Starting in 2004, some GM vehicles began using a new data network system called GMLAN. GMLAN networks use a new "Node Management Strategy." GMLAN is different from the older Class 2 network strategy where modules wake up and send a State Of Health (SOH) message approximately every two seconds. With GMLAN, only the modules involved in a given Virtual System of modules are "awake" and communicating at the appropriate time.
A Virtual System is any collection of modules and/or components that are needed to actuate a particular vehicle function. This function may be something like a cruise control system. This cruise control system may exist only through the interaction of the cruise control buttons, the body control module, the powertrain control module and the throttle actuator control module. All other modules may not be involved or needed in this function. If after eight seconds no messages are required of a particular module in order to accomplish a specific function, that module will return to a low power state and stop communicating on the data bus. This keeps messages to a minimum so the GMLAN bus can utilize greater messaging speeds due to less traffic.
As there are no SOH messages on purely GMLAN vehicles, there is no Class 2 Message Monitor Function in the scanner for this system.
Functional Diagnostics
Because certain groups of modules collaborate to accomplish particular vehicle functions, it only makes sense to keep this concept in mind when creating diagnostic steps to troubleshoot a particular function. GM has done just that by using the Tech2 in conjunction with the service manual diagnostics.GM calls this new concept Functional Diagnostics. With this concept in mind, you can expect that only certain modules within a particular functional system may set communication errors or other DTCs if a problem within a particular functional system arises. Let us look at how GM now tackles this task lacking the old Class 2 Message Monitor function.
Under the Tech2 Vehicle Controls Systems sub-menu, there is the DTC Information screen shown in Figure 1. This screen can reveal a great deal of information about the modules needed to create all vehicle functions.
ECU Name: This area lists the possible modules that could be installed on this particular vehicle. Notice the 3/19 displayed on the lower right of the Tech2 screen. This means the highlighted third module, the EBCM, is No. 3 of a possible 19 modules for that type of vehicle.
Status: All Modules begin the Status test with a Waiting state listed on the right (not shown). As the Tech2 highlights and queries the particular module, the status should change to Present (not shown) for a few moments. This signifies that the module is communicating during the time it was queried. As the module is being queried, the DLC Pin display area will list the DLC pin(s) the Tech2 is using to communicate with this module, pins 14 and 6 in this case. If the reporting module has any DTCs, the number of DTCs is listed here.
If after the Tech2 queries a module and the status reads a No Comm state, the module might be physically there but not communicating. In this scenario, there will be codes set in other modules that need the module in question to perform some functional task. Or it could be that this vehicle is not equipped with that particular module due to the option package purchased.
All defects will be listed in order of their priority. Modules with communication problems are highest followed by modules with the greatest number of DTCs.
Restart Button: Unlike the Class 2 MM test, the DTC Information test must be re-run in order to update the status of the modules over time as GMLAN modules only communicate their status when they are requested to do so by the Tech2.
View All DTCs: When this soft key is pushed, you will be taken to the DTCs Sorted By Priority view shown in Figure 2. Here DTCs and Symptom Codes are displayed.
Along with the typical five-character DTCs we are familiar with, B1517 in this case, there is something new added. After the DTC there is a two-character alphanumeric Symptom Code.
The first of the two characters in the symptom code is the category number. This pertains to general category of the fault. A 0 or a 1 means a general electrical failure. A 2 means frequency pulse width modulated failure. There are currently eight category numbers in use.
The second of the two-character code is the subtype character. This gives a more specific descriptor to the code. A 3 means Battery Voltage Below Threshold. In the case shown, we have a 03. Character 0 = General Electrical Failure. Character 3 = Voltage in the circuit was below threshold.
In our example, the 1/5 means the one code set in the BCM out of the five DTCs set in all modules in total.
Powertrain DTCs do not have Symptom Codes, as these codes are created according to OBDII regulations.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.