Allison HTB-748 Code 44

Discussion in 'Class A / Diesel Pushers' started by Bluesteelll, Sep 16, 2009.

  1. Bluesteelll

    Bluesteelll New Member

    I posted this in Talkback and got no replies. Maybe this is the sight I should have posted it in.


    My RV is equipped with a 1987 vintage Allison HTB-748 Transmission that is giving me an alarm showing error code 44. Allison shop said that code was for C Solenoid circuitry and replaced the solenoid. Not the problem since 100 miles down the road it happened again. Next advice was to clean the transmission cooler. Had it cleaned and professionally inspected. Still not the problem.

    I have found if I manually turn on the engine fan and drive the temperature down to about 175 degrees I can keep the Code 44 from happening. I have checked the wiring from the computer in the front of the RV to the rear - all is good.

    If I turn off the engine, let it rest a minute, and restart the alarm will go off until the next time.... I am at a loss of what to do and could use any help you can give me.

    Dick Hayden
  2. dbarton291

    dbarton291 Senior Member

    RE: Allison HTB-748 Code 44

    Replacing the solenoid is not the first step in this. The wiring harness needs to be checked for opens, shorts between wires and shorts to ground. The internal harness needs to be checked also. The terminals in the connectors need to be looked to make sure they are not deformed.

    The solenoid resistance should also be checked. Since it's new, it should be okay.

    Lastly it may be the ECU, but I think you've most likely got a wiring harness problem.
  3. Bluesteelll

    Bluesteelll New Member

    Re: Allison HTB-748 Code 44

    Thank you very much.

    We have done some of the things you mentioned and will continue working on it. I believe that it will be found to be a intermittant problem in the internal wiring harness. We are going to try a wire-around from the C solenoid directly to the inside of the plug. Also will check very carefully the plug, both inside and out for damage, wear and corosion.

    One other question, what is the temperature sensor just inside the plug and is it part of the inside harness? Since my problem is temperature sensitive, I can control it somewhat by keeping the coolant temp. down, I was wondering if that sensor could be faulty. I understand that the sensor is not available separately but is part of the internal wiring harness. Is that correct?

    Thanks again and I anxiously await your replay. It's nice to finally find someone who knows something about this transmission.

    Dick Hayden
  4. dbarton291

    dbarton291 Senior Member

    Re: Allison HTB-748 Code 44

    The last I knew, the temp sensor is only available with the internal harness. The temperature may not be the real problem, but possibly only masks the real problem which may be an intermittent electrical connection, particularly if this transmission has one of the molded internal harnesses not wires running inside tie wraps.

    There are diagnostic codes which should identify a faulty sensor. The sensor is the little black cylinder looking thing in the internal harness that is round on one end.

    Also look at the condition of the terminals closely. Sometimes they get damaged from probing them during troubleshooting.

    Don't forget to check the wires in the chassis harness going from the trans to the ECU also. I've seen this kind of problem stem from intermittents in either the chassis harness or the internal harness (and the associated terminals). Replacing the ECU is a last resort.

    Good luck.

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