GM computers

[Dale Ulan]

> 
> 
> 	Does anyone know of some sources or have some info on GM computers.
> My intentions are to change the intake manifold and cam on a GM 3.1 L V6, 
> turbocharge it and throw it in the back of a cheap '84 Fiero.  I would 
> also like to be able to adjust as much as possible from the cabin of the 
> car (A/F, spark timing, boost, etc?)  Now if one had a lot of money you 
> could buy stuff that would do this, but that takes all the fun out of a 
> do-it-yourself project.  What I'd like to know is about the different GM 
> ECM's, their inputs, their EPROM's, the location of the tables on those 
> EPROM's;basically, I'd like to know everything there is to know about GM 
> ECM's.  

You can easily identify the ECM type by the type of EPROM carrier used.
The EPROM will be hidden under a small 'sub-lid' of the ECM, and in some
kind of carrier.
A C3 PROM will be in a small black plastic chip carrier made by RN.
A P4 PROM will be in a large long chip carrier with a dual-row header
plug on the bottom.
The new ECM (P5?) uses on-board FLASH memory.

A C3 ECM is based on the Motorola 6801 processor. Different versions
of the C3 have different I/O on them, depending on the application. They
also have different software in there, too. For example, the 1988 to 1991
GMC pickups have the same ECM for both the 305 and 350 engines, however,
they have different PROMs. This ECM is probably not interchangable with one off
of a 1986 Firebird, even though they are both C3 ECM's. They have
different masked ROM, and different address decoding for the EPROM. They
usually have the GM timer chip set, which does spark and injector timing.

The P4 ECM is based on the Motorola 68HC11 processor. There are also many
variations on this basic ECM type. Same applies as above... except that
all of the software is contained in the 32k EPROM used in these ones... there
is no on-board masked ROM in most P4 applications that I'm aware of.

For both of these old ECMs, you'll have to figure out the mapping on your
own. I've done it for one firmware version only, and it's a lot of work.
Of course, the OEM algorithms are far superior to any aftermarket code, but
calibrating it can be a bear mostly because of a lack of documentation.

The tables are generally arranged with options and checksums first, then
spark advance basic tables, followed by a bunch of fuelling tables, which
include the large 'volumetric efficiency' tables, and finishing off with
diagnostic and idle speed limits.

The new processor is probably based on the Motorola 68F333 processor, which
is *very* difficult to deal with for most of us... at least for a couple
of months. I'd like to get my hands on one of the new beasts.... :-)

-Dale