Accessing GM Serial Data

[Donald Whisnant]

At 06:41 PM 11/27/95 -0500, you wrote:
>
>Hi, I've just found this mailing list in the last few days and would have 
>lurked for a while, but this thread is one of the main reasons I sought 
>out a DIY_EFI type of list. 
>
>I've got an '85 Fiero, and I'm interested in accessing the serial data 
>stream, but so far I haven't tracked down any specification. Looks like 
>the knowledge exists in this forum. Is there a FAQ with this info ? 
>The Fiero ECM is common to the Celebrity etc. and is the same as the 
>subject of the Email that's quoted here.
>
>I want to use the serial data stream in a dash computer whic also gives 
>access to the engine diagnostics, so purchasing off-the-shelf isn't an 
>option.
>
>My other interesting car is a Mazda Miata, which would benefit from the 
>the addition of a knock sensor/timing retard unit, especially when the 
>turbo goes in :-)
>
>Hope this query isn't one of those RTFM or rather RTFFaq queries.
>
>
>> >>The connector in my car seems to match this description.
>
>ditto
>
>> >>One of the readers sent me some interesting information of the GM serial
>> >>data communications protocol which seems to run at 160 baud and says that it
>> >>is a continuous data stream. Another reader has explained the format of the
>> >>data, 
>
>I'd appreciate a forwarded copy of this info.
>
>best regards from a new reader,
>
>Phil Logan
>Blue '92 Mazda Miata (manual)
>Red  '85 Pontiac Fiero 2M4 Sport Coupe (auto)
>Tan  '88 Hyundai Excel (manual)
>Highland Park, NJ
>
>

Phil...

Well, we are here trying to decide for ourselves exactly what pin and format
the data is in ...  Our conclusion so far is that it must be multiplexed in
with the MIL ("Check Engine") indicator, since most of the vehicles from those
years don't have a seperate "data pin" ...  As for the format, I believe it
was Dan Burk that mentioned something about the baud and "memory dump" format
for the data --- (his address is: ws6transam at voyager.net)  ...  I believe the
format info is also contained in the SAE HS3000 document with the other OBD-II
info ...  I've recently ordered a copy and expect it to arrive tomorrow --
So I should know for sure then...

I'm doing something very similar to you -- i.e. putting an on board computer
that does constant diagnostics monitoring, with reprogramming capabilities ...
However, I've replaced the '85 engine and ECM with a '93 engine and PCM
[on a Grand Prix]...  The '93 units have a separate data pin -- sorta like
my '95 firebird has [which I would also like to monitor/reprogram] ...

If I find out anything else, I'll post it...

Good luck...
Donald
dewhisna at ix.netcom.com