M923A1 Allison Hard downshift into 1st

[Artisan]

Mine downshifts hard as well. I am somewhat relieved that "they all do it" after
reading this thread, well, seems like most. So perhaps a filter change is in
my future and a change to DEX 3 at the same time.

I was most concerned for use on-road in the snow, seems like it would be an
invitation to loose traction coming to a stop...

 

[Hoefler]

I suspect its a modulator issue that makes these shift hard. I have limited my driving of mine due to hard shift. I did have it out this past weekend and found it to be relatively smooth when transmission is very cold. As she warms, she gets harsh. I plan on bring mine across town to an Allison shop versed in these mil transmissions and get mine resolved. These trucks are to fun to drive to just let her sit. I will report if and when I get mine serviced.
Pete

 

[Carlo]

My 931 does not make this hard shift either up or down in the 1st to 2nd or 2nd to 1st. BUT my 923a2 did. I noticed it had 10w when I bought it. I got it home and changed the fulid to dex3 and it changed it allot. I also removed the modulator and cleaned it and thatr helped more.

 

[Artisan]

Carlo is that Modulator something that requires a
rocket scientist to breakdown, clean and rebuild or is it
pretty straight forward and could a guy w/ average
mechanical skills be able to tackle it w/o issue?

 

[Scarecrow1]

Mine isn't bad compared to some other trucks I have driven ,so I live by the law of if it ain't broke don't try to fix it

 

[doolest]

Our M923 also shirts hard both up and down...talked to a gut that had drove them in the army about 10 yearsa back...he said he always remembered them shifting hard...he has a buddy that bought one a few years back that had the same issue...changed fluid to the Dex3 and said it made a difference...even more after the 2nd change...we haven't changed ours yet...maybe this spring when we take on some additional maintenance issues

 

[M35A2-AZ]

I have a M923A1 and a M931A1 and they both down shaft hard. I also have a M936 that came with what looks like Dex3 and it down shafts much better.

 

[smoke]

The reason switching to dexon 3 from motor oil helps is dexon has a friction modafier in it. That allows the clutches or bands to slip a little before they engage. In common words it cushion engagement some and that's fine when you are unload but when you are fully load(like military use the truck for) you don't want the trans to slip alot when shifting because it creates unnecessary heat and wears the clutches and bands faster.Plus the load itself put drag on the clutch and bands that cause it to slip some. All the buses I work on that have allison b400 and b500 we put 15w-40 motor oil in them. Only the zf trans and the allison hybiods units get a special synaethic fluid in them. Some of b400 and b500 go the whole 5 years between rehabs with no problems or rebuilt depend on the rebuilder in the shop. The buses run around the clock for public transit. Think of the motor oil as the b&m trick shift fluid you put in your hotrod's trans to make it chirp second gear.Just my 2 cents.

 

[Artisan]

WOW, that sure is something to chew on smoke...


After reading this it almost seems logical that if you are running heavy, you should stick w/ oil,
if you running light, DEX 3 might be better kinda deal, ehhh????

 

[Ozzie]

I'm fixin to try the Dexron III fluid myself. Modulator cable seems good.

 

[71DeuceAK]

Definitely curious to hear results from anybody's full Dextron flush (I've read you have to completely flush it, not just drain and refill when changing fluid types). I've ridden shotgun in a 923 that definitely had the hard downshift in that pulling-up-to-a-stop-sign kind of deceleration, very noticeable CLUNK. I'd consider doing it in mine I'm maybe getting if it actually worked out well, if I did I'd do a full write-up on it.

 

[74M35A2]

For anybody that had a hard downshift, have you throughly lubricated your modulator cable? Also check its adjustment. My trans was smooth, then started a harsh 2-1 downshift after off-roading. I lubed the cable well, and it returned to butter smooth.

Should not be harsh shifting, up or down. It is bad for the entire drivetrain. They are not designed to be this way.

 

[lindsey97]

I switched to oreilly's premium transhydraulic oil on all 3 of my keeper trucks, and they instantly started shifting smoother. It is 10 weight, and C4 rated. I unhooked my cooler hoses, removed the trans pan, and blew compressed air thru the cooler lines. Close enough to a full flush for me.

You can run any oil in the mt654cr as long as it is Allison C4 rated. You can even use TES-295 full synthetic if your wallet can stand it.

15-40 wt. engine oil is to be used off road, in 120 degree F operating conditions, loaded at slow speeds. Think desert conditions with no hard paved roads, Maximum rolling resistance. The reason allison lists the 15-40wt. for this application is that engine oil has the highest boiling point of all the approved fluids listed.

For those of you who insist on running 15-40wt. engine oil in cold climates, let your truck warm up for 30 minutes, or install a trans pan electric heater. If you fail to let the transmission warm up, you will be rewarded with a shorter transmission life.

I do have one truck currently that is giving shifting issues, it needs the modulator cable removed and cleaned. A small adjustment on the injection pump end of the modulator cable makes a big difference in shift quality. Make sure you have 2 springs on the pump end of the cable, and that the linkage is sliding/returning smoothly throughout the throttle travel.

 

[71DeuceAK]

Hey Lindsey97, would changing out the 10wt? that the military uses be something you'd do immediately then if winterizing a M923 for winters in Fairbanks, AK?

 

[lindsey97]

http://manuals.chudov.com/M939-Seri...654CR-Transmission/MTB-600-Mechanics-Tips.pdf

Read this Allison manual, specifically 1-6 pages 5 and 6.

 

[71DeuceAK]

Thanks, bookmarked!

 

[Willyk]

My tranny has 2013 stenciled on it. ( bought my truck in 2015) completely rebuilt by RRD. Never put a mile on it after rebuild. It sat for 2 years till I bought it. It too shifts hard.