Allison Transmission Won't Engage

Discussion in 'Class A / Diesel Pushers' started by lenhenderson, Dec 21, 2008.

  1. lenhenderson

    lenhenderson New Member

    Just drove Prevost Coach with T741 Allison Transmission 200 miles without a problem. Then stopped for a moment with engine running and put in nuetral. Now transmission won't engage --- neither drive nor reverse. Help please with suggestions.
  2. Pancanbob

    Pancanbob Senior Member

    Re: Allison Transmission Won't Engage

    Hi, I don’t know much (anything) about RV Transmission.
    Check the oil level, I assume it is an automatic.
    Next, check the linkage, something may have come lose.
    Be very careful, block the wheels, and make sure the hand brake is on!
    It may go into gear and start rolling!
    Good luck
    :laugh: :laugh:
  3. dbarton291

    dbarton291 Senior Member

    RE: Allison Transmission Won't Engage

    I assume you mean HT-741 transmission. Is the check trans light on? Any trouble codes?

    1. Make sure the engine high idle is not active.

    2. Sometimes they don't engage if the throttle sensor is misadjusted and the trans ECU thinks you're above a throttle position at which range is inhibited. If this is the case there won't be any lights or codes.

    3. If the engine is a DDEC engine, the trans ECU may be getting its throttle information directly from the engine ECM in which case, the throttle sensor will be be present.

    Need to define exactly what hardware you've got to limit the possibilities. If no lights or codes, I usually try to figure what the ECU could be thinking that would not allow drive to be engaged. Usually, there's an inhibit somewhere that's active or malfunctioning.
  4. dbarton291

    dbarton291 Senior Member

    RE: Allison Transmission Won't Engage

    correction to #2: if you have a DDEC engine, and the trans ECU is getting its throttle position info directly from the engine ECM, the trans throttle position sensor will NOT be present.

    You may also have a throttle pedal with two small throttle position sensors on it. One for the engine, and one for the trans. I have not seen many of these.

    There is no shift linkage on an HT-741. There is on an HT-740.

    Of course, the best course of action is to plug a diagnostic reader into the diagnostic connector and read what the trans ECU is thinking. I assume you don't have one of those, so start with defining what hardware you have and look for where the trans is reading the engine throttle position.

    Other possible inhibits that may be active are service brake that must be pressed before getting range, or neutral to range inhibit for a door open. I assume a Prevost does not have leveling jacks, but these are often wired into the system so you cannot get range if they are extended.

    Hope this at least gets you started.

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