loss of communicaton codes are common when voltage drops below about 10.5vdc, if you can plug in a scan tool and get/clear codes your data bus is working fine

 

Thanks mountainmanjoe for the reply. I started to look at the BCM. The BCM for my vehicle is NOT located behind the right driver side knee like the documentation states. It was located in front middle of the passenger knee bottom plastic plate. I took the BCM out of the holder. I pulled the X3 connector that has the data bus connections on it. Tried to start the vehicle with the connector removed. No PRNDM1 indicator and no crank. I plugged X3 back in, cycled the key and now it starts! I reinstalled all of the things I disassembled. It starts up every time. No codes are on the ECU now. I am kind of worried that this issue is going to come back to haunt me one day, but at least I know where to start looking. Thanks again for everyone's help!

 

thanks so much for posting your fix.....you could also put dielectric grease in those connectors to keep them from corroding ....im glad you fixed it....

 

If you have corrosion happening inside the vehicle the you have bigger problems.

 

Not fixed yet! Ugh!

This Chevy Express 4500 is a RV cutaway for a Coachmen Freelander 21QB. During the troubleshooting I accidentally stripped the brass positive cable screw. I replaced the stripped battery terminal yesterday and connected the house battery's back in and hooked back up the RV charging relay. The problem reappeared. It failed to turnover and the service airbag message appeared again. I unhooked the relay and after many tries was able to get it to start again. Seems to be very intermittent. I am looking back at the pictures of the scope screen for the CAN- low level. I noticed interesting voltage levels on the CAN Low bus line. Not sure if that is impacting the bus or not. I attached the picture so you can see that the low part of the CAN-Low line signal has a couple of different voltage levels. The CAN-High signaling seems to look fine. I will attempt more troubleshooting again tonight to see if I can create a fail/good start scenario to help a little more in finding the source of the issue. I wish I had a digital scope with 2 inputs and CAN bus decode, but I only have an older analog one right now with a single input.


CAN - low signal

 

I'd look at that charging relay. They can stick closed sometimes. And if the house battery is down, it'll draw down the main.

 

I think that if there were voltage problems, there would be codes to indicate that. There would also be a DIC message saying "Service Charging System" or something to that effect.

 

Yes the BCMs have been on vans in that location since 2003.

You're going a bit too deep with the scope on CAN. It won't help you as you won't be able to decode the CAN as it's encrypted, but the good thing is you don't have to. The software doesn't fail, so the only thing to worry about is continuity in the CAN harness, which is confirmed by the lack of "Loss of Communication" type codes in the control modules. Is PRNDM1 lit? Try wiggling the BCM-X3 again
And when I said C100/C101 I meant X100/X101 connectors (they're located near the horn/in front of the underhood fuseblock.

 

The CAN bus is not encrypted.

 

You're right. Sorry, my bad.