Corvette CCM Reverse Engineering Anyone?

[NomakeWan]

do you have any documentation of that at all ?

The 100 miles thing was actually confirmed several times over on Corvette Forums, including by, as I recall, people who had actually worked at GM during the C4's run. As for hard evidence of that as well as the ability to remanufacture CCMs, yes, there is this little tidbit to confirm it:

ccmreman.jpg

Is this info from the FSM? Mine haven't arrived yet. I'm not disputing this statement but it seems like if it can ask the PCM for message 41 it could also ask it for the VIN, and derive the engine code from that. Though I doubt that's the case.

Also wouldn't the VATS voltage / resistor code have to be set here?

Am I correct to assume the odometer is stored solely in the CCM?

This info can be assumed from several data points. First, the ALDL data request commands for both the PCM and CCM are public information, available right here on Gearhead-EFI's servers. If you sift through those, you will see that the parameters you are asking about are not part of any datastream. Additionally, they are not a part of the $41 datastream. As such, we can infer that the CCM is incapable of querying other modules (such as ECM/PCM) for the data that would be stored in secured locations.

Speaking of, yes, VATS resistor code is stored in the CCM. This value should be a part of the dump I posted, actually; that memory location is only prevented from access if no key (or the incorrect key) is inserted into the ignition. Since I only talk to the car with my car's proper key inserted, it should be there.

Also yes, the odometer is only stored in the CCM.

Here's a couple quick pics of the board (click for full res).



Though I also doubt the unit can self program the VIN and options, I have seen mention of a learn procedure for the VATS resistor value over on cf.

The three solder pads on the bottom-left appear curious to me. That said, as expected, they don't just point you at what pin you'd need to ground in order to override the EEPROM. That would be an insane security problem. Even I haven't seen any documentation pointing out where this pin would be. I'm sure GM knows, and I'm sure there must be some third-party remanufacturers who know, but I'm afraid I do not.

As for the VATS relearn procedure, you'll note if you go back and check that those threads were referring to people who had new or remanufactured CCMs installed, and were being told that this procedure had to be done within the first 100 miles.

Some interesting pin is d6 -diagnostic enable. Goes to DLC terminal 12. Likely when it is grounded or resistored like 4-10kohm it runs some tests.

It's not all that interesting. This is just the normal self-diagnostic pin on the ALDL connector (on a 94-96, this is pin 12, which you then jumper to pin 4, ground). When this pin is grounded, the CCM acts like a Tech 2, allowing the buttons on the dashboard to act as the Tech 2's buttons. Initially it queries ALDL devices for DTCs, then displays any available DTCs on the dashboard. Once DTC display is complete, you can either use the dash buttons to repeat the messages or can also switch modes to clear DTCs or run actuators. This functionality is detailed here: https://tech.corvettecentral.com/201...-trouble-codes

I wonder if diagnostic enable might be the "magic" pin.

Sadly not.