Stupid 5-Ton Allison Transmission Question

[Suprman]

When driving the m939 5ton you can definetly feel the torque converter unlock and lock when coming to a stop and speeding up. It is smoother if the engine idle is set above 500 rpms. Sometimes when coming to a stop I get a little buck as it takes a second for the converter to disengage. I can definetly see that in a panic stop the converter would not have enough time to fully unlock. I know the abs is the military patch for this and I always leave tons of distance and drive carefully so as to never be in a situation where I would have to lock the brakes. How come the military just didnt re-design the torque converter to disengage at a slightly higher rpm?

 

[LanceRobson]

The ABS has nothing to do with the transmission. It is there since the brakes were designed to stop a fully loaded truck and are very prone to locking when a poorly trained or inexperience operator stomps on the brake pedal in a lightly loaded condition. Add that to a high center of gravity and you can see why the M939 series truck accounted for a disproportionally high percentage of serious military MV accidents. In fact, at one point they were involved in ost of the turnoover accidents in the army even though they were a quite small percentage of the fleet.

The torque converter should unlock as a function of RPM and load. If you are concerned the best thing to do is contact an Allison transmission service center and discuss it with them. You may need an adjustment in the valve body or clean ATF.

Good luck.

Lance

 

[Suprman]

It was my understanding that in a panic stop the operator would lock up the brakes which in turn would stop the drivetrain. The torque converter would not unlock fast enough and the engine would stall and loss of power steering would occur. The same as if you hard braked in a vehicle that had a manual transmission and did not release the clutch. The driver would not be able to control the vehicle and would crash. The abs allows for the drivetrain to continue in motion while the operator is braking by preventing the wheels from completely locking up which could cause a stall condition. If I am correct about the problem, then why not change the converter to one that would unlock in the hard stop situation.
Will

 

[emr]

The ABS was an add on thing, And i have never heard of an ABS being connected to anything but causing a pulsing brake application, I just cant see how the ABS has anything to do with this at all, just wondering ?

 

[Suprman]

Abs pulses the brakes preventing the wheel from becoming locked hence the name anti-lock. Braking force is applied on-off-on-off rapidly. With no abs the driver slams on the brakes the wheels lock up and it's downhill from there. With abs the driver slams on the brakes but they don't lock, the brake application is pulsed to allow some wheel spin along with pulsed brake application. The theory is the vehicle will stop faster and under better control than just fully engaging the brakes and letting the tires drag the vehicle to a stop. In the 929 trucks, in a quick panic stop, the driver would over-apply the brakes and with no abs the wheels would lock. With the wheels locked and the torque converter not yet disengaged, the drivetrain would come to a halt, stalling the engine. No engine power equals no power steering which means basically no steering so panic stop loss of control you see where this is leading. So in comes the military and the add antilock brakes as a bandaid. But if you read about the problem it would appear that the torque converter is the real culprit, if it would unlock then the engine would spin free to idle and no stall. Not that abs is a bad idea it's great to have but retrofitting the converter would seem to have made sense.

 

[Suprman]

800 series trucks did not have this issue when the operator applied the brakes he would also depress the clutch disengaging the driveline allowing the engine to spin down to idle.

 

[LanceRobson]

I should have added that fresh transmission fluid often fixes the low speed delayed unlocking. If it were my truck the oil the DOD uses for the transmission would be gone and Allison certified transmission fluid would be in it. That would also tend to fix the late unlocking of the converter.

Given that the Allison transmission generally adds $8-10K to the price of a new medium duty truck I still can't see why the Army thought that using motor oil made sense. Every unit in the force with M939 series trucks also has humdingers which need ATF. Using motor oil in place of transmission fluid and saying it is to reduce the number of fluids a unit has to have on hand was an ingenious solution to a non-existent problem....

Likely a case of a staff officer with no field maintenance experience making a silly decision which no one bothered to subject to the always dreaded common sense test. And I'll bet the clown who made that decision got a Meritorious Service Medal for it.

