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GM has released new diagnostic software to help technicians identify a starting point in diagnosing faults in a CAN bus (GM also calls this a LAN) by looking at how connectors and wiring are working or not working.


The Data Bus Diagnostic Tool is an application that uses existing MDI functionality. You will use your MDI, a Techline terminal, and an active TIS license.


To download the software, click the GDS 2 icon on the TIS2Web home page. (Fig. 1) A prompt to install the software will appear. The software will be available through TIS2Web as part of a rollout that began October 13, 2014 and continues until October 27, 2014. If a prompt does not appear when clicking the GDS 2 icon, the software has not been activated for your dealership, but will be available shortly during the rollout.


F01 bus gds2

Fig 1


Diagnostic Aids


If there’s a high speed communication issue and 20 U codes set in every module, those DTCs aren’t very useful in narrowing down the concern. The new Data Bus Diagnostic Tool software will show counts of each module and how often it’s NOT communicating. This very quickly identifies the problem module, and provides a quick point of diagnosis for checking the connectors, terminals and wiring for that module. It’s proven extremely useful in field testing for these issues.


Localizing the Fault


The application actively queries all possible diagnostic addresses to see what modules are on the bus. Modules that respond are on the ‘good’ part of the bus. Modules that don’t respond are on the ‘bad’ part of the bus.


The application doesn’t know what modules a vehicle is equipped with, so it can’t report which modules are missing. Compare the list of modules displayed in the application against that vehicle’s schematics in the appropriate Service Information.


TIP: Under a hard short, no communication is possible, so no modules will be listed.


Determine Physical Layer Status


CAN buses have characteristic voltages under different fault conditions. By monitoring bus voltage, it’s possible to identify whether or not the bus is physically OK. It’s also possible to identify the type of fault that’s present (CAN LO/HI open, short, ground offsets, etc) by monitoring bus voltage.


TIP: No message data is taken into consideration.


Three-In-One Application


The Data Bus Diagnostic Tool is actually three sub-applications in one, including Detected State (Fig. 2, A), Measured Voltage (Fig 2, B), and Message Monitor functions. (Fig. C) Each one has its own tab on the application’s graphic interface.


F02 bus tabs

Fig 2


Detected State – Identifies which control modules are responding to diagnostic requests, and identifies the physical state of the vehicle bus being diagnosed (i.e., open, short, ground offset, etc). (Fig. 3)


In performing these functions, this tab does three things:


1. It provides a diagnostic starting point. All possible ECUs are sent simultaneous requests at a rate of 250 ms. The application tracks and displays the longest time it takes for each ECU to respond to a diagnostic request to help with intermittent faults.


2. It tries to identify what’s going on with the wires that make up the bus (i.e., OK, CAN LO/HI open, CAN LO/HI shorted to ground, ground offsets, enable line open/blown fuse/power faults, etc.).


3. It provides audible feedback. You can set the application to beep continuously when a fault is detected or when the bus appears electrically OK.


F03 bus detected

Fig 3


This is a list of states that can be detected.

• OK

• Single wire opens (ie CAN LO Open, CAN HI Open)

• Double opens

• Single wire shorts to power/ground

• Single wire open/short (i.e., the wire is broken and the far side is shorted)

• CAN LO shorted to CAN HI

• Ground offsets (i.e., bad ECU grounds)

• Power faults (i.e., open supply, blown fuse, broken enable line, etc)


Using the Detected State, if a bus has a break point in communication, some modules on the “good” side of the faulted bus will respond very intermittently. When diagnosing intermittent faults, pay attention to the ECU with the longest response time and use the electrical schematic.


Place a check mark on the schematic next to the modules noted on the Detected State test. Modules without a check mark are the modules that did not communicate during the Detected State procedure.


Measured Voltage — Provides a low speed voltage trace of the selected bus, CAN LO and CAN HI. (Fig. 4) Due to current MDI limitations, it refreshes at about 1 ms, so it’s not as fast as an oscilloscope. It also won’t let you see the individual bits in a CAN frame (CAN frames take about 0.25 ms to transmit). It is, however, very visually obvious when a fault is introduced or removed.


F04 bus voltage

Fig 4


Message Monitor — Passively monitors bus activity to help diagnose battery drain problems. (Fig. 5)


The biggest difference between Message Monitor and Detected State is that it this passively monitors the bus, while Detected State actively queries the bus. Actively querying ECUs will make some of them stay awake. Obviously, keeping ECUs awake would be a bad thing if you’re trying to diagnose a power moding fault.


F05 bus monitor

Fig 5


This tab will tell you which ECUs have communicated in the past 1.5 seconds, how long it’s been since ECU last communicated (if more than 1.5 seconds have elapsed), and how many times the ECU was the first ECU talking on the bus (the bus is considered ‘asleep’ after 30 seconds of idle time).


TIP: This tab works on Single Wire CAN only.


Audible Feedback


The application will beep when either a fault is detected or when the bus appears electrically OK. This is user configurable by selecting the Settings function. (Fig. 6) Different beep strategies can be used in different scenarios.


F06 bus beep

Fig 6


In the case of a vehicle with a suspected intermittent bus fault, you might want to make the application beep on faults so that it will beep when the fault is detected.


In the case of a vehicle with a continually faulted bus, you might want to make the application beep when the detected fault goes away.


The application reacts to bus changes quickly (approximately 100 ms). Wiggle connectors and listen for a beep.


TIP: Be sure to unmute your PC speakers and the application.


Tracking Intermittent Faults


The application tracks the longest time it takes for each control module to respond to a request. Control modules which take longer to respond than their peers were/are likely on the faulted side of the bus.


If the application is started on a faulted bus and a module responds mid-way through the test, it will appear with a non-zero time value in this field.


TIP: This number will not reset unless the reset (eraser) button is clicked or the bus being tested is changed.


More Information


U.S. dealerships: For more information about the new software, check out the TechTube video on the GM Center of Learning website at The software also was featured in the October 2014 Emerging Issues seminar.


Canadian dealerships: The new software will be featured in the November 2014 TAC Talk.


In addition, a user guide for the software is available in the Service Information.


If you have any questions about the Data Bus Diagnostic Tool, contact the Techline Customer Support Center at 1-800-828-6860 (English) or 1-800-503-3222 (French).


– Thanks to Chris Henley and David Gumpert