Some GM trouble codes, particularly those related to non-emissions-related systems (like transmission, ABS, or body control modules), may not be readable by basic, generic OBD-II scanners. These scanners typically only access the standardized OBD-II codes related to the engine and emissions control systems. To access these more specific codes, a more advanced, manufacturer-specific scan tool is often required.
As a rule I am speaking specifically about vehicles like the OPs vehicle which has no CANBUS like later vehicles. The OPs vehicle uses strictly the single wire SAE J1850 VPW protocol. Often referred to as Class II serial data.
There are phone apps that can handle this for those not inclined learn how to query modules manually. The phone apps will try the common node addresses for GM modules.
For those who decide to do the queries and subsequent decoding themselves there is just one byte in the query message that changes. The hardware is no different at all.
Transmissions when not handled by the PCM at node $10 or $11 are frequently found at node $18. ABS functions are handled by the EBCM at node $29. Body Control (BCM) would be node $40. Others are DDM node $A0, PDM node $A1, HVAC node $98, and so on and on.
I don't wish to hijack this thread with a lengthy discussion of code reading. Maybe just a screenshot to demonstrate?? Here I ask for current codes from a PCM (node $10) . It reported no current codes. I then alter the message header using an address for the BCM (node $40) and again request current codes. The BCM reports 12 current codes. Lastly I alter the header again to query a TCCM at node $1A. Again, current codes are requested. The TCCM reports 5 current codes. None of this is surprising as all 3 of these units are wired up in a drawer in my kitchen!!
The hardware is the same for all. Still using a $14 Veepeak unit.
Every action a scantool takes on these SAE J1850 VPW Class II Serial Data vehicles is accomplished by messaging. It looks similar to the screenshot above. And that messaging structure and electrical characteristics are set in stone by the SAE specification. There is no other backdoor or the like for any scantool to use.
So why are high end scantools so much more expensive if it is not in the hardware? It is the licensing of proprietary informations from the manufacturers. That is big bucks each year for the scantool manufacturers. This information is what is needed by the scantool in order to send the right messages to the particular vehicle under inspection .
And no doubt a high end scantool can be oh so handy and nice to have. But if all you need to do is scan for a code here and there and maybe monitor some live data using apps like Torque Pro or Car Scanner ELM OBD then these low cost dongles are adequate.
