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Originally Posted by Vaughanabe13
The lock module almost certainly uses serial communication on line "C", which could be relative to the 12V power, but more likely it is relative to an internal voltage regulator (I'm guessing 3.3V or 5V) and probably optoisolated.
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It's possible, but that would make the service manual wrong. The chart listed there is intended to be "what you see with a probe" on that line, and apparently it's 0V when it's been unlocked for 15+ seconds, 12V when it's locked (or recently unlocked), and then a signal a little negative pulse signal against a 12V background for the lock/unlock command. What makes no sense in that chart is: how does the BCM send the pulses against 12V to lock from the unlocked state, if the line is at 0V? :P Clearly, at the very least the chart just doesn't tell the whole story, but it does seem it's 12V and unlikely serial, just some timed pulses.
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It doesn't even matter what kind of protocol they are using (I2C, SPI, proprietary, etc.) because you could just use a bus pirate to record the streams that are getting received/sent from the lock module. Of course, this would require that you have a module that is actually working (and not intermittently). You could use a tiny microcontroller and a couple of relays to emulate a functioning lock. The use of flash or EEPROM for saving the switch state might not be the best solution, depending on how fast the micro boots up and how much time is available to send the "all clear" signal.
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Good point on the flash, but how does a relay fix things? Is there such a thing as an electromechanical relay that keeps its persistent mechanical state through power cycles?