Corvette CCM Reverse Engineering Anyone?

[NomakeWan]

Though this is not what I consider the holy grail at this point, the eeprom seems to definitely be used for storage of the odometer along with all the other configurables, both long and short-term. There appear to be no other non-volatile capable storage components on the board. The eeprom must be writable after the sacrosanct 100 miles, or where else would this data be stored? There are simply rules which we have yet to fully understand.

I have all the digital input pins mapped out to memory except the F10 seat belt switch. I'm learning as I go so things could change, but thus far the 8D section of the FSM has proven invaluable.

There should be something else, or some other way it's keeping track of the odometer. Folks over on the Corvette Forums have pointed out that when you program a "new" CCM via Mode 5, you can only get within 6-mile increments. Yet in-situ, the car is able to keep track of the odometer in 1-mile increments. So perhaps there's something to that there as well.

Sadly my '95 is quite irritated with me at the moment so I can't actually get reliable idle polls off of it. It'll be filled with irrelevant or missing data due to the PCM being pissed off. I'll still be doing an experiment to see if I can pretend to be the PCM and make the CCM and the dash happy, though, since I don't actually need the PCM for that. I can however point out a few patterns I found while messing around with it.

With the car having sat idle for a while, scanning the idle shows 10590000000097 for the C68 HVAC, and 4057000069 for the CCM poll. This just gets repeated constantly with no expected responses.

When the key is first turned on and the PCM actually responds to the first 4057000069 call, the next poll appeared as 40572089C0. Every subsequent poll appeared as 4057FFFF6B. After turning the key off again, the CCM poll changed to 4057A06366. I imagine that after I leave the car alone for over 30 minutes and the CCM is given a chance to go back to sleep, the idle poll will be back to 4057000069 until the next time I cycle the key.

EDIT: Confirmed. I left the car overnight, then plugged in and checked the idle comms, and sure enough it was back to 4057000069.