GM Network Diagnostic Opportunties

Martin Instructor Burnaby, British Columbia Posted   Latest   Edited  

In a previous post, diag​.​net/msg/m5on82rckc… information was briefly presented in regards to testing GM vehicle primary GM HSLAN and gateway isolated GM HSLAN networks, as connected through the GM Serial Data Gateway Module (SDGM) This is also known as the Central Gateway Module (CGM). 

Based on prior system experiences of HS GMLAN (CAN C) testing at the Data Link Connector (DLC), there may be mixed views or opinions on whether the measurement presented in Figure 1 here is "good, bad or ugly".

This is a routine terminal HS GMLAN terminals 6-14 resistance measurement with key removed and vehicle powered down. However, some technicians have recently been caught out assuming that this indicates an open circuit in the HS GMLAN. 

For those not ready to assume anything, the vehicle in question is a 2020 Chevrolet Bolt which is a current model Global "A" vehicle. However, as with some GM vehicles such as Cadillac XT4 and the 2019 new model pickup trucks that are Global "A" utilizing Generation 3 Serial Data Gateway Modules (SDGM), direct access to the primary HS GMLAN via the DLC is prohibited. 

This negates the ability to perform diagnostics via pins 6 and 14 using a Digital Multi-Meter (DMM), Data Bus Diagnostic Tool, or other means than a J2534 scan tool that is capable of interrogating the system. For GM dealerships the MDI 2 and GDS 2 are specified. For the aftermarket independent sector, "equivalent" tools that are capable of handling the protocols will suffice. 

Figure 2 shows a drawing of Global "A" 2020 Chevrolet Bolt DLC to SDGM data circuits, which should clarify the recorded measurement in Figure 1. 

Figure 3 depicts the Global "A" 2019 Cadillac XT4 DLC to SDGM wiring configuration, that will also provide the same measurement results at DLC terminals 6-14. 

Figure 4 represents the new model 2019 Global "A" pickup truck DLC to SDGM. 

Note: If concerned whether a vehicle is Global "A" or "B", as long as there is LS GMLAN at terminal 1 of the DLC, the vehicle electrical architecture is Global "A". Global "B" vehicles do not utilize LS GMLAN, or MOST Bus for that matter.

To illustrate this, Figure 5 depicts a typical schematic representation of LS GMLAN network topology and connection to the DLC for a 2019 K1500 (New Model) pickup. All LS GMLAN access is protected by the SDGM.

So, going forward it will be important to identify the network topology and level of accessibility into vehicle systems, whether any access with traditional tools at any vehicle network via the DLC is possible or not. One can no longer assume that the same diagnostic methods will work from one vehicle model to another, even within the same year of production. 

Figure 6 represents a partial network drawing of a current GMC Yukon from … model year showing the configuration of the Gateway Isolated LS GMLAN . While the Primary LS GMLAN branches (not shown) can be accessed via DLC terminal 1, the diagram clearly shows that there is no direct access to the Gateway Isolated LS GMLAN via the DLC. Accessing the Gateway Isolated LS GMLAN requires reviewing the schematic to identify opportunities to access the network with minimal, disassembly. The K56 SDGM lives to the left of the steering column, above other components, so is not in the most easily accessible location.

The Yukon T3 Amplifier connections can be accessed via an opening in the left rear quarter trim panel, behind some cargo area components. 

The wiring schematic reveals inline connector X275. GM vehicle "electrical zoning" identifies that connectors X200-299 reside in the area in the front of the cabin in line with the dash. X275 is located behind the right front kick panel at the base of the "A" pillar. It is easily accessed by removing the right front passenger door opening sill trim and combined kick panel. 

High Speed GMLAN accessibility via DLC

Previously when accessible at DLC terminals 6 to 14 if the primary HS GMLAN bus integrity was okay, 60 Ω would be measured with the system powered down (or battery disconnected). 

The DLC has been a familiar and convenient access point to the HS GMLAN many for the past several years, with the measured 60 Ω value being representative of the bus having two end of line 120 Ω terminating resistors in parallel, for a total bus resistance of 60 Ω.

With changes to a 3rd generation SDGM, access via 6 and 14 for system primary HS GMLAN diagnostics using a DMM or other means than the scan tool is no longer valid, since each DLC network loop at the SDGM is configured with two 60 Ω resistors in series, as shown in the Figures above for 2019 Cadillac XT4 and other represented models. 

Note: The 2020 Chevrolet Bolt that is the subject in Figure 1 above is still a Global "A" vehicle, as easily identifiable at the DLC by the presence of Low Speed GMLAN (LS GMLAN) at terminal pin 1 as identified in Figure 2. 

So, let's consider what value the 120 Ω measurement shown for the 2020 Chevrolet Bolt serves. Knowing that there are two 60 Ω resistors wired in series through terminals 6 and 14 connection from the DLC at the SDGM, measuring 120 Ω, validates the integrity of the circuit to the SDGM from the DLC. 

Global "A" vehicles have been around for a number of years, but Global "A" itself doesn't indicate how networks such as HS GMLAN (high speed CAN Bus), LS GMLAN or other networks can be accessed for testing. 

Connectivity is dependent whether the network being accessed, is a primary or isolated network and the generation of SDGM comes into play with increased security of Generation 3 SDGMs, effectively "slamming the front door" closed to previously accessible networks, using the DMM and DBDT.

