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Let's Break This Down- DSM A/T Diagnostic Basics

Posted by transdude, Oct 1, 2020

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  1. transdude

    transdude Proven Member

    Joined Jan 17, 2003
    Hawthorne, New Jersey
    For a number of years I have been providing tech support to The DSM Community, mostly via phone calls to my shop. I'll assume that some of you reading this may have even had this conversation with me.

    Recently, I had posted this info on one of the ATDSM FB groups. I've also explained this countless times to my customers.

    It occurred to me that this is something that I should put here for everyone;
    Transmission diagnosis needs to be Broken Down-
    Is it electrical issue or is my transmission bad?

    Lets get a diagnostic tree and chop it in half.

    For the most part, automatic transmission malfunction diagnosis falls into two categories:

    Electrical (including TCU/management)
    Mechanical (I'm including solenoids and actuators that reside inside the transmission pan here as well)

    To see what you need to be trying to fix, you need check the signal coming from the TCU and traveling down the shift solenoid wires (orange and a yellow wires)

    The procedure is to put test lights between both wires and ground- they need to change states on each shift.

    You can drive down the road or do this with the drive wheels off the ground.

    My preference is to extend test lights from the solenoid wires and stick them right under the wiper blades so I can see while I'm driving [insert legal disclaimer]

    A properly functioning system will look like this:
    1st Gear= A (orange) and B (yellow) are both on
    2nd Gear= A on B off
    3rd Gear= both off
    (also limp mode, get it?)
    4= A on B off

    If your test lights do not illuminate in this manner, your problem is electrical in nature.
    A computer controlled transmission has absolutely no reason nor does it even have the capability to function properly if it's not getting voltage in exactly this manner.

    This does not tell you what is wrong BUT it tells you what diagnostic path to take. Are you repairing the transmission or the vehicle electronics?

    Swapping a TCU when you have a stuck valve or burnt clutch pack will do 0 to fix your car, just as rebuilding your transmission when you have a solenoid wire shorted to ground will do nothing to fix your car.

    Remember, there are a hundred wrong fixes for something but usually only one right one- the one that actually works.

    Guessing and changing parts is a fool's errand and everyone has been guilty of it, myself included (but not for a very long time).

    This photo is courtesy of The Automatic Transmission Rebuilder's Association (ATSG)

    A Part II in which I will explain how to "force" the shifts may be coming eventually. This equates to making a crude shift box for test purposes.

    Attached Files:

  2. v8s_are_slow

    v8s_are_slow Proven Member

    Joined Sep 30, 2002
    Panama City, Florida
    Thanks for posting up, and helping when I've needed it. I know I was a couple of those calls. Lol.

    2K  20

    1998 Mitsubishi Eclipse GST
    10.949 @ 129.77 · 2G DSM
  3. transdude

    transdude Proven Member

    Joined Jan 17, 2003
    Hawthorne, New Jersey
    You're welcome

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