Today, a 2014 Silverado was towed in. The complaint was SES on, power steering inop at times, ABS light on, problems come and go. When I got in the truck, the Service Power Steering message was on, the steering had no assist, the SES and ABS and Brake lights were on. As soon as I moved the truck, the steering worked normally and I drove it inside. From the multiple complaints, it seemed to be a major communication failure. I checked for codes first and saw this....

[2014 Chevrolet Silverado 1500 LT, ECM/Inputs/Outputs Scan Data]

Quite a few codes, huh? Let's see what modules are communicating. Here's the module list on the high speed bus.

[2014 Chevrolet Silverado 1500 LT, ECM/Inputs/Outputs Scan Data]

This vehicle does not have I03 or JL1 so the only modules on the bus not shown are the HMI module and the EBCM. What about Bus activity?

[2014 Chevrolet Silverado 1500 LT, Waveform]

Well, we've got some obvious problems. The HMI is only on the high speed bus but the EBCM is also a gateway module for the HS Expansion Bus. Is this bus active? Let's see....

[2014 Chevrolet Silverado 1500 LT, Waveform]

UGH....nobody home....which modules show on the data list?

[2014 Chevrolet Silverado 1500 LT, Scan Data]

No EBCM here either. I was looking for EBCM communication on GDS2 and could not communicate with the module. I turned the key off and communication was established with both the EBCM and the HMI module for about one minute then they went silent again. I turned the key back on and still no communication. Let's try something. I turned off the key and communications were established again for about one minute. Interesting....I wonder what happens to the HSGMLAN when the key is rotated off. Let's see.

[2014 Chevrolet Silverado 1500 LT, Waveform]

Well, isn't that cute. Normal conditions with the key cycled off. When the modules power down, all communication is lost as expected. What's the next step? Well, there are some clues to look at.

Clue 1 - On the high speed module list, 2 modules are missing but the heading says "Control Module Not Awake."

Clue 2 - There is no communication on the expansion bus as the EBCM is down. Since it acts as the gateway module, the bus cannot communicate without the EBCM.

Clue 3 - The ECM has set U0299-00 High Speed Communication Enable Circuit as current.

Clue 4 - When the key is cycled off, communication is restored and the High Speed Bus waveform returns to normal as all modules are now communicating.

OK, time to use our most important tool between our ears and analyze the failure. Two modules are not communicating and the data tool tells us that they are not awake. The ECM says that a problem exists in the data communication enable circuit. G.M. low speed modules need only a 12 volt spike on the data line to awaken them, but high speed modules are different. They need a dedicated 12 volt enable line from the BCM to awaken. This should be a constant 12 volt feed with the key on, but our modules only awaken with the key cycled off. Why?

How do we find out why this is happening? I went to the data enable circuit for the EBCM and the HMI module at the BCM X4 connector, pin 23. Back-probing the pin, with the key turned on, there is a very short 12 volt spike then no voltage. When the key is turned off, there is a steady 12 volt feed until the BCM powers down as seen on the HSLAN waveform. Pin 22 is the communication enable circuit for the TCCM, ECM, and ATC. Checking this pin, when the key is cycled on, a steady 12 volt output is seen, as expected.

Maybe the signal is being pulled down... unlikely but I can prove it easily. I swapped pin 23 and 22 in the X4 connector. Now, with key on, I have normal communication with the EBCM but no communication with the ECM until the key is turned off. This must therefore be an internal BCM failure. A new BCM solved the problem and everything is fine now.

This shows why, with network communication issues, it's very important not to jump in too quickly. just look for the clues first and analyze the failure before replacing components.

James from Florida

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