Hoping to get some better insight on the MEC for GM modules. I understand that it is 255 ign cycles before the module is locked out from factory. I dont really understand the programming side of this. When using a used module, are you needing to flash the MEC to 255 for it to accept a new calibration or vin, or set it to zero to do so? Do you know which modules will need this, is it global a only? What is a symptom of a MEC exceeded, would that be a errored out programming event? 

Thank you 

 

There are something about MEC:

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The Vulnerability Flag And Manufacturers Enable Counter (MEC). The Manufacturers Enable Counter (MEC) and Vulnerability Flag are variables which are used to bypass a nodes security system during development or during portions of the node and vehicle manufacturing process. The variables manufacturers_enable_counter and vulnerability_flag are used in the pseudo code of the applicable diagnostic services described within this specification to represent the MEC and Vulnerability Flag respectively. Reference the SecurityAccess ($27) service within this specification for more information on accessing node security.

 

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The Manufacturers Enable Counter (MEC). The MEC shall be supported during ECU development and production by all nodes which implement the SecurityAccess ($27) service. The MEC is a single byte in permanent (EEPROM or equivalent) memory which allows a node to remain unlocked as long as its value is not $00. When the value of the MEC becomes $00, security shall be enabled (provided that the Vulnerability Flag is not $FF) and the SecurityAccess ($27) service shall be required to access security. Production ECUs shipped to the vehicle assembly plants shall have the value of the MEC initialized by the ECU supplier to a value specified in a CTS, SSTS, or supplemental diagnostic specification referenced by a CTS or SSTS. Service Replacement ECUs shall be shipped to the dealerships with the MEC already set to $00. The MEC must be programmed to $00 at some point in the vehicle assembly plant process (typically, the MEC is programmed to $00 at the conclusion of a passed Dynamic Vehicle Test). Note: The MEC is programmed with the WriteDataByIdentifier ($3B) service. Refer to Appendix C to determine the Data Identifier (DID) number for the MEC. A node shall not allow the value of the MEC to change once it becomes $00, unless SecurityAccess ($27) is successfully initiated. The ability to allow writing a new value to the MEC for any specific ECU which supports the MEC shall be negotiated by representatives from Service and Assembly Verification, and the responsible DRE, and documented in the CTS. The platforms should employ a backup mechanism to ensure that a node will lock itself in the event that the vehicle somehow manages to make its way out of the assembly plant with one or more nodes unlocked.

 

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Example Implementation of the Manufacturers Enable Counter (MEC). In this example, the MEC is used as a counter which decrements by 1 each time the ignition is cycled. The MEC is initialized to a calibratable value ($FE for this example) by the supplier before the node is shipped to the vehicle assembly plant. In this scenario, any node which was not locked prior to the vehicle leaving the assembly plant would become locked after a calculated number of ignition cycles (254 - the number of ignition cycles which took place at the vehicle assembly plant). If all nodes on the vehicle which support a MEC also have an ignition input, then each node would decrement the MEC by 1 each time its ignition input transitions from a high voltage state to a low voltage state (e.g., Run to Off transition). If the vehicle has one or more nodes which support a MEC, do not have an ignition input, and can remote off, then a more complex method of decrementing the MEC is required.

Note: It would not be desirable to decrement the MEC each time the node remotes off as this approach greatly increases the possibility that the node would lock itself before all assembly plant programming and testing processes have been completed. To solve the issue of decrementing the MEC for the nodes which have no ignition input and remote off, a MEC master is used. The MEC master is a node which does have an ignition input and is capable of communication after the ignition input goes to a low voltage state. The MEC master detects the ignition transition and generates a bus wake-up. The master then transmits a message which contains the value of the MEC. (See Note below.) The nodes which were remoted off would wake-up and then synchronize their MEC with the value provided by the MEC master. When synchronizing the MEC, nodes which already have their MEC at $00 would not modify the value. This prevents the possibility of replacing a single node on the vehicle (the MEC master) and unlocking all nodes on the vehicle.

Note: This message may need to span subnets based on the vehicle configuration.

 

I hope this info will useful for all of us.

Thank you for sharing,