U0073 & U0101 Network Errors - Can't communicate with ECM, TCM, BCM and ABS

[Steve Shaw]

Hello,

I will try and keep this short and to the point.

2008 2500HD Sierra 4x4 6l90 WT (Very few modules, manual windows)

Driving along, the trans goes in to limp mode. Throws a P0700, P2544, U0073, U0101. Try clearing the codes, they come back. Unhook battery, clear codes they come back. Limp truck back home, gauges start acting up, door locks freak out. I changed out the battery with a new one and cleaned up the maxifuse connections. I also just by passed the maxifuse as well.

Scan truck with a snap on solus scanner. Now it will not communicate with ECM, TCM, ABS and BCM. Still communicates with other modules. If I unhook the battery, hook it back up I can communicate with everything for maybe 30 seconds, then they all lose communication again. Just long enough to read all the modules (most

The High Speed GMLAN is solid at 60ohms. I reset all of the connectors, and pulled/tugged on all of the wires I could. Resistance is solid at 60 ohms, so it does not seem to be a bad connection, at least on the GMLAN side.

My guess right now is that one of the modules is causing issues. What happens if I jump the GMLAN wires (+ in to + out and - in to - out) at each module? I think it should just bypass that module, and the rest should at least communicate properly, correct?

I do not believe it is power or ground issue as I get the exact same results each time I reconnect the battery. Everything connects long enough to read each module, then they go dark after about 30 seconds. Bad ground or power should cause it to be a lot more sporadic. Either way, I am going to go check for 12V and good grounds at each module right now.

Thanks for any help,
Steve

[Steve Shaw]

Update

Here is what I found today

-Pin 6 and 14 on DLC are both reading 9V with everything plugged in.
-BCM unplugged - DLC, Onstar, and ABS I get 1V on Pin 6 and 14.
-BCM unplugged. At the BCM connector, ECM and TCM I get 10V on the GMCAN + and - wires.
-DLC (BCM Connector), ABS unhooked, should only be OnStar module I get the right ~2.4V on both wires. Hook ABS back up and it goes back to ~1V on both.

Tomorrow, I will unhook the TCM connector and start testing there to isolate TCM. There is a heat shield on the side that needs to come off. When I rebuilt the trans 2 years ago, I had to replace the connector as 2 wires were exposed and nearly broken. I will double check all of my splices while I am at it.

I do not think this is a bad connection or ground issue. Of course I will still check everything to make certain, if I can isolate the culprit module(s) then I can dig in deeper. Connection issues tend to be intermittent, this was instant, and has not been an issue in the past. When this happened, I was cruising on a smooth highway. No sudden bumps or jolts. The only weird thing was voltage was running high at 15.0V, it is usually rock solid in the 14.0V range, when this happened. (I was logging it with Torque at the time)

Question
- If a module is correctly functioning you should see ~2.4V on the GMLAN + and - wires, assuming it is isolated from the rest of the network correct? It would appear this way, because these were my results when the OnStar Module was isolated. I did a little testing on my 07 Tahoe, and it appears this way also.

Please let me know if there is a flaw in my thinking. I am going to continue down the 2.4V from each module means it is not the problem, line of thinking.

Thanks again,
Steve

[tbarn]

2.5v is what is normal with the circuit "idling". Seems as though you have a short to voltage be it wiring or internal to a module. Given it's age it is anybody's guess. Given that communications work for a short time indicates to me the culprit is probably a module. Here are a couple of wiring diagrams that may help.

[Steve Shaw]

Thanks tbarn. I have been using diagrams to work through this. The alldata interactive diagrams are pretty useful. The plan is to isolate the problem module tomorrow. 

 

As all the testing had been done with Key On, Engine Off, I am guessing the modules are at idle? Either way, once isolated, I am also going to try and communicate with that module through the scanner. 

 

Thanks again,

Steve

[dna9656]

You guys are in some pretty deep waters....

[Steve Shaw]

Tell me about it. I am about to start on the trans connector. 

 

Again today, when I first started it I was able to pull codes from all the modules. Cleared the codes, and re checked the codes. It wouldn't communicate with the tcm and bcm. Reset that, lost communication with everything again.

 

My best guess right now, its the tcm. Why? Its the hardest and most expensive module to replace. 

 

Thanks again for any help,

Steve

[Steve Shaw]

FIXED - The trans connector had some water in it. The lower pins were corroding. The aftermarket connector I used, when I rebuilt it, must be sub par and letting water in. Cleaned everything up and added some dielectric grease. All my voltages fell back to into line. 

 

Moral of the story, check your connectors, power and ground first. 

 

Thanks for the help,

Steve