Class 2 Chatter?
 

Do most OBD-II interfaces allow you to just hook it up, not send any data, but just sit there and watch the chatter data? I don't know if that's the right terminology for Class 2, but I want to be able to just watch and log the conversations going on between the different modules and decode it.

Do most OBD-II readers do that? I know I am not looking for a "Code Scanner" because I don't care about codes.

I think you may be confused about OBD-II. You might want to read up more on it. My understanding is that you get the OBD-II serial / usb interface, then request / recieve data from the ECU.

I think you misunderstand what I want to do.
I don't want to send any messages on the Class 2 bus right away. I want to see what the existing messages on the bus look like, for example, I want to see what the Class 2 command the climate control sends to the PCM to enable A/C compressor looks like. I want to see what the BCM to cluster commands look like, etc. Then I can reverse-engineer them. That will let me experiment with sending commands on the bus that "look like" other devices, but first I need to learn the device ID, format, etc. of the sender so I can emulate its function.

I don't know if that makes sense, but that's what I want to do.

there are several programs that do as you are requesting. Autotap is one of them.

These applications allow you to change some settings, but they have to be intentional by you. If you wish to simply sit back and watch then they can do that too. I chose AutoTap (autotap.com) because it had the most impressive gui for the price.

Hope this helps,

Marc

Thanks! I will look into it.

More info
 

I've been looking into trying to decode class 2 data as well. Information is naturally scarce since I'm sure GM isn't just gonna hand out a decoder. I ran across a peice of hardware a while back that plugged into a computer serial port and did some basic decoding. I'll try to see if i can run across it again. But yeah if you keep up with this project keep everyone posted, I am definitly interested. I'd like to hear if you found anything else too.

This is exactly what h have been working on. see my thread here:

http://www.mp3car.com/vbulletin/engine-management-obd-ii-engine-diagnostics-etc/71604-doing-more-obd.html

I can give you whatever information I have so far, as long as you make whatever you figure out public.

I havn't done a whole lot of research on specific readers, but the elm chips have the ability to monitor the bus and send data to it. I use an elm scan 5, but if you are only interested in GM, you could save a few bucks and use http://scantool.net/products/elmscan_vpw.htm

The greatest source of info i have found is:
http://www.hptuners.com/vpw.htm

from there you can start to figure out specifically what is what. What car do you have? I can give you some pointers?

I also have the service manuals for all GM cars which are sometiems helpful at giving hints.

Ok. So i have a 2004 Cadillac CTS, no luxury package or anything hence no GPS, no XM radio. Currently I have an old laptop running Delorme streets USA, and assuming I have a copilot I have a pretty good GPS setup, and that is where the adventure begins.

After finding this site I fell in love with the idea of adding a computer hardwired in the dash. Originally this whole fixation came from just accessing the input for the XM and adding an 1/8" audio cable (which I did) to plug in peripherals and run the audio through the car speakers, but the dealership couldn't get the band programmed.

On a side note, I am an industrial design and mechanical engineering student (dangerous mix); read this as fabrication skills galore, with an idea of what good interface is, but no programming knowledge whatsoever. I've seen the post of the other CTS carputer and that got me thinking about the possibilites. http://www-personal.umich.edu/~mattbai/

This guy had a good idea, but naturally I'm the guy that wants to go a step further and integrate that whole dash face into the computer setup (no buttons, toggles, switches, etc.) These buttons mostly have nothing to do with the ECU (they all control the radio and whatnot), but the DIC does have the engine info as well as some other nifty info that all has to come from the ECU. There is also the steering wheel controls, and OnStar; I dont know if any of those pipe into the bus but those need to be considered also. So as of now I haven't invested in a scan tool and i would certainly like to, so that I could continue with this ongoing project.

In the end what does this all mean...I, like a few people here, would like to unlonck the mysteries of OBDII, and add to the intricate complexities of my life for fun:nutz:

As far as what I know about OBDII has been mostly due to this forum. I'll look in to the elmscan.

Thanks for the reply.

If you don't already have it, i think you really need the factory service manual. While it doesn't directly describe the communication protocols, it does give detailed wiring info, and tells what talks to what. You can infer ALOT of information form it. There is a guy on ebay who is selling bootleg GM service DVDs for $19. Do a search for "techline". I bought on and its well worth it.

Most of the DIC info is sent to the DIC from the ECU via class 2. This includes fuel econ, batt voltage, oil life, and tras fluid life. The DIC also has a VSS( vehicle speed sensor) input that it uses for trip odometer, and average speed. Regarding the MPG the service manual says this:

It is unclear whether or not "distance" is sent as part of the class II info or is derived from the VSS input.

I uploaded a few schematics out of the manual for you. Sorry they are soo big. I had to convert them from the TIFF format the manual uses and was too lazy to resize them.
Steering Wheel Controls - 2.6L/3.2L/3.6L (LY9/LA3/LY7)
Steering Wheel Controls - 5.7L (LS6)
"radio Power, Ground, DLC, and Audio Signal Communication"

DIC
DIC 5.7L

Along with the pics, i also uploaded a few other pages that i think might be of interest. Look in the folders
http://www.pictar.net/pics/CTS/

I hope all of this helps

I'm going to do an experiment here and allow everybody to access the entire GM manual. If it doesn't bog down my lowly cable modem internet connection and i don't get any threatening legal notices i will leave it up. This is running off of my laptop which is often not at home etc so don't expect it to work 100% of the time

http://fasnorkis.kicks-***.org:9001

enjoy