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

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Nyearmy

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ABS was retrofitted due to wheel lock ups causing the engine to stall..ABS works have had ot on all my commercial tractors and trailers since 1997. ABS works great on trailers in snow,,trailers do not jack knife with ABS unless you are a real dumb driver.
 

74M35A2

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Hi all. New here, but wanted to answer a repetitive question I see amongst these pages. I am an automotive engineer, responsible for engine calibrations.

These engines stall upon driveline lock due to the lack of engine's ability to compensate at a super fast rate. Basically, when the wheels, axles, driveshafts, and transmission output shaft are suddenly driven hard to zero rotation, this pulls engine rpm down very quickly. So, engine rotating mass (inertia) quickly heads to the negative direction. Because the engine is mechanically controlled, it can not open the throttle quickly enough to compensate this fast decreasing rate of crankshaft rotation speed loss. The engine rpm quickly drops past target idle speed and simply continues all the way to 0rpm. The fuel pump governor commands to add fuel once below target idle speed, but it is too late and unrecoverable, plus it is trying to recover a dying engine with only idle fuel quantity, which is not nearly enough.

The torque converter locking is not the direct cause of this. This can happen in unlocked converter states as well. The act of the torque converter locking does exaggerate this even further though.

In the automotive world, we watch for this condition and we suddenly blip the throttle open to prevent the impending engine stall. This is called an anticipatory compensation, to prevent a situation we know is coming soon. Same with a throttle bump when the A/C compressor turns on, all in an effort to keep the rpm (tach) stable as a rock.

Hope this helps, at least a little anyway.
 

Suprman

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It is confusing because this is not the conclusion the military came to when adding the ABS they went with the driveline dragging the engine down with the torque converter still engaged. ABS modulated the braking pressure allowing the wheels to roll in theory at a controlled rate compared to being brake locked and skidding/sliding. I was also under the impression that it is very hard to stall a diesel engine engine unless you drag down the drivetrain. In an ideal setting the moment the rpms drop below the converters engagement point it should unlock and the engine should spin free to idle. If the converter behaved like a normal vehicle converter the engine could not be suddenly driven down like described above.
 

74M35A2

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No confusion, it all coincides. The purpose of ABS is twofold: 1. Prevent front wheel lock so steering is maintained during panic braking. 2. Keep the vehicle's rear from passing the front.

The engine stalls because of the rate at which the engine is decelerated by the transmission. If done more gradually, it is obviously fine.
 

castirondude

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Yea i think the converter doesn't release fast enough, it stalls the engine, then loses oil pressure so the engine remains stalled, then you lose power steering and have to shift to neutral to start the engine again. That's too much stuff to do when doing a panic stop... particularly for inexperienced drivers...
LA
 

castirondude

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Darn it, lost most of my text when I tried to post...
I was saying one time I came downhill in the rain in my (not military) dump truck, with the bed empty and towing a 30k trailer with a pintle hitch & dolly. Using the jake brakes coming up to a switchback I tried to brake and it took almost no brake pressure to lock the drivers. I had to turn off the jakes so I could get even braking force on all axles. Fortunately that truck has a standard so the engine would start running again on its own. The trouble with the automatic is that once the engine stalls it won't start again, you have to shift to neutral and start with the switch, but surely you will need both hands to turn the wheel so in a panic situation you will be screwed...

This trailer is a 1961 TTTI military trailer. I noticed the air brakes took like 3-4 seconds to release from fully on to fully off. Modern trailers are definitely quicker to respond. So this is another factor.
 

quickfarms

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Darn it, lost most of my text when I tried to post...
I was saying one time I came downhill in the rain in my (not military) dump truck, with the bed empty and towing a 30k trailer with a pintle hitch & dolly. Using the jake brakes coming up to a switchback I tried to brake and it took almost no brake pressure to lock the drivers. I had to turn off the jakes so I could get even braking force on all axles. Fortunately that truck has a standard so the engine would start running again on its own. The trouble with the automatic is that once the engine stalls it won't start again, you have to shift to neutral and start with the switch, but surely you will need both hands to turn the wheel so in a panic situation you will be screwed...

This trailer is a 1961 TTTI military trailer. I noticed the air brakes took like 3-4 seconds to release from fully on to fully off. Modern trailers are definitely quicker to respond. So this is another factor.
Does the trailer have its own air tanks?
 

