Allison MT643 4 Speed Transmission

[Charles B]

I am new to the forum & this is my first post.
Have a 2000 Winnebago Journey with a Cummins ISB 275 with an Allison MT643 4 Speed Transmisson.
When traveling in a 45 MPH speed zone with D selected the transmission will shift back & forth between 3&4 (very hard shifts).
My solution has been to put the selector to 3 & hold it in 3rd gear.
Any comments forum members might have about this transmisson would be appreciated.

[Charles B]

I am new to the forum & this is my first post.
Have a 2000 Winnebago Journey with a Cummins ISB 275 with an Allison MT643 4 Speed Transmisson.
When traveling in a 45 MPH speed zone with D selected the transmission will shift back & forth between 3&4 (very hard shifts).
My solution has been to put the selector to 3 & hold it in 3rd gear.
Any comments forum members might have about this transmisson would be appreciated.

[Pusherman]

Charles, first welcome to irv2.com. You will find many knowledgable people here to help you with your questions.

My previous coach was a '00 Journey with the 275 Cummins /MT-643 4-speed Allison transmision. When I first picked up my coach it was experiencing hard-shifts out of sequence. Took it to a FL shop where it was diagnosed to be a bad Engine Control Module (ECM). Apparently it was the ECM telling the coach when to shift. Once repaired the transmission would never 'hunt' between gears. Shifts were pretty smooth, and would only downshift when it was needed. At 45mph, it should stay in 4th securely, unless on a grade.

I would first start by calling 1-800-FTL-HELP and describing the conditions to them. They may then direct you to either a FL, Cummins, or Allison shop who can repair the problem. My guess is it will either be a FL or Cummins shop who can fix it. But it might be worth talking to Allison as well. If I remember correctly, the ECM is $200-300. In my case, it was covered under warranty.

One other thing to keep in mind is that the MT-643 tranny should have its fluid and filters changed ANNUALLY for this transmission. I spoke to an Allison rep, and he told me to stick with the Dexron instead of TranSynd, as you will still have to change the fluid annually, and the TranSynd is about 3X more expensive.

Might be worth first having your transmission serviced by Allison before you chase the ECM route. If you don't know when it was last serviced, have this done first.

P.S. This is actually a pretty good transmission for medium-duty. Just needs annual PM service.

I used to get 9-11 mpg with this coach towing a Saturn.

Good luck and let us know what happens.