Retrieving ECM data with PC

Mon Jan 16 21:31:01 GMT 1995

John T Stein <JSTEIN at dpc2.hdos.hac.com> Picked up his electron pen and wrote:
| I am trying to find software (PC) to read out, decommutate, and 
| display data stored in a General Motors engine control computer.  
| Does anyone have, or know of the existence of any such software?

Delco manufactures the Tech-1A, the definitive scan tool for GM products.  I 
have no idea how much it costs, but I believe it is in the $1000 range.  It is 
stand-alone hardware that plugs into the ALDL and prints/records the frames as 
they come out.  It can be set to trigger on various conditions.

A company called diacom makes software to run on a PC that hooks to the ALDL 
via the parallel port, not serial.  I believe it costs $400 or so.  I have 
heard that it can be purchased thru NAPA auto parts stores, and via mail order 
from speed shops.  I have been told that it offers nearly the same 
functionality as the tech-1a, with a better presentation format.  I could find 
out more info about this if you are interested, since someone on the vette net 
recently purchased it.

If you are interested in a Buick GN/Ttype or similar, Ken Mosher has written a 
program which he sells for $100 to read the ALDL info and print frames.  This 
only works with a certain version of engine computer which sends data at a 
much lower rate (see below).  You can check out the GN/Ttype list to get more 
info on this.

| 
| Any other information about the format and organization of the 
| telemetry available at the ALDL connector; e.g word sizes, baud 
| rates, data order, etc. would be appreciated as well.

There are at least two types of engine computers, which use different data 
rates.

All that is commonly known about the faster one (I should say, all that I have 
been able to figure out) is that the baud rate is 8192, TTL level.  The UART 
in a PC cannot be programmed to 8192 exactly, but a divisor of 14 yields 8228, 
which should be close enough, esp. since the UART in PCs uses 16x 
oversampling.  The TTL voltage should drive an RS232 port with a very short 
cable, if not a MAX level shifter could be used. I have no information on how 
the data is partitioned, not even if it lies on byte boundaries or not.  I 
would assume it does.

You have to know what type of engine computer you are dealing with.  The 
computers in the buick GN-ttype cars uses a much lower baud rate, about 110 or 
so, and it is self clocked in some manner.  Apparently the computer does not 
send data continuously, but sends a bit whenever it finds the time.  I believe 
all corvettes (84-up) use the faster data rate.  Other cars I have no idea.  
In a few years, the ALDL will be gone, replaced with OBD-II, a standard 
interface across car manufacturers.  The specs will be public, so if you want 
to roll your own, it shouldn't be too hard.  The 94 Corvette is partially OBD 
compliant.

| 
| I would expect the ALDL data rate to be low enough to allow the data 
| to be received by the PC serial data port.  I would also expect that 
| the only hardware needed would be a line receiver for the ALDL data 
| and a suitable driver to sink the ALDL test pin to ground.  Are these 
| assumptions valid?

Here is what I know/have guessed from the corvette service manual.  ALDL data 
is present all the time.  The engine computer can be in one of three modes, 
ALDL, 10K, and backup.  ALDL mode is normal mode, ie you can collect data 
while you are driving, etc.  10K maintains engine rpm at 1000, and disables 
some features, like O2 feedback, etc.  Backup mode cuts the ECM out all 
together, and simulates the fallback mode that takes over should the ECM 
totally bug out.  I am not sure how to set these modes, but I believe 
different resistances from the test pin to ground will do it.

| 
| I am sure this has all been done by someone already, does anyone 
| out there have any experience they could share with me?

It's been done, but unfortunately it's only been done commercially, and no-one 
wants to talk about it.  I have tried calling GM about it, but haven't ever 
managed to get ahold of someone who knows what I am talking about, much less 
knows if the info can be released or not.

| 
| Thanks, 
| 
| John
| 

Anyone else?

--steve