939 Allison Problems

[gvschafer]

Have been driving the recently aquired 925A2 quite a bit lately, and have been noticing a problem. When trans is shifted fron neutral to drive (1-5) the trans seems to "drop" in so hard that it stalls the engine of the truck. When I first got truck a few weeks ago there was no problem. Then when it first happened it was only when truck / trans was cold, right after start up, then would go away. Now, even though the truck will drive when trans is warmed up, there is still a noticable shudder and engine hesitation when shifted into drive (almost stalls). All seems to function 100% in reverse, and once truck is in drive there is no other problem. It drives and shifts fine. Only when transitioning from neutral to drive is there a problem. My question for you more experienced members is has anyone had this problem before? Any suggestion as the to cause or cure?

 

[davidkroberts]

dont know if you have done the check from the TM yet but it is below. you can download the manual from jatonka's website

37. ROUGH SHIFTING
Step 1. Check manual selector for proper operation.
If defective, replace manual selector (para. 4-73).
Step 2. Check shift cable for binding and proper adjustment.
Replace or adjust shift cable as necessary (para. 4-75).
Step 3. Check if modulator is sticking.
Replace modulator if necessary (para. 3-145).
Step 4. Check if modulator cable is kinked or out of adjustment.
Replace or adjust modulator cable as necessary (para. 3-145).
END OF TESTING!

i couldnt find anything else to help and my Allison has been trouble free since ive had it

 

[jwaller]

might be the TCC is lockiong up on you. R&R fluid with the proper allision fluid and not the milspec motor oil.

also replace the filter while you are there.

 

[R Racing]

If you do all that and still have problems . check your fuel lines . you could perhaps have a air leak and sucking some air and that can cause 2 problems. 1 a higher idle. and 2 way easy to stall the engine.just food for thought.

 

[wreckerman893]

When I was in a unit that had those trucks there was a problem with the bolts that held the transmission to the bellhousing (I know it's an automatic) backing out and causing the transmission to bind and buck. Might want to crawl up under there and give them a check.

The ones I drove always shifted roughly......the worst was downshifting but they can jump on the upshift.

When you think it is getting ready to shift back off the throttle a hair....that helps sometimes.

 

[GIJoeCzar]

Help, I've got a M923 in Columbia, S.C. with the same problem - truck runs fine, but when shifted into drive the truck stalls. So, this isn't hard shifting that I know of, because I can't even get the truck to move. Any help appreciated.

 

[Nonotagain]

Help, I've got a M923 in Columbia, S.C. with the same problem - truck runs fine, but when shifted into drive the truck stalls. So, this isn't hard shifting that I know of, because I can't even get the truck to move. Any help appreciated.

Try shifting the truck into low gear and see if the problem goes away.
A valve body that's sticking can cause the transmission to shift into high gear which will cause a stall.

 

[GIJoeCzar]

I never thought of that! Are you suggesting attempting to start the truck in the lowest gear (bypass neutral safety switch)???

 

[Nonotagain]

I'd only try that in a safe location only.

You should be able to start the truck and then shift right into low range.

Has this truck been modified with ABS brakes?

 

[Chief_919]

FWIW guys, while you are at it never use reverse in low range with these trucks, you will destroy the transfer case.

Most trucks should have a lock out now that prevents you from putting it in reverse while in lwo range, but not all units applied the MWO.

 

[sandcobra164]

Being a 925, I'd suggest PTO linkage. I know I've run into this on Guard trucks in the past. Operator will have PTO engaged, transfer in gear and for whatever reason, it kills the motor instantly and rather harshly when you shift into gear. If the linkage is close to being engaged, it may be intermittent.

 

[FASTNOVA]

I never thought of that! Are you suggesting attempting to start the truck in the lowest gear (bypass neutral safety switch)???

Try to push down on your transfer case range selector lever located next to the driver seat make sure that the button on the lever is not stuck, it being pushed in will cause the engine to die out as soon as you put it in gear.

 

[spicergear]

I've noticed that when driving and coming to a stop, it will downshift a little slower than the slowing speed you're traveling so it's almost like for a moment you're in too high a gear causing the engine to bog a little til it bumps down. Another thing you can do is just bump the hand throttle up a notch until everything warms up and it starts shifting more smoothly. Mine M931A2 will shift hard when cold then shift very politely once nice and warm. I've read all the case breaking stuff from low range and reverse and looked on mine but it didn't have the MWO applied so I'm real careful about goosing it in low range.

 

[GIJoeCzar]

M939 Transmission problem

Thanks for all of the ideas everyone; I'm certainly going to try this stuff before replacing the transmission!

Here's the truck:

http://joe.czarnik.net/pictures/v/M...s+For+Sale+M939+M35+M923+M809+M813/1983+M925/

 

[gvschafer]

Well looks like the source of my problem was the button on top of transfer case shifter. Thanks Fastnova! It would have took me forever to figure that one out! Steelsoldiers is the best mil vehicle resource out there! It was sticking in just enough to cause the random stalling.

 

[FASTNOVA]

No problem, glad you were able to figure it out.

 

[ARYankee]

I'm having the same problem of it stalling instantly when put in any gear on my M936. I can put the transfer case in neutral and put it in gear and I can tell that is loading down but it doesn't stall out the engine. It started to act up on my way back home from recovering it. I ran over some train tracks that jarred me and the wrecker fairly good. After that, when I would come to a stop it would lurch forward every so often and began to become difficult to take off. I think that I may have jarred something loose or possibly broke something. I haven't had much time to get to look at it between work and our uncooperative weather. I was hoping to come up with a few things to look at and test.

 

[Artisan]

I am not an Allison mechanic but I will tell you that after I towed 8- M939 series trucks up and down hills for a couple of days w/ my MT654CR TRANNY it really "tamed" it. I am now a firm believer that these trans are new mostly new and should be "worked in". Now it shifts GREAT. It stills shifts like a bull pulling a lil red wagon but now it does it just a tiny bit "nicer"... It Is What It Is