Serial data protocol on production autos

[m_mcdonald at marx.ENET.dec.com]

>  I am trying to get information on the ODB-II communication protocol used
>  in new production autos.

"New production" includes a few 1995 model year (MY) and all 1996 MY
automobiles and light trucks.

> I would like to write code to monitor/modify vehicle functions.
> There are tools available which do this, however, I am
> an engineer and thought it would be interesting to do it on my own.

Welcome to the club.  I don't feel like paying Snap-on Tools or other
companies $1,000 and up for their products.  I'm trying to monitor
(but not modify) my 1993 Ford Escort's non-OBD-II data stream using
a connection to a laptop PC.
 
> There are some ISO standards on "low-speed serial data communication"
> used in vehicles, but they are very expensive, and I am not sure if
> it's the information I need.

I believe the standards are largely those of the Society of Automotive
Engineers (SAE).  The Environmental Protection Agency (EPA) has decreed
to all manufacturers that they will follow these SAE standards as they
open their OBD-II emissions-related data streams to scan tool
manufacturers (Snap-on Tools, etc.) and to "any person engaged in the
repairing or servicing of motor vehicles or motor vehicle engines."
It turns out that "emissions-related data" includes most, if not
all, data related to engine performance.  It does not include the
data stream that Mercedes computers send to electric motors to raise
the convertible top automatically when rain falls on leather seats ;-).

I bought the standards manual from the SAE.  It cost $65.00 (ouch!)
but it contains vast amounts of information that is extremely helpful
to people such as you and other people on this mailing list and me.
The SAE is in Warrendale, PA with a 412 area code.  They accept plastic
and will send you HS-3000 (or is it HS-3500?) in a couple of days.  The
manual has a title with words about OBD-II diagnostics for cars and
light trucks.

>  Any information would be helpful.  If this request doesn't fit into your=
>  group, I apologize.

Hope this information helps.  Yes, I think your request fits into this
group because of its advanced do-it-yourself nature involving automobile
engine electronics even if the request isn't explicitly about fuel
injection.
 
> Thank you in advance,

> Chris Shaw.

Marll McDonald   KB1AGM
m_mcdonald at marx.enet.dec.com