For the past several years, we have enjoyed convenient access for testing of the HS GMLAN at DLC pins 6 and 14. With SDGM Generation 3, that access has been eliminated. The networks are now secured beyond the SDGM. However, on the vehicle side of the SDGM the networks still remain in the same configuration with end of line termination resistors inside modules, or as separate components. 

HS GMLAN has lived at pins 6 and 14 as the primary high speed network, as mandated and continues to reside there right through Global "B" network introduction. What changed since 2017 model year was the introduction of the Serial Data Gateway Module (SDGM), also widely known as the Central Gateway Module. 

The SDGM was first introduced on GM vehicles in 2017, to protect unauthorized access into vehicle systems through OnStar and "infotainment" systems. This was referred to as the "dirty" side or "back door" entry, since flash drives with corrupt files could result in issues and also any "beamed" in services via OnStar could wreak similar havoc. So, the SDGM isolated vehicle operation critical systems from access.

At this point, the "front door" (DLC) remained open as an entry point for physical attacks using purpose designed "dongles" that could also be corrupted. Closing the front door was the next logical step in the security process.

At this point in time, there are several carry over models that may or may not allow access to some vehicle networks via the DLC. E.G. 2020 Terrain and Yukon still allows access to the primary GM HSLAN at terminal pins 6 and 14, using a DMM, the Data Bus Diagnostic Tool (DBDT) or DSO. However, Gateway Isolated networks cannot be accessed via the DLC on these vehicles, except when using the scan tool. Since the DBDT will be no longer be applicable due to these ongoing changes, GDS 2 will incorporate network diagnostics.

A quick way to understand whether traditional tools can access the affected data bus via the DLC is in the description that is provided when following a DTC diagnostic path. If the result identifies " Scan tool does not communicate with...", the system can usually be tested using traditional tools. 

If there are DTCs U007A, U007B, U007C, U007D or U0007E, those refer to Gateway Isolated Networks. 

I use a simple analogy for any DTC with U007(X), as being a "James Bond" code. In other words, much like the famous elusive spy, the networks that exhibit these DTCs when faulted, are hidden behind the Serial Data Gateway Module and cannot be diagnosed from the DLC using conventional means such as DMM, DBDT or DSO.

To diagnose these DTCs will currently require gaining access to the affected network on the "vehicle side" of the SDGM. There is nothing different about the construction of these networks, other than the SDGM provides a "firewall" function that closes the "front door" path via the DLC.

Most current platform GM vehicles are still Global "A", with only the 2020 Cadillac CT5 being Global "B". Whether Global "A" or "B" alone, does not define the diagnostic approach. Vehicles with Generation 3 SDGMs will not allow access to the primary HS GMLAN or other LAN via the DLC using a DMM, DBDT or DSO. 

While GM dealership specifications are to use GDS 2 with MDI to access data through the SDGM, aftermarket scan tools that are capable of accessing and displaying the data and scan tool functions correctly, should work and not impose the costs of purchasing new equipment. 

Global "B" vehicles will also require the MDI 2 interface in GM dealerships since the MDI 1 will not function for every need. So, whatever tools of choice are in the independent sector, they must be capable of meeting system requirements. 

The DLC shown in Figure 7 represents the layout of a Cadillac CT5, which is the first Global "B" GM vehicle, as officially known as the GM Next Generation Digital Vehicle Platform. That's quite a "mouthful", so we can assume that Global "B" will be way that most technicians will refer to the new network architecture. The 2021 Corvette is also Global "B" architecture and most GM vehicle models by year 2023 should be Global "B". More information here: gm​.​com/masthead-story… 

Ray Diagnostician
North York, Ontario
Ray Default

Thank you Martin, excellent post as usual!

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Martin Instructor
Burnaby, British Columbia
Martin Default

Thanks Ray. We've been so used to diagnosing some systems using familiar methods for so long, that keeping in tune with changes that are coming fast, will be necessary.

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Maynard Technician
Elmira, Ontario
Maynard Default

Thank you Martin, very informative. As for scantools authorized to access SDGM.... will this be a similar scenario as we experience with Chrysler? It will be primarily OEM tool that gets access and it is authenticated through tool subscription and serial numbers and so on? And as for after market scan tools, maybe access is not possible without applying to have the tool added to the safe list by…

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Martin Instructor
Burnaby, British Columbia
Martin Default

Hi Maynard. Thanks. I honestly don't know how the scan tool thing will work out. It might well be similar to the Chrysler scenario with Snap On Tools. I guess time will tell on that. Since GM uses Snap On Business Solutions for the Electronic Parts Catalog, I would expect some form of relationship might be developed for the scan tool, but perhaps that is wishful thinking. Moving forward, since…

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Brian Instructor
Parma, Ohio
Brian Default

Thank You, This is a great lesson for me on some of the newer stuff that should be showing up soon. Fantastic information and presentation.

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Martin Instructor
Burnaby, British Columbia
Martin Default

Thanks Brian. It is especially important for us all to stay on top of it, when looking at the "full feature" CT5 Global "B" DLC terminal pin assignments stuffed with FD CAN and Ethernet.

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David Owner/Technician
Milton, Ontario
David Default

As usual Martin: THANK YOU VERY MUCH for taking the time to post this. I found it both helpful and interesting. Again THANK YOU. Take Care. Best Regards. Harley

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Martin Instructor
Burnaby, British Columbia
Martin Default

You're welcome Harley.

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