Intermittent Communication Tips?

Joseph Technician Lexington, South Carolina Posted   Latest  
Question
Electrical
2008 Chevrolet Suburban 1500 5.3L 4-spd (4L60-E)
Gauges Drop
Ac Stops Working
Low Speed Network (Pin 1)

Hey Everyone, I know this is child's play for most of you but I am having trouble nailing this one down and I am starting to question my approach.

By the way the only comm codes were between the airbag module and rollover sensor. as well as no comm with the radio due to an aftermarket head being installed I assume. I looked at the tsb that describes these issues due to high speed comm problems High side is stable during the event

Customer complaint was driving gauges drop and Ac stops working. It took several test drives to duplicate his concern and my suspicions were confirmed the low speed network was being pulled high (12V), which I feel is a module failing ,when the symptom was occurring. Unfortunately for me the network only goes high for between 5-10 seconds at a time and I have not been able to isolate the modules in that time.

Due to it only happening for a short amount of time this is the approach I have taken

1. Pulled the jx300 connector to try and split the network in half because of the codes in the airbag modules on that branch. symptom still present.

2​.​Disconnected easy to get to modules on the other half of the network, (window switches, hvac module). same symptom 

3​.​Getting desperate started the truck pulled the JX200 connector and started scoping the comm lines individually, I know I should have just got the pico out and put it on multiple modules at the same time but at this time I was able to duplicate the concern by driving it pretty easily ( I thought) and was sure I was about to nail it.

Something I noticed is with JX200 unplugged and tapped into pin J that goes to the modules associated with onstar I had no activity on that pin, I am not sure if this is normal or I just don't understand the system well enough. 

If anyone has any comments or critiques on my approach I would really appreciate it. Whether I am on the right track and I need to keep slogging it out or I am going about it all wrong and need to step back or any other tips you might have to help me get this one fixed.

Thank you everyone for your time!

+2
Billy Mechanic
Deming, New Mexico
Billy Default
 

Isolate the modules by disconnecting jx200, jump term a and d and see if scan tool communicates with IPC. If it does then use another jumper (fussed 3 amp) and test other modules individually and see if lose scan data or high voltage. I have replaced a lot of that era IPCs and also make sure the aftermarket radio is off the low speed lan (disconnecting the communication wire from the radio…

+2 Default Ð Bounty Awarded
Caleb Technician
Mishawaka, Indiana
Caleb Default
 

If your worried about the high speed scope it and make sure it's okay when the problem occurs. Then you can focus on the low speed. Low speed is a 0-5 square wave . Even though the radio is no comm that doesn't mean it's not on the network. That radio from the factory does not have a hard wired igniton input to turn it on. That wakeup is done thru low speed. There is some sort of aftermarket…

+1 Default Ð Bounty Awarded
Billy Mechanic
Deming, New Mexico
Billy Default
 

Thank you Caleb you had the words that failed me about the radio. back when I was in the trenches and a GM came in with aftermaket radio that would be the first to disconnect for diag,and a lot of times it solved the problem. Have seen a lot of no communication solved that way.

0 Default Ð Bounty Awarded
Caleb Technician
Mishawaka, Indiana
Caleb Default
 

Lol yep same here Billy ! Aftermarket junk tied into any vehicle network always makes for a good time!

+1 Default Ð Bounty Awarded
Angel Technician
Boston, Massachusetts
Angel Default
 

Great response Caleb, easy to suspect aftermarket accessories but its important to know how the system is supposed to work normally, and sometimes that information can be hard to come by with aftermarket resources.

0 Default Ð Bounty Awarded