2012 Chevy Cruze intermittent stall
2012 Chevy Cruze
Lady luck is a welcome friend. This car had an intermittent stall complaint and I was just about to call a bad PCM when the real issue suddenly surfaced. If you look at the initial code list and then the list I got after a code clear and duplicating the stall, it sure seems to be some type of high speed bus problem.
While scoping the can bus I pulled and pushed on the PCM connectors, multiple harnesses etc. I tapped on the PCM too and every once in a while the car would stall like the key was shut off. I immediately saw a bus issue when it stalled and zoomed in for a better look. You can see both CAN lines suddenly get pulled down together. Usually when I see CAN lines act like that it's a bad transceiver in a module.
Looking at all the data and trying to reason it out, it seemed pretty likely that the PCM was the culprit. I attempted to pull the PCM up so I could test power and ground. The car was running and when I went to unlatch the plastic retaining clip, the car suddenly stalled. I started it and tried again and I found I could make the car stall every time i moved the pry bar towards the battery. Now with the engine off I started more carefully moving and manipulating wires. When I moved the thick cable coming off the power distribution block I heard relays clicking. This cable wasn't loose and the battery cable wasn't loose. After some checking and wiggling I found it!. The large cable in the back of this distribution block had a loose nut and I could move the terminal by hand.
Here's the CAN bus when I moved the loose terminal. It looks pretty much the same as when it stalled. I tightened the nut and cleared all the codes and it was fixed. It obviously was interrupting power to the PCM. A simple fix that I almost missed but today was my lucky day.
Here's a couple quick videos for demonstration. This is with the engine of and you can hear the relays clicking.
You can see both CAN lines suddenly get pulled down together. I can’t! Could we zoom in a little bit more? (are these rare spikes actually good packets, is the space between them just a lack of packets on the bus?)
How's this? I also added a tdms file to my original post. That's when I was moving the loose cable
The second picture shows a different moment in time than the first. Zooming muuuch into the first picture shows the can line shorted together though as well. Great you added the data files. Thank you sir.
I still don't see how it can be shorted together lines. The pulses are 0.4ms in length and go to normal (3.5, 1.5) voltages: diag.net/file/fffy25sw3… To me it looks that outside of the pulses the lines are just not driven. Unfortunately, eScope's sampling rate does not allow to zoom in more. One thing is that the issue was identified as unpowered PCM, even though the PCM itself (and CAN…
In between the pulses where both lines go below 2.5 is where I thought the issue was. In hindsight it is probably the bus shutting down because the module lost power.
I zoomed in on the phone screen and took this for shorted lines…..
Great post Bob!! I am on a similar repair. Equinox theft recovery that they took a sawsall to the power dist block and wiring trying to steal a battery & cat. Same power distribution on top of the battery, now have no RF headlamp as of late today. Great scope captures & info!
Outstanding post!! Thanks again as usual Bob, really good stuff!!!
Nice write up, sometimes it’s better to be lucky than good.
Electric steering related always wakes me up , I've found Alternator B+ cable nuts the same way on previous worked on GM with that wiggle .
Thanks for posting Bob! There are also issues with the short cable that runs from the positive battery post to the power distribution block. There is a TSB on the Malibu for excessive voltage drop on that short cable.
Looks like when the battery was replaced ( autozone looks newer) that fuse box is removed to replace battery and it never got tightened properly when reinstalled. Great find and thanks for sharing how you found it!
So I’m wondering; after the engine is running, does it need the CAN bus to stay running? All of it’s sensors go directly to the ECM. Outputs(drivers) go directly to components. Other than if it has a FPCM, what data would it need from the bus to sit and idle? If it does need the bus, wouldn’t every hard fault of the bus cause a stall?
That's an excellent thought. I also wondered if a CAN bus fault could stall a running engine. I think by itself, a can bus fault wouldn't stall an engine or we'd be dealing with a lot more erratic stalling cars. That's one reason I wanted to check the PCM closer. Test powers and grounds etc.