no ecm/tcm communication

[mhowald]

I have a 2006 Chevrolet C5500 LLY/ Allison , motorhome application, that is in derate mode due to communication issues between ecm and tcm. It has been in the hands of a shop for a bit over 2 years and no resolution. The shop has replaced the ecm and tcm with known working units and issue persists. The wiring harness has been inspected, connections cleaned, ohmed out and all appears ok. Of course we are missing something. Any ideas?

 

[2004LB7]

sounds like you need a bench harness to test them and see if they are bad or there is something i the harness you / they missed

 

[Chevy1925]

I have a 2006 Chevrolet C5500 LLY/ Allison , motorhome application, that is in derate mode due to communication issues between ecm and tcm. It has been in the hands of a shop for a bit over 2 years and no resolution. The shop has replaced the ecm and tcm with known working units and issue persists. The wiring harness has been inspected, connections cleaned, ohmed out and all appears ok. Of course we are missing something. Any ideas?

well, you basically gave us nothing to go off of. if there is a comm issue, there are codes. specially if its in a "limp". hwo do you know its in a limp? what is the rig doing? why has it taken this shop 2 years?

we are not there, you gotta paint the picture so we understand what you are asking us.

 

[mhowald]

I understand that the details are missing. I will see if I can get the codes from the repair shop.

 

[NoHeadNed]

I have a 06 5500 with the same issue. Code U0101 ECM lost communication with tcm. I have this similar problem, I have replaced the internal harness, the pressure switch assembly, because the temp sensor was the only thing that did not check out, ohmed all wires from trans to tcm, checked for short to grounds and opens. Found an open in the temp switch. I changed the PSA with internal temperature sensor. Fixed the open on that circuit. Also changed the harness for safe measure.

Backstory, Allison condemned the tcm. I have a brand new TCM and my guy cannot program the tcm because of a communication error. I really need help figuring out where else to look.

 

[Ron Nielson]

The first check is to see if the ECM has powers and grounds You need a wiring diagram to see which pins on the ECM are power or ground. Also need to check the communications lines. Your ECM needs a complete electrical circuit and some communication also. GMUPFITTER publishes ELECTRICAL manuals for many trucks but trucks '12 or older are extremely difficult to find as they have lost the formatting to keep the years separate. Try Googling and see if you can find what you need

 

[NoHeadNed]

Thank you! Merry Christmas! I am going to find the schematic for the ecm harness today, my programmer says it is most likely a wire between the ecm and tcm

 

[NoHeadNed]

How do I check communication wires?

 

[Ron Nielson]

The best way to check is with an oscilloscope, but it's unlikely you have that. See if there is any voltage at all. If they're dead, that's not good. Or at least make sure they're not broken, and have continuity., try a test light with low-watt bulb.

 

[mhowald]

My dtc code says that the ecm has lost communication with the tcm. Does anyone know if the 2006 c5500 medium duty with LLY duramax uses the canbus for ecm/tcm or class 2 wiring?

 

[Ron Nielson]

https://www.scribd.com/document/387160397/2006-chevy-6-6-c5500-MD-Electrical-pdf#

You may be able to find your info here

 

[mhowald]

I went to the link you provided and searched for the 2006 medium duty diagrams. It looks like the header says 2006 but provides diagrams for a 2005??

 

[Ron Nielson]

There were years when GM did not produce a new electrical manual because the wiring diagram didn't change. This is one of those situations.

 

[2004LB7]

Like Ron said. I think 2003 through 2007.5 are basically the same. If they don't have one for a particular year then the one preceding it is probably the correct one. In your case the change to the electrical happened mid 05 so that would be the new diagrams. Then another change mid 07-08