Lance

 

[erixun]

I should have added that fresh transmission fluid often fixes the low speed delayed unlocking. If it were my truck the oil the DOD uses for the transmission would be gone and Allison certified transmission fluid would be in it. That would also tend to fix the late unlocking of the converter.

Given that the Allison transmission generally adds $8-10K to the price of a new medium duty truck I still can't see why the Army thought that using motor oil made sense. Every unit in the force with M939 series trucks also has humdingers which need ATF. Using motor oil in place of transmission fluid and saying it is to reduce the number of fluids a unit has to have on hand was an ingenious solution to a non-existent problem....

Likely a case of a staff officer with no field maintenance experience making a silly decision which no one bothered to subject to the always dreaded common sense test. And I'll bet the clown who made that decision got a Meritorious Service Medal for it.

Lance

Just to add to the conversation, it was not the Army or some knucklehead, that decided to use 10w oil in the transmissions. This comes directly from Allison Transmissions themselves. On page 5 and 6 of this "Mechanics tips" from Allison, it shows that the use of SAE 10W or SAE 30 oil is preferable over ATF in MT 654CR transmissions used in all M939, M939A1, M939A2, and USAF R11 refuelers vehicles in "severe duty and offroad applications." It also cautions the use of other Dexron fluids in the transmissions. So the transmissions were designed to function with oil. This is why I keep my fluid spec.

Here is the link: http://www.allisontransmission.com/documents/test/MT1357EN.pdf

Hope this helps.

 

[Carlo]

When driving the m939 5ton you can definetly feel the torque converter unlock and lock when coming to a stop and speeding up. It is smoother if the engine idle is set above 500 rpms. Sometimes when coming to a stop I get a little buck as it takes a second for the converter to disengage. I can definetly see that in a panic stop the converter would not have enough time to fully unlock. I know the abs is the military patch for this and I always leave tons of distance and drive carefully so as to never be in a situation where I would have to lock the brakes. How come the military just didnt re-design the torque converter to disengage at a slightly higher rpm?

Look at this another way.. What if your car motor shut off when you locked the brakes in a curve? It does not happen. When there is to much pump presure on a allision this happens. Truck trans's are not as forgiving or driver friendly as an car's auto trans. Lowering the pump presure helps as long as your not hauling HD loads. In my case my 900 series trucks are used 90% empty with no load.

 

[Suprman]

It would seem that castrol transynd dextron 3 c-4 will meet all the requirements of Alison for general use and extreme duty I will get some and see how it drives

 

[bsteinborn]

This is probably another stupid question. I recently bought an M939A1 with no ABS. I've had a deuce for a couple of years. But the 5 ton has me spooked with this braking issue. I've had to stomp on the brakes with the deuce in the past, sometimes people in front of you do strange things. No worries though, push in the clutch and stomp on the brake. So with the 5 ton I was wondering if it is like any other automatic, can you place the truck into neutral at any speed? Or does the truck have to be at rest to shift into neutral? Can't remember. Or will this frack the transmission. I love manual vehicles. I was reading this post, walked away while thinking about it when the neutral option question popped into my head. I made my wife practice putting the car into neutral at highway speeds when all these runaway cars were in the news. Guess few people know this. Like I said this is a dumb question. Thanks.

 

[Suprman]

I dont see why you couldnt shift it into neutral at speed. It may not be a good idea to put it back into drive while moving i would think it best to come to a stop first. I think the moment you have to slam on the brakes and you start to loose control everything will go out the window and you wont be able to put it into neutral in time to make a difference. With a manual trans your body is trained to press clutch when braking it becomes automatic.

 

[Carlo]

This is probably another stupid question. I recently bought an M939A1 with no ABS. I've had a deuce for a couple of years. But the 5 ton has me spooked with this braking issue. I've had to stomp on the brakes with the deuce in the past, sometimes people in front of you do strange things. No worries though, push in the clutch and stomp on the brake. So with the 5 ton I was wondering if it is like any other automatic, can you place the truck into neutral at any speed? Or does the truck have to be at rest to shift into neutral? Can't remember. Or will this frack the transmission. I love manual vehicles. I was reading this post, walked away while thinking about it when the neutral option question popped into my head. I made my wife practice putting the car into neutral at highway speeds when all these runaway cars were in the news. Guess few people know this. Like I said this is a dumb question. Thanks.

