allison won't downshift

Discussion in 'Class A / Diesel Pushers' started by waynekir, Apr 19, 2009.

  1. waynekir

    waynekir New Member

    I have a allison 6 spd in a 1993 southwind with a cummins 230hp. The trans starded downshifting hard and would not downshift at full throttle. I replaced the throttle position sensor and had the ECU reset. I worked great for one trip. Now its doing the same thing again and it even locked me out of getting it in gear (after resetting the key several times in finally went into gear). Arrow do not show any trouble code. I think it may be some type of communication problem. Anyone had this happen ? thanks
     
  2. dbarton291

    dbarton291 Senior Member

    RE: allison won't downshift

    sounds like the key to this one is to try to find out what the ECU is thinking. I would take it to a service outlet and connect Allison DOC. Find out if the ECU is seeing the 100% throttle signal when you step on the throttle.

    Then check to see if the ECU is commanding the downshift.

    You may have an intermittent in the wiring from the TPS to the ECU, or a TPS mounting arrangement that doesn't pull the TPS properly. That would prevent the ECU from getting the proper throttle signal.

    If the ECU is getting a proper TPS signal and is commanding the downshift, but the trans is not going, that's another set of potential problems.
     
  3. waynekir

    waynekir New Member

    RE: allison won't downshift

    Thanks. When Allison installed the new TPS, they had a problem with the ECU reading the correct values (the reason they thought the TPS was bad). After taking the ECU out a reinstalling it, they were able to get the correct TPS values. I have a feeling that there is a problem in the harness/connector or the ECU itself. Interesting, all the time the ECU never had an error code. I am going to take the ECU out and see if there is anything obvious. If that dosent work, sounds like I will have to take it back in to Allison.
     
  4. dbarton291

    dbarton291 Senior Member

    RE: allison won't downshift

    To set a code, the TPS values have to be outside a set of parameters that clearly indicate a failure. If an intermittent is present that never reaches a voltage value on either side of its voltage extremes, there will not be a code.
     
  5. waynekir

    waynekir New Member

    RE: allison won't downshift

    I think that's the problem, I pulled out the ECU, unplugged it and cleaned the connector and reinstalled and now it will down shift. For some reason the way the wiring harness is mounted it's too short and it is almost impossible to get it out of the mouting box (very poor design, I was temped to cut an access hole to be able to reach the back of the ECU). I think this may have caused excessive pressure on the connectors and has damaged one of the pins causing the sometimes faulty connection. I am not sure how to trouble shoot this. I would think I could use on ohm meter and check each pin, but it may not show a bad reading unless its at a certain angle or has pressure on it. Its not too comforting to use the motorhome knowning at any time it could lock me out of gear. I guess I am just getting old, but the electronic eqipment is great when it works and almost worthless when it fails. Our boat has two mechanical 6BT motors that will always run as long as the motor gets fuel. I have seen seveal of our friends with newer electronic motors that will run great one day and outright stop another because of a faulty electronic componet. Again thanks for your help, sounds like you really know the Allison's well.

    Wayne
     
  6. dbarton291

    dbarton291 Senior Member

    Re: allison won't downshift

    I hear you 100% about the electronic stuff. It sure is different than the mechanical days. There's days when I really miss points in a distributor and carburetors and such.
     

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