks safer in young, inexperience hands. http://www.transchool.eustis.army.mil/ts_safety/PUBS_Files/SafetyPubs/gao 939 rollover.pdf One of the interesting points I saw was that the trucks were originally intended for 80% off-road and 20% on-road duty, but the numbers ended up reversed because of the national guard and other units that have to travel a great deal on the highways.
 
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duncan

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That accident stats report is a very interesting read, thanks. The drop in accidents is quite amazing, especially if you see that all they did that first year was brake fixups and ABS.

So on your truck you're going to mount a complete aftermarket ABS system then? How does that work? Attach speed sensors to the wheels, and have electronic regulators in the brake air lines or something I assume.
How will it deal with, say, a single brake line for the rear 2 axles that only splits at the last moment?
 

cranetruck

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I'd like to see a response to the "stall test" posted above. My 8x8 is also an automatic, but never heard of an engine stall problem. Is this a transmission problem that is indirectly fixed with ABS? It'll be a while before I try a brake test like this on the hilly and windy roads around here.
Next time you try this Chris, use a different gear selector setting.
 

duncan

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It would fix the stall yes, because the wheels wont be locked by the brakes with ABS. They keep rolling thus allowing the engine to turn as well.
 

cranetruck

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Unless the transmission is mechanically locked, the torque converter should provide the slip...but this is where an expert comment would be desirable. The stall test described above demonstrates this "slip", I would think.
 

steelsoldiers

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That accident stats report is a very interesting read, thanks. The drop in accidents is quite amazing, especially if you see that all they did that first year was brake fixups and ABS.

So on your truck you're going to mount a complete aftermarket ABS system then? How does that work? Attach speed sensors to the wheels, and have electronic regulators in the brake air lines or something I assume.
How will it deal with, say, a single brake line for the rear 2 axles that only splits at the last moment?
I don't want an aftermarket kit. I will be looking for the complete ABS MWO kit. It has a sensor on the left rear hub to detect lock-up. The rear-rear and the front-rear axles have separate relay valves that receive the signal from the treadle valve. There are electric solenoids controlled by the ABS ECU that modulate the signal to the relay valves I believe. I will re-read the ABS TM and let you know for sure.

Adding this kit to my truck is not priority #1, but if I find one I will snatch it up for later application.

Bjorn, I will try locking the tranny in 4th gear to see what happens with the panic stop.
 
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