General Specification Electrical Function gmw3110 General Motors Local Area Network Enhanced Diagnostic Test Mode Specification

9.3.2.6.3 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 
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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.