First off its not a dumb question at all. I dont know how easy it would be finding netural if your truck has locked up the brakes and also trying to stear at the same time. I thought I knew allot about 900 series trucks but its the first time I have heard of a non ABS. I was under the idea all 900's had been modified to ABS.
In any regards putting the trans into netural at spped does not hurt BUT putting it back into drive will cause the trans to "hunt" for a speed to match the motor. It could drop down into a lower gear you would not want at the speed your going. Only for a second till the trans feels your going faster and she up shifts to the correct gear.

 

[bsteinborn]

Did some looking in the TM. If you want to shift the transfer case from hi to low and vice versa, slow the vehicle down to 22 mph, put the vehicle into neutral, shift the transfer case and then shift from neutral back into gear. Didn't see anything else. If slamming on the brakes locks the torque converter and causes the engine do die one should be able to practice dropping the vehicle into neutral before slamming the brakes. At least the engine should stay running providing power steering.

 

[Artisan]

IMO everyone whom buys these trucks and dislikes the hard shifting, usually shifting from a higher range to a lower, (not TC shifting, tranny only) with 2nd to 1st the most notable in the hard shifting, I am of the opinion you should connect a like truck w/ a tow bar and pull it up and down hills using all gears up and down hills for a good 150+ miles. I found my tranny to be a much "nicer" downshifting unit after I towed 7 M979 TYPE TRUCKS. It did not break anything but it definitely took the edge off it. Additionally I listen to "some" folks and on this list, I read a gents input on the subject and his credentials and input were noteworthy. I deduced from this gents input that if you are going to work your truck hard then stick w/ OE Oil, if you want a mall cruiser throw DEX3 in it... FWIW...

 

[Suprman]

 

[Triple C]

Personally, I wouldn't worry about shifting into neutral, if you have time to do that, it isn't an emergency stop. I don't know about your truck but there isn't a noticeable change in tactile feel between forward, neutral and reverse so in a panic shift, I would worry about sending the truck into reverse and that would just add to the problems at hand. Years ago, when I drove class 8 trucks there were always quirks to each truck one drove and you just had to get used to them - same thing here. When I first started driving my 925 it bucked and jumped on shifting but I soon found out a lot of that was me and not the truck. As I got to know the unit and began to figure out how it worked the shifting wasn't nearly as noticeable. Give it time (and practice.) If nothing else, it's a good excuse to go drive the truck

 

[originalstarhauler]

A question here. Why would you want to disengage a clutch when trying to stop? You want to use all the friction you can to stop. You down shift in a manual trans to stop a heavy load. Why do you think they put Jake Brakes on a truck, to use the engine as a breaking force. Ride a brake down a mountain and it might get to set a speed record!

 

[1 Patriot-of-many]

I dont see why you couldnt shift it into neutral at speed. It may not be a good idea to put it back into drive while moving i would think it best to come to a stop first. I think the moment you have to slam on the brakes and you start to loose control everything will go out the window and you wont be able to put it into neutral in time to make a difference. With a manual trans your body is trained to press clutch when braking it becomes automatic.

And then in a panic you overshoot neutral...OH NO!

 

[1 Patriot-of-many]

First off its not a dumb question at all. I dont know how easy it would be finding netural if your truck has locked up the brakes and also trying to stear at the same time. I thought I knew allot about 900 series trucks but its the first time I have heard of a non ABS. I was under the idea all 900's had been modified to ABS.
In any regards putting the trans into netural at spped does not hurt BUT putting it back into drive will cause the trans to "hunt" for a speed to match the motor. It could drop down into a lower gear you would not want at the speed your going. Only for a second till the trans feels your going faster and she up shifts to the correct gear.

Me too. But apparently some slipped through the cracks.