Impala Auto Stop/Start Failure

William Technician West Virginia Posted   Latest   Edited  
Case Study
Electrical
2019 Chevrolet Impala LS 2.5L (A LCV Gas) 6-spd (6T45) — 1G11Z5SA6KU139664
No Codes
Stop/Start Failure To Restart

This is my first case study (so take it easy on me) from my old dealer. I am going to share some of the weird things have seen as I no longer work there. This vehicle came in with the customer's concern of a failure to restart on auto stop/start events. I test drove the vehicle and it restarted every time on auto stop/start events. I scanned for codes and found none. I, therefore, began the same way always do when I am unable to duplicate the customer's concern, the basics. BOC, which stands for Battery, Oil, and Coolant. The oil and coolant levels were fine as expected so focused on the battery. At the dealer, we use the GR-8 battery charging station for battery testing as this is what GM mandates. I performed a SOC (state of charge) and SOH (state of health) test on both the main and auxiliary battery, in which both failed. Easy peasy. I changed both batteries, performed the battery module relearn, and test drove the vehicle for an hour to let BCM complete the learning of the battery SOH. I had no issues so I released the vehicle.

Here is where the fun begins. The vehicle returns on the hook 3 days later. The customer states the vehicle does not start will just click like the battery is dead. I go out to the vehicle and low and behold it starts the first time. I drive it into one of my bays and scan for codes again. No codes. I attempt to start the vehicle approximately 10 times and on about the 8th time, I get this (see video) over and over again. Now, we are getting somewhere. The customer concern is confirmed. I used GDS2 to scan for codes and found U0100 Lost Comm with ECM as current in the Parking Brake Control Module, Power Steering Control Module, Body Control Module, Human Machine Interface Control Module, Electronic Brake Control Module, and the Chassis Control Module. I decided to use the Data Bus Diagnostic Tool to get a quick look at the can bus waveform and found this (see picture 1). Now that is a messed-up CAN waveform! What is happening there? Well, luckily I have seen that only one other time on a 2008 Chevy Cobalt 2.2. That vehicle had a TCM with a bad ground making the module pull up the CAN GMLAN network voltage. At that point, I decided to remove the ECM X2 and X3 connector covers and test the ECM grounds. Using a piercing probe, I tested the grounds using Voltage Drop. 6.5 volts on both. I then provided the ground a path (fused) directly to the battery negative and the CAN waveform cleans up (see picture 2). Noisy but correct and the vehicle started immediately. The fix ended up being the entire engine harness as it was difficult to isolate the fault with the harness on the vehicle. I later found but unfortunately did not take a picture of the almost completely broken splice between ECM X2 pin 73, X3 pin 73, and G121 (see picture 3) causing the intermittent bad ground. Hope this helps others on their journey! Thanks for this great place to share information and experiences with meaningful people.

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Robert Mobile Technician
Michigan
Robert
 

William Very nicely done. You did a great job explaining your diagnostic approach and providing pictures to substantiate the issue at hand. It is tidbits like this that help others to build upon their diagnostic skill sets!;

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Randall Technician
Pennsylvania
Randall
 

William, I love that you mentioned BOC! I can’t remember how many vehicles I have seen where those basic checks hold valuable clues and yet are ignored by others. I DO regret not keeping a journal. Keep sharing!! Awesome fix and great post. Good luck on your new job! Thank you. Randy

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Bentley Manager
California
Bentley
 

Thanks for the case study!

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Martin Instructor
British Columbia
Martin
 

Good job ensuring the basics were not overlooked! The Data Bus Diagnostic Tool is very handy. I've been using it since it was made available in 2014. Between the Measured Voltage and Detected State, it's a great window into the system, that provides info at an easy to view pace, with little if any manipulation required. I used it yesterday morning, to demonstrate an intermittent condition…

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William Technician
West Virginia
William
 

Once you figure it (the DBDT) out it truly is an amazing tool, turns hours into minutes. I like it especially with a scope and BOB at the same time so I can see who is communicating. The audible alert makes it easy to identify faults and the scope just adds more depth. So far the only VIP diags I have had to do were for SDGM failures and were fairly easy with system knowledge. Been reading a…

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Martin Instructor
British Columbia
Martin
 

I absolutely agree on the DBDT. Even now, many techs don't use it and are amazed when they see how useful it is. The same goes for reviewing session logs, although I can understand that a bit more. If a DTC or symptom diagnosis leads to a resolve, there is usually no need or time to review session logs, unless TAC or engineering require it. Between other work I'm prepping for a VIP hands-on…

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William Technician
West Virginia
William
   

This is an awesome conversation. Haha, I review session logs all the time with intermittent conditions especially. I will clear the codes (normally I wouldn’t) and compare different fault events to make sure the fault presents itself in the same way. This validates if the fault is the same (or not) and repeatable. Thank you so much. The DBDT slides you shared below are incredible. They will…

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Kody Educator
Oklahoma
Kody
 

Good write up and very helpful information. I have never used the gm data bus tool but looks very useful for sure.

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Martin Instructor
British Columbia
Martin
   

Kody, here's some Data Bus Diagnostic Tool (DBDT) pdf slides I put together a while ago, that might help in understanding how various conditions are displayed using both MDI 1 and MDI 2. Note: If a Bus is offline when the DBDT is turned on, the main screen area remains blank, but the header field identifies the condition. The DBDT doesn't know how a vehicle is equipped, but can recognize…

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Maynard Technician
Ontario
Maynard
 

Nice demo Martin!

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Greg Technician
Colorado
Greg
 

Really helpful stuff, thanks Martin! I've been successful identifying modules using Adam Robertson's techniques with the PICO serial decoding function. Nice to know there are tools out there that make it quicker. There are also functions (math channels) that allow you to see the result of Hi/Lo differential to see if the network can use a signal that looks unusable. great discussion, great…

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Martin Instructor
British Columbia
Martin
 

Hi Greg. Thanks. You will likely be interested in the linked post below, related to how HS CAN network diagnostics change with the installation of three Generations of GM Serial Data Gateway Modules (SDGM), Gen 1, Gen 3 and VIP (Vehicle Intelligence Platform). The Changing Face of CAN Bus Diagnostics on General Motors Vehicles – Diagnostic Network The Data Bus Diagnostic Tool will have little…

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Josh Technician
Virginia
Josh
 

Thanks for sharing!

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Raymond Mobile Technician
Michigan
Raymond
 

William, nice find thanx for sharing ,after u grounded circuit the voltage are correct but pattern had a lot of noise , it’s amazing how these module still work and signal can look like that , just my observation. So my question is what is this data bus diagnostic tool ? Do u have a part#? Does it have to be used with a laptop? Thanx will

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William Technician
West Virginia
William
   

It is basically a powerful tool for BUS diagnostics on GM products. It is available with the GDS2 diagnostic package. A MDI or MDI2 is required. It is available on acdelcotds​.​com/subscriptions Martin has another post with some great shots of it in use. It is a little difficult to explain the tool completely, but it shows what modules are communicating in real time on a single or multiple…

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Eric Owner/Technician
Wisconsin
Eric
 

Nice job William. Keep them coming!

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