Allison mt 643

Discussion in 'General RVing' started by gon2sea, Jul 28, 2014.

  1. gon2sea

    gon2sea Junior Member

    i am converting from a mt542 to a mt643, i have a mt 643 but need help regarding controls, specifically the modulator, its location and its purpose, any help will be appreciated.
  2. dbarton291

    dbarton291 Senior Member

    The modulator opening is on the right side of the transmission. Left and right are defined as if you were sitting on the transmission with the torque converter facing forward. You will see an opening with a small pin sitting there. There's a clamp and bolt that holds the modulator in place and the modulator has an O-ring on the part of it that fits into the transmission case. The modulator connects to the fuel lever on the engine and tells the MT643 valve body what your throttle position is.

    Modulators for the MT are made by Morse and Felsted. Make sure you connect the modulator cable to the engine throttle with a yield link which will allow the engine to return to idle if the modulator cable sticks. You also have to make sure you buy a PUSH or PULL modulator depending on what direction your fuel lever goes when you step on the throttle. The PUSH or PULL designation defines whether the modulator cable is pushed or pulled as the engine is moved toward full throttle.

    I hope this helps.

    Oh, also make sure you get a quality selector and adjust it properly. An improperly adjusted selector can contribute to partially applied fourth and/or forward clutches causing premature failure. Good luck.
  3. gon2sea

    gon2sea Junior Member

    the transmission i purchased was removed by a the wires were cut right at the sensors, the modulator was chopped off at the vacuum fitting, the oil hoses were simply cut as well... thanks for the info, after several hours of research last night i found the modulator location and what was left of it on the transmission, i can run the transmission either way, with a vacuum modulator or cable operated, the price of parts will end up being about the same, just shy of $200, the vacuum set up is more complicated / harder to install so i am curious as to what system works best... Vacuum or cable.
    the transmission is the "rare" mt 643R with retarder....low miles and only $200.. its all going into a 1977 AVCO motorhome that is also being converted to 4WD.
  4. dbarton291

    dbarton291 Senior Member

    You can only run the MT643 with a vacuum modulator if you have a gasoline engine. If you have a gasoline engine, the vacuum setup is very easy. One manifold vacuum hose down to the modulator. If you have a diesel engine, forget using vacuum. You must use mechanical. The cable operated modulator, when adjusted correctly, works very well and is a better indicator to the valve body of the operator's throttle position. I much prefer the mechanical setup.

    Are you sure it's an MT643R? There were very few of those made. It's much more common to have an MTB643 which has the output retarder on the back of the transmission. An MTB will have an accumulator cover on the back of the transmission that is stamped "Cover Under Load". If you have the MTB, did you get the retarder accumulator with the transmission?

    The MT643R has an input retarder and will require either a linkage to operate the retarder or something to prevent it from coming on if I remember correctly.

    The MTB643 can be plumbed to either use or not use the retarder, but the output retarder on the MTB is a great brake saving feature and great for controlling speed going downhill.
  5. gon2sea

    gon2sea Junior Member

    Hi, you are right, my mistake, it is a MTB643. since i could not find a converter i purchased a standard mt643 with converter and adapter that was mounted behind a cummins 5.9 . i assumed the torque converter would be the same between the mtb and the standard 643... the pump cover and the seal is different between both transmissions.... any info or knowledge about that...?
    also, you mentioned a retarder accumulator... what is the purpose, the only connection is see are two very large oil line and a 1/4" air supply.
    if you would rather email me at home i can send pictures a swell
  6. dbarton291

    dbarton291 Senior Member

    Ah, my mistake. The area on the back of the MTB643 that says "cover under load" is the retarder accumulator pistons. I was thinking world transmission six speed. Those use a separate accumulator.
    There were a few different torque converter pump hubs, bearing and bushing combinations over the years. It's not an MT vs MTB thing. It's the generation of parts.

Share This Page