rockfishon

I have a 2015 Chevy Express 4500 with a little over 18,000 miles.

Symptom - When trying to start engine, turn key , dash goes dark and nothing happens, the engine does not turn over. Stop turning key and "SERVICE AIRBAG" is displayed and dash chimes 33 times. I used a Snap-on Solus Ultra to read the codes.

Troubleshooting so far:
Battery is good. Checked all fuses in engine compartment and under drivers seat. All test good. Looking for ideas on what to check next.

canucklehead

The codes point to a bad pcm or can bus issue.....
The fourth one is rear wheel speed sensor, but can also be caused by a faulty control module or wiring problems.....

rockfishon

Thanks for the information! Do you know where I can find the wiring pinout of the electronic brake control module connectors? I would like to verify power and ground to the module.

rockfishon

I think I found it on page 59 of the Body Builders Service Manual:
https://www.gmupfitter.com/files/med...2015_en_US.pdf

rockfishon

If I disconnect the ebcm, should I be able to start the vehicle?

Artie1

No EBCM fault will cause no start condition, so you can safely ignore the C0055 for now.
The three U codes are important and most likely have one common cause. The ECM, BCM, TCM modules are unable to communicate with each other. Finding the reason could be tricky, time to study that electrical diagram posted above and verify power, ground to the modules, disconnect those C100 and C101 connectors and check for continuity on the data wires. Automotive electrical troubleshooting experience required.
As a side effect, PRNDM1 indicator on the instrument cluster will not be illuminated while no communication is present. So when it lights up you know you found the problem.

LCAC_Man

seized/shorted starter, loose cable connection at the starter, bad battery ground connection at the motor/frame

rockfishon

Thanks for the info Artie1!

I do not see a C100 or C101 connector in the Body Builders service manual.

The PRNDM1 indicator is illuminated and seems to work when shifted. The head lights and FTP and brakes lights all seem to function properly as well.

Troubleshooting 1/18/19:
I disconnected the connector to the Airbag control module under the drivers seat. I got different symptoms when I tried to start the vehicle. I then reconnected it and disconnected the EBCM on the drivers side rail under the vehicle. When I tried to start the vehicle, I got the same exact symptoms as when it is connected. I measured 12 volts on both pins 1 and 11 and 18 and 28 on the EBCM connector K17. The ground was good to the ECBM as well.

I am going to hook a oscilloscope up to the CAN bus data pins and see if the correct signal voltages for proper bus communications are present.

Troubleshooting 1/19/19:
I hooked the scope to pins 6 and 14 on the OBD2 interface under the drivers side dash. I turned the ignition on. I saw the expected wave forms with no strenuous signals. I tried to start the vehicle and this time the started engaged and the engine turned over but it still did not start. The "Service Airbag" message was no longer displaying. I disconnected the connection to the OBD2 connector. I tried to start the engine again and it turns over but did not start. I tried again and the "Service Airbag" message re-displayed. I reconnected the OBD2 connector to the scope and was able to get the "Service Airbag" message to go away again.

With the engine wanting to turn over, I disconnected the EBCM connector again. I got the same exact symptom as when it is connected, engine turns over but does not start.

Next step, I am going to try to see if there is any data on the SAE J1850 bus pins on the OBD2 connector.

Troubleshooting 1/20/19:
I connected the scope to pis 2 and 10 on the OBD2 connector to measure data. I did not see any signals on those pins.

I disconnected the battery so I could measure the resistance of the High speed bus, pins 6 and 14 on the OBD2 connector. I read 60.4 ohms which is the correct resistance.

rockfishon

Any Ideas on what to test next? Is there a way to tell if the EBCM is functioning correctly? I do not own the scan tool but have to borrow it.

mountainmanjoe

DTC U0100: Lost Communication With ECM/PCM "A"
DTC U0140: Lost Communication With Body Control Module (not Brake Control)

Check continuity to them.