Engine codes for '84 T/A

[FIScot at aol.com]

    Subj:   Engine codes for '84 T/A
    Date:   96-01-21 13:31:47 EST
    From:   ws6transam at voyager.net (Daniel R Burk)

  > Does anyone have the hexadecimal engine codes for the GM ALDL computers
  > as used in '82 through '92?  I have an '84 that I want to begin
  > communicating with, but I do not know which addresses I need to poll to
  > receive the engine diagnostics data.  So far I have struck out in
  > finding them via the random method.  Also, the ALDL interface seems to
  > have the pins in the wrong locations than the "official" designation.

    You have a slow ALDL box.  It is 160 baud, and it only sends data, it
    does not receive.  Sometimes GM did not have a separate ALDL data pin,
    and the data comes out on the CEL line.

  > Here's what I mean:  OFFICIAL:  for the 12 pin ALDL port.

    [deleted]

  >     The connector pinout is arranged in this fashion:
  >             F E D C B A
  >             G H J K L M

  > The communications protocol seems to be to pull the diagnostics line
  > high, and send the hex command "F4" down the data line.  (Your choice
  > of baud rate.) You then release the diagnostics line and wait for your
  > data.  I base this information on old documents I found at work
  > involving automotive data acquisition.

  > Any help would be appreciated.

    Toss what you have been reading?  That is incorrect as stated.  It is
    part of the 8192 baud protocol to send something similar, but that has
    no application in your ECM.

    There is no easy way to reverse engineer this.  One way is to get the
    code out of the ECM's ROM and disassemble it, and figure out what it is
    sending.  Another is to learn all you can about what it is sending (get
    a factory manual) and use another scan tool and try to figure out what
    is what from the outside.

    Good luck, Scot Sealander   FIScot at aol.com