So, the fobs register with the Lift Gate Module? Does the LGM control all doors actions thru the fobs? I am confused. I would like fobs for my girl but need to "understand" how it all works in "laymans" terms.
Are you saying the existing actuators, doors and liftgate, will be fine and no hard wiring is needed?
I still as of yet cannot definitively say that the BCM needs flashing for keyless entry. And I cannot yet determine if there were different LGMs with and without keyless entry capabilities. More research and perhaps experimentation is needed.
I see all manner of conflicting information online. I saw one ebay listing for a used LGM that stated it was without keyless entry but they included an image of the RPO sticker that clearly showed the keyless entry code AU0.
Then another official looking site lists an LGM that again says without keyless entry but goes on to list the various models it supercedes and there in that list is an entry for a model that definitely does have keyless entry.
In short, I have yet to find a parts listing that clearly states different model LGMs with and without keyless entry.
Now to the matter of how it works and why no additional wiring would be needed.... I will try to explain. I may over simplify a bit but it will still be somewhat confusing nonetheless.
@mrrsm posted a listing of various programmable modules in GM vehicles. Scattered throughout the vehicle you have a bunch of microcontrollers (modules) that serve various systems. The PCM handles the engine and transmission, the HVAC module handles the heating and cooling, the TCCM handles the 4wd system, PDM handles the front passenger side door, DDM is the drivers door module, LGM handles everything at the liftgate/hatch and so on. Even the radio and the instrument cluster are modules.
In days of old when you flipped or pushed a switch you actually sent power to some device like a door lock or what have you. That meant there had to be separate wires for each operation from the switch location to the controlled device location That added up to a hell of a lot of wire, cost and weight.
Then electronics and microcontrollers came on the scene. And they can communicate with each other. Now you could could just put a microcontroller (module) here and there in locations where stuff had to be controlled and instead of running separate wires you just run one communications wire to the modules.
In these GMT360/370 vehicle all of the various modules are interconnected on a single wire network. Every message from every module is heard by every other module on the network. I might liken it to a group chat/text. No private messages, it's a big party line.
If some function like unlocking a door needs to happen you just need to send a message out on that big party line instructing whoever is responsible for that function to do it. No separate wires needed. The module already has power and ground at all times and is also connected to the devices that it is responsible for. Like door lock/unlock actuators or window defog grids and so on.
So how does the keyless entry actually work?
The key fob sends out a radio signal when a button is pushed. The radio message contains the key fob ID of some sort and the details of what button has been pushed.
The LGM has the radio receiver for the keyfob radio signal frequency. The LGM will check the key fob ID to confirm that the particular key fob is recognized and registered to this vehicle. If yes then the LGM puts a message on the vehicle network to "wake up" the network from a sleep state. Even when the key is off the modules still have power and are monitoring the network for traffic.
The BCM receives the "wake up" message and broadcasts a "power mode" message to the network. This notifies all modules what the desired operational state of the vehicle should be.
The LGM follows up the "wake up" message with a functional message that details which key fob (driver 1, driver 2) was used, and what function is being requested (lock, unlock, etc.) and so on.
The BCM is programmed to act on this second message sent by the LGM by responding with a different functional message directing the relevant module(s) to perform the action requested.
The relevant modules(s) get the functional message the BCM sent and perform the function then report back the results of that action.
All of this usually happens in less than one second.
Mixed in among the messages are a host of other messages about vehicle security, interior lighting and all manner of other stuff.
I keep using the term "function" because that is how these messages are addressed. These messages are not directly sent from one module to another specific module but rather to "functional address". There are functional addresses for nearly every operation of the vehicle. "External Access", "Door Locks", "Tires", "Mirrors", you name it. The modules are programmed to act on functional messages that deal with devices or actions that they are responsible for.
Clear as mud, right?
So for a vehicle to have a working keyless entry system the LGM must have the radio receiver circuitry and the programming to go with it. That part is simple enough because I think a used module can be just swapped in without VIN specific flashing. It may set a code if the swapped LGM has features the vehicle does not have. Like rear window defog grid or rear wiper. But it won't set the CEL or the like.
And the BCM must have the programming to act on the message(s) from the LGM that deal with keyless entry. That part is much more of an issue. It's not so easy to add capabilities to a BCM. The BCM DOES retain the vehicle VIN number and uses it for security.
