Engine codes for '84 T/A

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    Subj:   Re: Engine codes for '84 T/A
    Date:   96-01-25 08:09:17 EST
    From:   gmd at tecinfo.com (george dailey)

  > Dan, I also wish to crack the enigma of the GMC CCC serial data
  > protocol.  Here is what I know.  Listed below is the "official" pin
  > description from Chilton repair manuals.  Notice that GM does not
  > distinguish between M and E.  One source has wrote that M is used in
  > conjunction with E for 'high speed communication'.  NOT!  My '89 350
  > TPI set up did not have an E connection, just an empty slot.  This
  > leads me to believe that it is E or M and not both.  My personal tests
  > indicate that serial data will only be transmitted when B is grounded.
  > This test was done by connecting the ALDL to my PC using Procom
  > terminal software.  All I got was unreadable data .  It appeared in
  > about 3 paragraph chunks that repeated over and over.  I'd be willing
  > to bet that it's ASCII.  I tried different setting but never got
  > anything readable.  GM computers can only transmit data ( up to '92 I
  > think)

    I understand why you (and others) are having a hard time!  ;-) This is
    not a flame, but there is *ALOT* of misinformation in the previous
    paragraph.  The 89 Camaro can do BOTH slow and fast ALDL speeds.  When
    you ground the ALDL pin B, the ECM goes into slow ALDL mode.  In that
    mode it is only transmitting data at the slow rate. (Unless of course
    if you ground the pin while already in the fast mode, then it ignores
    the ALDL pin.)

    C3 ECMs can only transmit, and only in the slow mode.  The fast ALDL is
    limited to the P4 and newer ECMs.  P4 and above require much more
    knowledge to get the ALDL data.

    Basically, the E pin is slow data, and the M pin is fast data.  But
    this too is not hard and fast.  Sometimes both are connected together.
    In the above case, the fast and slow data are both on pin M.

    To enter fast ALDL mode, you have to listen to the ECM, wait for the
    correct data to be sent, and send it the correct message that it is
    expecting.  Won't spit out all that data without it!  But even this is
    not universal.  I have seen a P4 that only transmits in fast mode, and
    does not receive.

    The data is not in ASCII, but regular 8-bit (mostly) unsigned integers.
    There are some 16-bit values that come out.  The factory service manual
    explains alot of what the data is that comes out, and all you who want
    to figure this out need to RTM.

  > What's needed to crack the code is a program that reads a serial data
  > line and prints it as raw binary data.  Then, any electronic hacker
  > worth his weight in semiconductors can figure out what's going on.  I'm
  > still assuming it's ASCII though, I sure as hell don't know for sure.

    I think it is much easier to read the EPROM, figure out the code, and
    then know what is going on.


  > If one of our suave programmers can write the program, I would be
  > willing to connect it to my hardware and share the information that the
  > ECM regurgitates.  If anybody's interested, the program needs to do the
  > following in general:


   [text deleted]

  > The user then pulls up the ASCII data file in a word processor and
  > tries to assemble it .  3 endings are possible.

  > * The code is cracked and we will be immortal in the eyes of all GM car
  >   owners.
  > * The code is cracked and GM marks us for termination.
  > * We can't make heads or tails of the data and admit defeat.

     I'll bet #3.

  > PS a company called DIACOM makes software that can read the serial data
  > and print the engine diagnostic information to a IBM PC/AT via RS232
  > for $300.00 800-888-4146

    For 300 USD you will spend a lot less overall to just buy it!  How
    many hours will it take to figure it out?  A mere 500/600 hours of
    labor?  This is 50 cents an hour.  You better be doing it for the love
    of it.   Also, remember the code changes model to model, year to
    year...  There are too many variables..... Buy DIACOM and save some
    grief, unless you like to hack..... now that is another story! <Big
    Grin!>

    Scot Sealander   FIScot at aol.com