on GMT-800 vehicles, you shouldnt run into buffer-full errors, especially with the improved STN-xxx chips. Even though GM made very widespread use of J1850 (class 2) on 2003-2006 GMT-800, its still only 10.4k…so ELM/STN chips are usually ok for them.
99-02 only had like 3 or 4 modules on the bus, but in 03 they transitioned to a true fully-multiplexed electrical architecture, with 10-15+ modules all on the bus. Really there wasnt anything (besides the sunroof) on the 03+ trucks/SUV’s that was NOT controlled via data in some way. It was actually very impressive for the time when it was introduced. Ford/Chrysler didnt catch up to GM’s level of databus use/integration on their trucks/SUV’s until at least 4 years later lol.
But anyway…grab yourself a BCM, ECM, instrument cluster…you can also add driver/passenger window switches, radio etc… They’re all really cheap, I wouldnt pay more than $40 for a BCM, $75 for a PCM, and maybe $150 for an instrument cluster. www.mouser.com has all the connectors you’ll need. And then just get yourself a 12volt bench power supply, an OBD port, and get to wiring it all together…
You dont NEED multiple modules in order to get them talking. You could just power up that spare cluster you;ve got there and start messing around…
Id grab a chinese knock-off Tech 2 (the gm dealer scan tool) as well…theyre like $250 bucks. Then you can watch messages that the Tech 2 sends out when you use the Tech 2 to command the truck to do things (fold the mirrors, windows, horn, yaddyayayda)…and just copy those messages and incorporate them into your code.
Class 2 is super easy to reverse engineer because the header tells you everything you need to know. Its 3 bytes.
[message priority] [intended destination ID] [sender ID]
So you basically just watch the sender ID and you can tell which message is coming from which module. Id have to look up all of them, but off the top of my head, $40 is the BCM, $60 is the IPC, $80 is the radio, $A0 is the DDM (driver door module), $A1 is the PDM (passenger door module), $58 is the SDM (airbag module) $97 is the VCIM (onstar).
I think the HVAC module is $86 and ABS is $56, I cant remember.
your Avy is gonna have:
RSA (rear seat audio)
RSE (rear seat entertainment, if you have the DVD player)
TCSM (transfer case shift module)
EBCM (electronic brake control module, the ABS)
SDM (sensing diagnostic module, the airbags)
MSM (memory seat module)
VCIM (onstar module)
I think thats it…Escalade/Denali will also have CD changer and Bose amplifier on there. (I know your Avy probably has Bose, but the Escalade/Denali gets a nicer bose amp that is “smart” and communciates on the bus)…and Duramax diesel trucks have a separate TCM (transmission control module).
European export tahoes have a TDM (theft deterrent module) as well…european export 02-06 SUV’s got a true immobilizer with a transponder in the key because ECE regulations stipulated that all vehicles needed to have an encrypted transponder/immobilizer in the key. USA trucks just had the crappy Passlock ignition cylinder, which was controlled by the BCM, no separate TDM.
The USA trucks/SUV’s didnt get a transponder key until 2007, when the new body style came out (GMT-900, they also ditched Class 2 and switched over to CAN)