M923 hard downshift

Just curios if there is a way to make the downshift on the allison trans a little less harsh when it goes from second to first. Mine also seems to shift up through the gears so fast that it makes the engine bog down a bit. Thanks

Some people have put Dexron III in the transmission to help. I did that and I think it may have helped a little but didn't solve the problem. I have 3 M939A2 trucks and 2 of them seem to shift just fine from 2nd to 1st and one of them is pretty hard even after using the Dexron III. My next step to invstigate the modulator valve but I have no idea if that will help. I made some adjustments on the modulator but it didn't seem to change anything so I question if it is defective. Since 2 out of 3 seem to shift smooth, I gotta believe I can get the third to shift OK.

Don

Its the nature of the beast. Thats why the trucks have ABS now if you slam the brakes the drivetrain can drag the engine down. Part of it is the torque converter takes time to unlock. There are a few posts about this.

If you simply search your thread's title, you will get plenty of reading on this topic.

Here's a sample of what is out there. http://www.steelsoldiers.com/showthread.php?97945-M923A1-Allison-Hard-downshift-into-1st

I went through this exact issue with my 925A2 this past weekend. WildChild467 and I went to the local mud bogs, and after, I had an extremely harsh 2-1 downshift that was not there earlier. The trans shifted gravy smooth before through all 5 up and down. Occurred at about 7mph on the downswing to a stop. Thought it was the torque converter not wanting to unlock, but it would do it in neutral also while rolling up to a stop.

Check your modulator cable at the injection pump linkage. Make sure it is free to move the entire length of the oval slot in the throttle lever. For my issue, I found that the modulator cable was not self returning to its home position 100%, as I could manually push it back further. I pulled the seal back on the cable, and sprayed WD-40 down the cable housing as best as I could, while pulling the cable core out and letting it snap back to seated position about 20 times.

Took the truck for a ride, and the harsh 2-1 downshift was gone. I'm not so sure it is "nature of the beast" as others say (I buy stuff from Suprman, so no offense there), at least for me it was more of a factor of the modulator cable being 24 years old. It should be spring loaded on the trans end under the cover where the cable terminates. This cover has tamper-proof 3 slotted screws, but start with lubing the cable and checking its travel on the return stroke. Also if it is shifting early and bogging the engine down, move the adjustment to max position at the injection pump end. Mine was set that way and the shift points seem great. I can force a downshift by mashing the pedal, etc.....

If you modulator cable is adjusted best it can be, the spring return is working on it, and it is lubed and can return to its fully seated position, then yes it may be nature of your beast.

Thats a good point I didnt even consider that my modulator might be bad. I checked out the TM and I believe it said that when the accelerator is held to the floor that the shifter linkage should also be at the end of its travel, and I know mine still has room to move after wide open throttle so I might try to adjust that as well to see if it helps any

74M35A2 said:

I went through this exact issue with my 925A2 this past weekend. WildChild467 and I went to the local mud bogs, and after, I had an extremely harsh 2-1 downshift that was not there earlier. The trans shifted gravy smooth before through all 5 up and down. Occurred at about 7mph on the downswing to a stop. Thought it was the torque converter not wanting to unlock, but it would do it in neutral also while rolling up to a stop.

Check your modulator cable at the injection pump linkage. Make sure it is free to move the entire length of the oval slot in the throttle lever. For my issue, I found that the modulator cable was not self returning to its home position 100%, as I could manually push it back further. I pulled the seal back on the cable, and sprayed WD-40 down the cable housing as best as I could, while pulling the cable core out and letting it snap back to seated position about 20 times.

Took the truck for a ride, and the harsh 2-1 downshift was gone. I'm not so sure it is "nature of the beast" as others say (I buy stuff from Suprman, so no offense there), at least for me it was more of a factor of the modulator cable being 24 years old. It should be spring loaded on the trans end under the cover where the cable terminates. This cover has tamper-proof 3 slotted screws, but start with lubing the cable and checking its travel on the return stroke. Also if it is shifting early and bogging the engine down, move the adjustment to max position at the injection pump end. Mine was set that way and the shift points seem great. I can force a downshift by mashing the pedal, etc.....

If you modulator cable is adjusted best it can be, the spring return is working on it, and it is lubed and can return to its fully seated position, then yes it may be nature of your beast.

Click to expand...

My M925 has had the same hard downshift since purchase, this is good info and I'll check the condition of the modulator feedback cable.
Thanks
JC

Thank you very much 74M35A2 I appreciate you taking the time to explain, and will check that on my truck

Not everybody likes me here, but I do buy from many, and try to offer help when possible to anybody on a subject I had experience with.

I have been meaning to post the results of my recent Allison MT654CR 2-1 downshift harshness resolve since I saw so many had similar complaints, so your timing was a match to do so.

My has a hard 2-1 shift as well. I am going to check the return on it.
Thanks for explaining the possible fix.
Pete

Do yourself a HUGE favor.

Get a medium towbar and tow another 5ton for about 100 miles. Up and down
and flat and forward and even some reverse.

Thise trannys are made to HAUL HEAVY HEAVY LOADS, they are not cushie
civilian nice trannys! THEY are MONSTERS and trying to tame them is real hard
BUTSOMEHOWEVER. That "edge" is now gone from my tranny after I towed 7
other like trucks for about 140 or so miles. These are TACHTICAL MILITARY TRUCKS
not 1/2 ton chevys...

Been there, done that.

Artisan said:

Do yourself a HUGE favor.

Get a medium towbar and tow another 5ton for about 100 miles. Up and down
and flat and forward and even some reverse.

Thise trannys are made to HAUL HEAVY HEAVY LOADS, they are not cushie
civilian nice trannys! THEY are MONSTERS and trying to tame them is real hard
BUTSOMEHOWEVER. That "edge" is now gone from my tranny after I towed 7
other like trucks for about 140 or so miles. These are TACHTICAL MILITARY TRUCKS
not 1/2 ton chevys...

Been there, done that.

Click to expand...

That hard 2 to 1 downshift is not as hard on the human driving the vehicle as it is for the universal joints and gearing in the differentials. It is also tough on the transfer case. When sizing a gear reducer for an industrial application, one must use a much larger service factor-or gear box when there is shock loading at a given horsepower. Bottom line-shock loading mechanical stuff will reduce its life.
Pete

Hoefler said:

That hard 2 to 1 downshift is not as hard on the human driving the vehicle as it is for the universal joints and gearing in the differentials. It is also tough on the transfer case. When sizing a gear reducer for an industrial application, one must use a much larger service factor-or gear box when there is shock loading at a given horsepower. Bottom line-shock loading mechanical stuff will reduce its life.
Pete

Click to expand...

Now add 10 Ton and tell me all about it...with all due respect indeed sir.

I adjusted the modulator cable and the shifts improved alot

Will have to do this once the truck is up and running!