Kawgomoo

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On the stall issue. Are only trucks with the p7100 pump experiencing this? You may be wrongly accusing the allison auto if thinking its slow to unlock.

Reason i say this, is my cummins, with a dodge auto tranny, likes to stall if you get off the throttle too suddenly and on the brakes. Its a weird event, everyone on the planet says it won't/doesnt/cant happen. but my truck does it all the time. ive learned how to work around it, and keep the truck running at all times. but the first time i stalled it in a panic stop pulling 10,000lbs behind me. surely sucked.

also those videos are loaded tests. the wet skid video you can clearly see the driver inducing a skid just before the brakes are applied. not saying abs doesn't help, but that video is loaded and obviously a sales tool. the motion he is doing would be known as a feint {Scandinavian flick} in the road racing community. its used to induce oversteer going into a corner {all wheel drift}
 

RustyM923

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On the stall issue. Are only trucks with the p7100 pump experiencing this? You may be wrongly accusing the allison auto if thinking its slow to unlock.

Reason i say this, is my cummins, with a dodge auto tranny, likes to stall if you get off the throttle too suddenly and on the brakes. Its a weird event, everyone on the planet says it won't/doesnt/cant happen. but my truck does it all the time. ive learned how to work around it, and keep the truck running at all times. but the first time i stalled it in a panic stop pulling 10,000lbs behind me. surely sucked.

also those videos are loaded tests. the wet skid video you can clearly see the driver inducing a skid just before the brakes are applied. not saying abs doesn't help, but that video is loaded and obviously a sales tool. the motion he is doing would be known as a feint {Scandinavian flick} in the road racing community. its used to induce oversteer going into a corner {all wheel drift}
MW pump here. I had the ABS light on at one time and locked the brakes inadvertently (split second lock...as much as that's possible with air brakes haha) at about 45 mph. The engine did stall abruptly. Steering was near impossible, as well. It was kind of scary actually.

(Mountain bikers swerved into my path of travel)
 

Fordnut85

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Maybe i can shed a little light on how the tc lockup function works. Imagine a tube with a piston in the middle, on the left side you have transmission modulator pressure (throttle input) and on the right you have govenor pressure (speed input). When you are into the throttle on acceleration, modulator pressure overcomes govenor pressure pushing the piston to the right and keeping the tc in an unlocked state until govenor pressure builds enough to overcome modulator pressure moving the piston back to the left locking the tc. Likewise, during deacceleration modulator pressure is low therefore keeping the piston to the left keeping the converter locked. This is not a common problem with most MT series allisons however i suspect valve body setups on these units might exagerate the symptoms. Hope this helps :) I would conclude that as stated in a previous post it is a normal event being exagerated by the lockup function and the particular valve body setup and the "best" solution would be ABS.
 
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Csm Davis

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On the stall issue. Are only trucks with the p7100 pump experiencing this? You may be wrongly accusing the allison auto if thinking its slow to unlock.

Reason i say this, is my cummins, with a dodge auto tranny, likes to stall if you get off the throttle too suddenly and on the brakes. Its a weird event, everyone on the planet says it won't/doesnt/cant happen. but my truck does it all the time. ive learned how to work around it, and keep the truck running at all times. but the first time i stalled it in a panic stop pulling 10,000lbs behind me. surely sucked.

also those videos are loaded tests. the wet skid video you can clearly see the driver inducing a skid just before the brakes are applied. not saying abs doesn't help, but that video is loaded and obviously a sales tool. the motion he is doing would be known as a feint {Scandinavian flick} in the road racing community. its used to induce oversteer going into a corner {all wheel drift}
No if you will read the whole post you will see this all happened before the 939A2 was built.
 

sandcobra164

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I do have a question. I installed 14R20's on my truck recently and now the ABS seems to be obtrusive. Did they calibrate the ABS controller differently between the trucks on 11.00's? Mine still works, light stays on 3 seconds and goes off. Truck came on 11.00's but now the ABS is locking and unlocking under what I consider moderate braking when in a hurry. I really don't get in a big hurry but am impressed that it can lock a wheel up from time to time. It never did that on the dualed 11.00's.
 

VPed

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Maybe you just need to air down a little on the rear tires. no load and TM recommended pressure = a small contact patch on my truck, and I have a long wheelbase version so I have a little more butt weight. I usually run 40-45 psi on the rears for long distance highway and 25-30 around town.
 
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