oceanbrave,

I will be studying the information you sent and thanks. My hummer's DLC is serial data 2 communications and not CAN. I used my DMM to find out the voltage and it was between 1 and 2 (#3 in my opening post). Page 35 of the document (Network Comms & Diagnostics V2.pdf) you sent references in bullet #2 that Class 2 is active all of the time the ignition is on; therefore, voltage varies between 0 and 7. What does this mean compared to the voltages I found? Is it referencing low and high signal voltage levels based on the J1850 Protocol Variable Pulse Width for GM Vehicles?

Page 11 references the Universal Asynchronous Receive and Transmit (UART) which uses a fixed pulse-width switching between 0 and 5 volts. It is obvious from the picture that a lab scope was used to show pulse width. How is this applicable to my issue if at all?

Please excuse the questions. I am learning and educating myself, as a non-automotive technician, and if I can avoid taking this beast into the service center, that would be ideal.

Respectfully.

 

 

Thanks!

 

hummerz love Eric the car guy, so positive, so practical

ZackMan Apologies it should have been page 13.

J11850 Protocol is VPW. Some of the H2 devices e.g the Air Suspension Module use a specific Data Link Controller chip (Motorola MC68HC58) and not a UART as such but operate similar. The MC68HC58 data sheet shows the BUS levels switching between 0 to 7V as per page 13.

You seem electronically aware, so you'll know that the average BUS voltage will depend on communications activity, for example a square wave (but won’t be) the average would be 3.5V.

With the engine running and the average Min was 0.8V and average Max 2.7V, so your values are very similar. For reference the waverforms look like the attached, one is a short time frame the other longer.

To me it does seem like you have valid Serial Comms 2, so its point towards the Tech2.

I too have one and the normal way through the menus is to hit "Power On" -> wait for splash screen -> "Enter" -> "Diagnostics" - > "2006" -> "LD Truck, MPV, Incomplete" -> "Hummer" might be worth checking this aspect.

 

oceanbrave,

Thank you for providing this information. I did try another scanner on the DLC and the same failed communications occurred. The DLC wiring diagram you sent does show the purple wire (pin 2) going into the SP205. Maybe there is a loose connection. Two scanners exhibiting the same results leads me to believe the serial 2 communications is faulty.

This is is a guess because I have not physically accessed the SP205 yet.

 

for me the XM module was killing my serial 2 data. so have you found any of your trucks options not working?

 

bronxteck,

I’ll need to check that one out. What led you to determine that the XM module was killing your serial 2 data? What did you do to correct the issue?

 

I would not get any XM radio stations at all not even the free ones. removing the module fixed my problem serial 2 communication issue. i use a FM transmitter to listen to BT audio. so i wont miss XM but it was outdated any ways like the onstar module both analogue not digital.

 

On that vein I have seen similar problems with certain aftermarket devices too. I remember a remote start that I had on one truck that while functioning just fine, put so much noise and chatter on the data bus that no scanner could be used until it was first disconnected.

 

Thanks!