Letting the Real data lead you
Seems like every one of my Case Studies is a communication problem. I learn so much though so I think its important to share. I get called to look at this 2011 Equinox. Concern is no comm with the EBCM and the Power Steering Rack. The car starts and runs fine, of course there is no power steering assist. I plug in the scan tool . The BCM has U0121 and U0130 and U1814. I can not communicate with EBCM or the PSCM myself. The U1814 is Powertrain Wakeup Communication Signal short to ground. This circuit is really just a fancy IGN power feed to certain modules. It is a BCM 12v smart driver output. It tells all the modules in the circuit to wake up and begin communicating. Im not concerned about the no comm modules at this point because if the wakeup is not there they wont communicate anyway. I pull up a diagram and take a look at the layout. I check marked the modules that communicate green and a red X the ones that can not communicate. diag.net/file/f2c4ji4dh…
I dont really see any common denominator to tie the EBCM and PSCM together besides the fact they are on the same wakeup circuit referenced in the U1814. Because the shop already has the EBCM connector accessed, I connect there. Sure enough the wakeup signal is stuck at grd with little repetitive like 50mv blips that I believe is the BCM trying to self test the circuit. I unplug the EBCM with no change. I unplug the PSCM with no change. Hmmmmm.......a quick visual of the harness doesn't look like anything is grounded out anywhere. I think to myself I wanna go straight to the BCM an see if it's shorted there also. I connect at the BCM in the second connection in the diagram and it's also stuck at grd. This point in the diag is why I wanted to make this post. In my mind I'm starting to get confused because even though the other 4 modules in the diagram are communicating the wakeup is still shorted . I don't understand how they are still talking. I decide not to read to much into wether they comm or not. I decide to focus on the fact that the wakeup is shorted an it could be anywhere. Unplug the telematics, no change. Unplug the fuel pump control module, no change. Unplug the Rear Diff control module an I see a quick 12 volt and then back to 0volts. Sweet I clear the code and now my signal is back at 12v and the EBCM and Power Steering are back online. After quickly jumpering the Rear diff module the car starts and runs and Power Steering is restored. Ive added a waveform that shows me RECONNECTING the Rear Diff Module.
Its important too note it takes at least 2 minutes for the BCM to actually turn the circuit back on even AFTER disconnecting the Rear Diff Module. This can also be accomplished by clearing the code. This type of circuit character is similar to any sort of smart driver. Its important to know this when looking for a short. You may unplug a module and when the circuit does not immediately return, you may think " okay thats not my problem". But it may be and you just need to clear the code or give the module time to turn the circuit back. Anyway I just it was interesting that one of the modules communicating was the issue.
Thanks Caleb. Your last paragraph reminds me of searching for a shorted 5-volt reference on older Jeeps. They would often be a PCM no-comm tow-in ("no-start" complaint). It was necessary to cycle the key after disconnecting EACH sensor. And often times, what were indicated as separate 5V sources on the WD were all the same source internally, so that could fool us too.
Caleb, excellent work and presentation!
Thanks for sharing good diagnostics strategies
Great teachable case study Caleb - thanks! BTW - some modules are woke up by bus messages AND/OR a discreet wake up / ignition input while others are strictly woke up via a discreet wake up / ignition input. With GM I’ve seen mostly the discreet wake ups on chassis related modules, bus and/or discreet on power train and bus only for infotainment module. Sometimes explains why some modules come…
Nice work. I like how you stated "This point in the diag is why I wanted to make this post. In my mind I'm starting to get confused because even though the other 4 modules ..........." It is these points, forks in the road, of our diagnostics that make or break the job. I am trying to communicate with others these stategies along with no putting on the blinders.
Great write up and info. Thanks
Thanks everyone. It was definitely a fun one ! I personally enjoy communication problems.
Very good post. I the the exact same issue on the exact same car about 5 months ago. Rear differential module was pulling the wakeup line down from the bcm. No pscm or ebcm communication.
Interesting....so simply disconnecting and reconnecting was the answer? No flash, no replacement?
No reread my post carefully. Yes the rear diff module was junk.