Lots of questions about Saturn PCMs

[Shannen Durphey]

Uh-oh.  Another newbie asking "small" questions. : )

In the world of GM, Saturn is a different beast.  They have many "close
but different" ways of doing things.  They do not share GM's parts
network, they don't use GM warranty programs, and they didn't use
traditional GM suppliers when they started up.  That's changed some, but
the fact remains that you can't rely on Saturn to do things the GM way.
For example, they use an unusual 5000 pulse per mile speedo signal, rather
than the more common 4k pulse gm signal.

You pcm's part number is the service number.  I can't see your label, but
it's most likey 21023810.  The "master" calibration is probably 21008768. 
Within that master calibration there are often several calibration files. 
For one calibration we find:

A 21008881;Automatic Transmission
Contains:
21008878
21008879
21008831
21008868
21008651

I have no further information on this.  But it does appear that the pcm
you found has most of the original programming, and that these part
numbers above are "newer".

All GM pcms are programmed with the VIN from the factory, and the GM
dealer tools will program the VIN into a replacement pcm as part of the
programming process.  I've been told it's required by OBDII regs.  Some
companies rely on this info to make sure their reprogrammers can only be
used on one car at a time.

GM is extremely upset that they have to share pcm/scantool info.  Whenever
and wherever possible, they don't.
The mileage counter may be part of OBDII required freezeframe data.  This
data keeps track of mileage since a trouble code was set, and records
various data from the engine when the trouble code occurred.  I'm sure you
would find it a great help if you could get a dealership to plug a tech 2
into your car and show you some of the readouts and screens.  Just knowing
what it can do may shed some light.

Shannen

Gary A. Allen, Jr. wrote:
> 
> This is my first post to this discussion group.  Since I'm a newbie,
> I'll
> do a a self introduction and then ask my newbie questions.  I became
> aware of this discussion group from a digest of postings from DIY-EFI
> that started on 7 Feb 1999 and ended on 1 Jan 2000.  I am unaware of
> what
> has been posted here since 1 Jan 2000 (apologies in advance if my
> questions
> were recently answered or now considered off-topic).
> 
> My area of interest is OBD-II PCMs for the Saturn.  I own a 1996 Saturn
> SL-I with a 5 speed transmission and a benchtop PCM that I pulled from a
> 
> junked 1999 Saturn station wagon that had an automatic transmission.
> I communicate to the PCMs through my PC's RS-232 serial port using a
> Multiplex Engineering J1850-VPW adapter that came with the GMTOOL
> software purchased from CarComp Inc.  I also own a complete set of
> GM factory service manuals for the 1996 Saturn.  I soon learned that
> the engine parameters reported by GMTOOL were only a small subset of
> what was listed in the GM service manuals.  I then began writing my
> own scan tool software in the C programming language under DOS
> (yesterday for the first time I got some of my software to work
> [badly] under Linux).  I wrote my software based upon what I could
> extract from GMTOOL (its binary database is encrypted), learn on the
> Internet (downloadable codes for other scantool adapters) and from
> the SAE HS-3000 handbook.  The HS-3000 handbook was the most useful
> information source.  However I discovered that GM stuck to the SAE
> standard
> only when it was a "legislated" standard, i.e. SAE J1979.  Where ever
> the SAE standard says "the manufacturer may optionally...", GM always
> optioned to follow a proprietary standard.
> 
> I've since been able to write programs that call almost all of the PIDs
> called
> by GMTOOL.  My codes can convert the raw data for most of the legislated
> 
> PID responses to something understandable, i.e. degrees of advance,
> volts,
> Pascals, etc.  I have also written "battering ram" codes that call
> long lists of GM proprietary PIDs which produce responses that I mostly
> don't understand.  I only run these battering ram codes on my benchtop
> PCM
> (NEVER on my '96 Saturn).  My benchtop PCM has the following label:
> 
> -----------------------------
>   21008768                            21023810
>   AUTOMATIC             SERVICE NO.
>   POWERTRAIN CONTROL MODULE
>                        [bar code]
>               *868768M172558G7J*
> -----------------------------
> 
> I also pulled the main wiring harness from the same '99 Saturn wreck
> that
> I got the PCM from.  I power the PCM with an old PC/AT power supply and
> have multiple switches and variable resistors attached to the wiring
> harness to fool the PCM into thinking it is still attached to a working
> Saturn (I also use these to trigger DTCs).  What I have learned about
> the
> Saturn PCM is almost insignificant compared to what I do NOT know.  For
> example I am not sure what the part number is for my benchtop PCM.
> I know if I send the PCM a Mode 0x3C PID 0x09 then it will come back
> with
> 0x21008768 which also appears on the PCM's label as shown above.  Is
> 0x21008768 the part number or the specific PCM's unique serial number?
> I have not found a PID that will generate 0x21023810 which also appears
> on the label.  However I can get other numbers that are in the same
> format:
> 
> Mode 0x3C PID 0x08 ---> 0x21008760
> Mode 0x3C PID 0x09 ---> 0x21008768 [appears on PCM label shown above]
> Mode 0x3C PID 0x0A ---> 0x21008758
> Mode 0x3C PID 0x0B ---> 0x21008772
> Mode 0x3C PID 0x14 ---> 0x21024625
> 
> The Mode 0x3C PIDs are interesting.  For example I have found that
> PIDs 0x01 - 0x04 will give me the VIN number.  PID 0x62 seems to
> be the number of kilometers/miles since the PCM was last cleared,
> I have not yet figured out the conversion from the raw hex to miles
> or kilometers.  Does anyone know how to do that?  PID 0x63 and 0x70
> produce these interesting bit patterns that change everytime the car
> is run.  However I am clueless as to what they mean.
> 
> Other things that baffle me are conversion factors.  For example GMTOOL
> says that Mode 0x22 [2 byte PID] 0x11 0x9D is barometric pressure in
> "inches of mercury".  What conversion formula do I use to convert the
> raw hex from the PCM to "inches of mercury"?
> 
> Ultimately I would like to access my benchtop PCM's flash memory,
> download its executable code, disassemble it, modify it and then reflash
> 
> the PCM's memory with the modified code.  Needless to say, I have no
> clue how to do this.  Does anyone out there have a clue?  I tried
> playing with Mode 0x27 "Security Access Mode" as described in "Enhanced
> E/E diagnostic test modes - SAE J2190".  My PCM should have gone into a
> 10 second time delay when I gave it two bad password attempts but it
> didn't.  It only behaved approximately as described in J2190.  I suspect
> 
> the security access provided by Mode 0x27 is actually a decoy with
> actual access provided by some undocumented mode.  A disturbing Mode
> command that I found with my battering ram code was one that crashed(?)
> my PCM:  Modes 0xA1 and 0xE1 [Do NOT run either of these Modes on a car
> that you are actually driving!!]  If I invoked either of those commands,
> 
> my benchtop PCM became immediately unresponsive and remained silent
> until
> I cycled its power.  Anyone know what those Modes are supposed to be
> doing (other than act as booby traps)?
> 
> Finally does anyone know what the processor is for my PCM?  Somewhere I
> read that it is the Motorola MC68332.  However after I opened the PCM
> up, I found none of the chips had the correct pinout for a MC68332.
> 
> Thanks in advance for whatever help you can provide!
> 
> Gary Allen
> 
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