GM Class 2 network Diagnostics Video

Richard Instructor Palmetto, Florida Posted   Latest   Edited  
Case Study
Trained By Techs
Network Communications
2003 Chevrolet Trailblazer 4.2L (S LL8) 4-spd
No A/C Power Windows And Locks Inoperative

I recently gave a network communications class and found that many techs have a really hard time wrapping their head around what is going on with network communications.

"If you don't understand it, you can't fix it" is a mantra I've heard from many instructors (including myself) throughout my career.

In this video, Keith Perkins demonstrates how to properly identify which module has caused complete network failure on a 2003 Chevrolet Trailblazer. 

I realize for many of you, this may seem like a simple, typical GM network issue but, short simple videos like this provide a great resource for techs just starting to get their feet wet with network issues and help them understand how it works. 

Hope you enjoy, let us know what you think!


Jim Educator
Freeport, Illinois
Jim Default

Great video! Teaching network communication(s) operation and diagnosis to entry level students/technicians is a challenge. This video showed me another way to explain and demonstrate operation, diagnosis and verifying the repair.. Good stuff!

+5 Default Ð Bounty Awarded
Keith Instructor
Collinsville, Oklahoma
Keith Default

Thank You for sharing my video Rich, I also would like to open the discussion to what are other methods to pinpoint this type of concern more efficiently? I by no means am an expert, the video was not created to show a completed analysis or repair, just the method and approach to honing in on the issue efficiently.

+6 Default Ð Bounty Awarded
Chris Diagnostician
Commack, New York
Chris Default

Good job Keith. Keep them coming.

+6 Default Ð Bounty Awarded
Michael Owner/Technician
Cartersville, Georgia
Michael Default

Hi Keith, I think your diagnostic procedure is spot on for most CAN systems. Having combs in this vehicle makes it much easier to diagnose because everything is there to test. Having a dead module or wiring issue is relatively easy to diagnose. Where I struggle is when your voltages, resistances, and scope patterns are correct but you are still getting communication codes. I have no idea how

+2 Default Ð Bounty Awarded
Keith Instructor
Collinsville, Oklahoma
Keith Default

Thank you Michael, Yes, communication codes without visible or "scope-able" failures can be hard to analyse, I usually rely on the ability to replicate the code, and track the conditions in which it sets. I find most often now if the network is sound, and no "dead" modules are present, my "U" codes are from voltage supply issues, such as weak batteries.

+1 Default Ð Bounty Awarded
James Technician
Plant City, Florida
James Default

I see in the video you stated that the Class 2 network has a 7 volt bias provided by the BCM and ECM. The Class 2 protocol uses a 7 volt pull up signal, not a pull down signal and the normal state of a Class 2 line is zero volts.

+3 Default Ð Bounty Awarded
Keith Instructor
Collinsville, Oklahoma
Keith Default

100% correct James, the issue with it being pulled to zero was the car was running, at that time there would have been some activity on the network, but there was not. This indicated a problem.

0 Default Ð Bounty Awarded
Daniel Technician
Salinas, California
Daniel Default

Good video

+3 Default Ð Bounty Awarded
Brian Diagnostician
Willoughby, Ohio
Brian Default

Keith, Is there any significance to why this issue happened during the aftermarket radio install. Is it possible the radio could have damaged the suspect module? Were precautions made the to customer to maybe not use that radio or to have it properly tested and installed by a professional? Or, do you think this was just a coincidence? I have personally bugged networks in many ways and have

+1 Default Ð Bounty Awarded
Keith Instructor
Collinsville, Oklahoma
Keith Default

Unfortunately the radio was completely missing when the vehicle was brought to us, as in no radio or additional wiring as installed as it was all removed before it arrived. I believe the radio itself may have not damaged the module, but improper installation or testing of the circuits may have. I know we all know how dangerous a "Powered Ice Pick" (Power Probe) can be in the wrong hands. So, I

0 Default Ð Bounty Awarded
Merle Educator
Nyssa, Oregon
Merle Default

Great video Richard. I shared it with the automotive instructors that I mentor. Thanks for sharing.

0 Default Ð Bounty Awarded
Bill Technician
Rosetown, Saskatchewan
Bill Default

Nice efficient diagnosis. Excellent work. I wonder if there may have been any change to the operation of that module trying a global reset.

0 Default Ð Bounty Awarded