M939/A1/A2 ABS vs. Non-ABS Trucks - Page 2 - Steel Soldiers::Military Vehicles Supersite

cranetruck · 01-07-2010, 13:16

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.)

D-Man · 01-07-2010, 16:47

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 · 01-07-2010, 21:36

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.

duncan · 01-08-2010, 03:33

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.

steelsoldiers · 01-08-2010, 06:04

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/upl...r%20Manual.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.mi...20rollover.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.

duncan · 01-08-2010, 06:21

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 · 01-08-2010, 06:33

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 · 01-08-2010, 06:35

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 · 01-08-2010, 07:01

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 · 01-08-2010, 08:36

Quote:

Originally Posted by duncan

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.