Chassis CAN drama

Caleb Technician Mishawaka, Indiana Posted   Latest   Edited  
Case Study
Network Communications
2016 Chevrolet Tahoe LTZ 5.3L (C L83 Flex) 6-spd (6L80)
U0077 U0126 U0121 U0073

So this 2016 Tahoe comes in from one of our sister stores. They have just installed a new transmission( i dont know why). Upon completion they have various U codes stored mostly in the Chassis CAN bus and the Stabilitrack light flashes intermittently. They apparently arent having any luck so I get it. I do a full vehicle scan and clear them and then do it again. The codes that come back are in the attached photo. There are 4 separate CAN networks on this vehicle. Regular Powertrain at pins 6 an 14. Chassis CAN at pins 12 an 13. Powetrain expansion CAN just from ECM to FPCM. Object detection CAN at pins 3 an 11. But all the networks function exactly the same as any regular CAN network. The manufacters just have them split up for various reasons above my pay grade haha. Anyway moving on, with the car running everyone is happy and I can comm with all modules. I disconnect the battery and do a quick ohms check across 6 an 14 and 3 an 11 . Both 60 ohms. Ok moving on. Reconnect battery and hookup scope to breakout box. I observe when i move the shifter the Stabilitrack light flashes. I monitor pins 6 an 14 on the scope when moving shifter and the bus stays active no issues. Because the EBCM is on Powertrain an Chassis CAN i want to cover all my bases. I then monitor 3 an 11 Chassis CAN on scope and it pulls the bus to ground like clockwork everytime I pull on the shifter. I disconnect the battery again cause Im impatient and dont want to wait for the bus to go to sleep. Another ohms check on pin 3 to ground shows 0 ohms when pulling shifter. Ok cool more than likely this a direct short to ground and not inside a module. I hoist the car just to make sure the shift cable isnt rubbing on a harness to apease my manager because hes sure the problem was created during the trans swap. A quick visual reveals no issues at this time. I pull the column panels off and dont see anything near the shifter. I can wiggle the harness and it shorts to ground everytime but i cant see the short. Getting slightly frustrated Im beginning to doubt my diag. But I tell myself no trust your test results and move down to the base of the column, and viola! There she is rubbed just touching on the base of the column blue/yellow Chassis CAN high wire. I push up and my bus goes to ground. Sweet I repair and reassemble. Next! see attached diagrams and scope shots. 

Brian Owner
Parma, Ohio
Brian Default

Nice find , great information.

0 Default Ð Bounty Awarded
Elias Owner/Technician
Poway, California
Elias Default

Way to stay on it!

0 Default Ð Bounty Awarded
Ray Diagnostician
North York, Ontario
Ray Default

Excellent work!

0 Default Ð Bounty Awarded
Joel Diagnostician
Soquel, California
Joel Default

thank you for sharing !!

0 Default Ð Bounty Awarded
Andrew Technician
Commack, New York
Andrew Default

Good job in many ways! You have my thumbs up! I have some comments that may be helpful to you and others: Understanding the number and layout of the vehicle CAN networks is very important if you have determined there is definitely a CAN problem (gj!). Avoiding testing the CAN networks with disconnected modules is a good idea, for example the terminating resistors are not always 120, they can

+4 Default Ð Bounty Awarded
Brian Owner/Technician
Amanda, Ohio
Brian Default

Great Diagnoses, thanks for sharing!

0 Default Ð Bounty Awarded
Martin Instructor
Burnaby, British Columbia
Martin Default

Good job Caleb. Beyond the quick "history" review of prior repairs, when nothing was obviously related, the CAN bus diagnostics worked like a charm! Watch out for the newer platforms with isolated gateways that measure 120 Ω across the bus connections to the DLC, which is now isolated from direct connection to each bus. That's going to catch anyone out who assumes that the diagnosis is the same

+1 Default Ð Bounty Awarded
Angel Technician
Boston, Massachusetts
Angel Default

Great case study Caleb. Thanks for sharing

0 Default Ð Bounty Awarded
Jimmy Analyst
Québec, Quebec
Jimmy Default

Would love to include this case study in a textbook for author James Halderman. I'm doing research for him and this is exactly the type of interesting info he likes to include. This is for a textbook to help train the next generation auto techs and would be for students. Would that be oaky?

0 Default Ð Bounty Awarded